Advanced Marksmanship Sniper's Hide Marksmanship - Articles Archive

[JT505]

Ocular Lens Focus and Parallax Adjustment
In Blog by Rich - February 8, 2015

There is often some confusion when discussing the topics of ocular lens focus and parallax adjustment. People confuse the terms for each other and they misunderstand what it means to have a parallax free image. We are going to discuss how to properly set the focus of the ocular lens and we are going to discuss what parallax is and how to remove it with your scope. People don’t realize just how big an effect that parallax error can create when shooting and they often misadjust the parallax as they try to correct problems with the focus of the ocular lens. Let’s break the topics down and discuss how to get it right.

Ocular Lens Focus
What is the ocular lens, how do you focus it properly, what’s the purpose?
The ocular lens is the lens at the rear of your scope that you look through with your eye, hence ocular lens. What people often don’t realize is that it has its own focus adjustment and that you probably aren’t focusing what you might think. The whole point of focusing the ocular lens is to get a crisp and clear image of the reticle. Not the target. Just the reticle. The best way to set the focus on the ocular lens is to look through the scope at a well lit white wall, or in the backyard looking at clear sky. Set the parallax knob to infinity, which is the little figure eight symbol.

You change the focus of the reticle by adjusting the diopter.

Now look through the scope and look at the reticle. Is it clear? Or is it fuzzy? If it looks fine, look at something else, a car on the street or what your wife is making for dinner and count to five. Now look through the scope again, does it still look clear or is it fuzzy? Your eyeball will adjust and focus an out of focus image remarkably fast. So you have to sort of snap your eye behind the scope and get a quick read on whether or not the reticle is clear. If it isn’t, and even if it does appear clear, try adjusting the ocular lens focus. Most of the time this is accomplished by grabbing the whole eye piece and screwing it either clockwise or counter clockwise. If you have trouble, look for a locking ring, like a giant lock nut, just in front of the ocular lens. That holds the adjustment of the diopter from accidentally changing.

When adjusted properly you should be able to snap your eyeball behind the scope and get a crisp and clear view of the reticle immediately. Little changes can go a long way so go slowly and see what looks best. This is important because your eye will have to work harder when you shoot if it’s constantly trying to refocus the reticle for you. So make sure you get this set right, then leave it alone.

Parallax Adjustment
You probably hear this one a lot, but do you understand what it means?
The absolute best way to describe this that I’ve heard is Frank Galli of Sniper’s Hide and his method.
Draw a circle on a piece of paper with a pen. Now hold the pen vertically between you and the paper. Close one eye and line up the pen with the dot, then switch eyes and notice how the relationship between the pen and the dot changes. That is an image with parallax error. To remove parallax error, you want the target and the reticle on the same focal plane. Take the pen and touch the tip to the dot, now do the same drill closing one eye and alternating. That is a parallax free image and it is what we want to replicate with your rifle scope.

In order to accomplish the same task with your scope use the parallax knob, sometimes referred to as the focus knob, on the side of the scope. Sometimes there will be a ring in front of the ocular lens that you can twist for parallax adjustment, if you aren’t sure, consult your scope’s manual.

I start by focusing the image in the scope so that it is crisp and clear. However, it’s important that you understand a clear and focused image may still have parallax error present. In order to check, look at the position of the crosshairs of the reticle in relationship to the target. Now very slowly and gently, don’t move the rifle, slide or roll your head from side to side or up and down.

If you have a truly parallax free image the crosshairs will appear to remain in a fixed position.
For example if you slide your head slightly to the right, it will look like you are looking at the target from an angle, but the X of the crosshairs will still be on the bulls eye. If there is parallax in the image you will see the X of the crosshairs move laterally away from the bulls eye. If that happens you need to readjust the parallax knob and try again.

Understand that breathing and shooting, not to mention running around or moving through a rifle match or battlefield, will force your head to move around a bit on your stock. If you have a parallax free image, this won’t cause you problems. If you have parallax error present in the scope image, you will get shifting point of impact issues with regard to the point of aim.

Wrapping Up Ocular Focus and Parallax
Most of this stuff is pretty easy once you get the hang of it but having a good understanding of what adjusting the diopter on the ocular lens does versus what the parallax knob does will help keep you from confusing the purpose of the knobs. Once you understand what the functions of the different knobs are you can much more easily tweak the adjustment of the scope for maximum performance.

Remember, adjust the ocular focus with the diopter until the reticle looks crisp against a clear background as soon as your eye lands on it. You only adjust it once, then leave it alone.

The parallax knob is how you get the target and reticle on the same focal plane.

 

[JT505]

Parallax- How to set Up a Scope
By Paul Coburn

I've answered questions about scopes and parallax about 900 times, and it's always a long drawn out thing, going several e-mails, and a few phone calls. It doesn't seem to make any difference how long the guy has been shooting, this one always keep screwing guys up. OK... here goes, and it's gonna be a long one.

Parallax is:
When the image of the target, and the reticle, are not in EXACTLY the same plane, and by moving the eye up and down, or side to side, either the target OR the reticle appears to move in relation to the other.

You might see the target move and the reticle stay still.
You might see the target stay still and the reticle move over it.
Both are exactly the same, and which one you see is only a matter of your OWN perception.
It is NOT possible to have parallax while moving up and down, but NOT have it when you are moving side to side.
If you think that is what you have, you have other problems. Either you are moving the rifle, or you have eye problems.

How to set Up a Scope - This is the only way to do it.

First:
Screw the eyepiece out (CCW) all the way until it stops. If you wear glasses, put them on.
Hold the scope up and look OVER the scope at the sky and relax your eyes.
Then, move the scope in front of your eye. The reticle should look fuzzy.
Turn the eyepiece in 1/2 turn, and do the same thing again.
You will have to do this several times before the reticle starts to look better.
When you start getting close, then turn the eyepiece 1/4 turn each time.
Do this until the reticle is fully sharp and fully BLACK immediately when you look through the scope.
Then back off one turn and do it again to make sure you are in the same place.
Then LOCK the ring on the eyepiece.

Second:
Set the scope down on something solid where it can see something at a long distance. Half a mile or longer is good.
It can be on the rifle or rested in sand bags at the range, but pick something at least 1000 yds away, even further if possible.
If the scope has an "AO" Adjustable Objective, then set it for infinity and look at the distant object and move your head from one side to the other, or up and down if you prefer. If the reticle seems to move, there is parallax. Change the distance setting and try again. If you are very careful you can move your eye and adjust the distance at the same time seeing which direction gets better.

(Something to know - the graduations/calibrations on the AO of a scope are approximations ONLY. They get you close. If shooting is critical then check for parallax by moving your eye and adjusting until there is NO movement of the crosshair/target image).

With front objective adjustments you can turn them either way without worry. BUT with side adjustment scopes the adjustment must ALWAYS be made from the infinity end of the dial. Turn the adjustment all the way until it stops (past infinity) and then start turning it in a little at a time until there is no parallax.

If you "overshoot" the proper setting, you can't just turn back a little. You must go back and stop at the end of the dial and start over again. "AO"s dials are locked in place, and if the indicated distance doesn't match the real distance, there's nothing you can do about it. Side focus dials are not locked in place. Once you have found the setting for infinity on the side focus models, then CAREFULLY loosen the screws and set the dial so that little sideways infinity symbol is lined up with the hash mark, so it is calibrated. You can also make little marks on the dial or stick on a paper tape for other ranges instead of using the round dots that don't match any range. Now you can set it to infinity, but remember that you MUST turn the dial all the way past infinity to the stop, EVERY TIME before going from a close range to a longer range.

If you are set for 500 yds, you can go directly to 100 yds, but if you are set for 100 and want to set it to 500, you MUST go all the way back to the stop, and then go to 500. This is because there is a fair amount of backlash (aka SLOP) in this wheel linkage to the focusing cell, so you can set it only from one direction to make sure the slop is always on one side. The other problem with it is, even if you decided that you wanted to calibrate from the other end, the recoil will push the cell back. SO you must ALWAYS set these dials from the infinity end of their scales.

To make it easy to remember:
I always start from the end stop when I change range, no matter which direction I'm going in. It adds about 0.023 seconds!

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Post Parallax questions here -Management of Parallax Adjustment
http://www.snipershide.com/shooting/forum/s...lax-adjustment

 

[JT505]

Parallax - What’s happening inside the Scope
By Paul Coburn

There are several things that go on inside a scope, and in the eyes at the same time. Some of them work against each other. Some terminology first... And we'll leave out lenses that are there to correct some optical or color errors, but don't have anything to do with image forming. We'll start at the front of it all, and work back.

1 - The "Object". The "object" (target) that you are looking (shooting) at.

2 - The "Objective". The front lens is called the "Objective".
It forms the first image of the "object" we are looking at (that’s why they call it the Objective). It is the lens that "captures" all the light that is solely responsible for the image quality of the scope. If the objective is poor, you can't fix the poor image later. This lens is usually made of two different types of glasses (called "elements") sandwiched together and is called an "Achromat".

The Achromat is fully color corrected for blue and green. The red wavelengths are partially corrected but have what is called "residual color errors". These are very minor. This is the normal type of objective used in shooting and spotting scopes. In quality, they can vary from bad, through sort of OK, to pretty damn good. If one of the elements is made of an "ED" glass, or a "Fluorite" (CaF) glass, the two element lens can be very good to outstanding. In some instances, objective lenses are made of three elements, and all three colors (blue, green, and red) are completely corrected. This type of lens is called an "Apochromat", and this is the finest lens that can be bought. The best of these can also have "ED" glass, or Fluorite as one of the elements.

3 - The "First image plane". The Objective focuses the light to make an image of the subject, just like a camera lens. This image is upside down, and right/left reversed. This is the first image plane, but NOT the "First image plane" that is talked about when shooters talk about reticles.

4 - The "Erector lens". If it is a group of lenses it is called the "Erector cell". Because the first image is upside down/wrong way around, we (as shooters) can't use it, so we flip it around with a simple optical group called the "erector cell". This cell gives us a new image that is right way around, called the second image plane.

But this cell has another very important job. Moving this cell causes this second image plane to move, so micrometer spindles are put against the cell, to get elevation and windage adjustments. The total amount of elevation/windage available in the scope (MOA from bottom to top) is determined by how much the spindles can move the cell. The amount of movement "per click" is simply determined by the thread of the screw, and the spacing of the detents on the spindle.

5 - The "Second image plane". This is the second real image plane in the scope, and this is the image plane that shooters call the "First image plane" when talking about reticles. In a fixed or variable power scope with a "First image plane reticle", the reticle would be placed in this image plane.

6 - The "Zoom group". In a variable scope with standard (non-magnifying) reticle, the zoom group of optics would follow #5. This group of lenses can change the size of the image plane in #5 and then form a new (third) image plane behind it.

7 - The "Third image plane". In variable power scopes, this is the plane that the reticle is placed in. By being here, it allows the image to change sizes, but the reticle to stay the same size. In the context of reticles, this is the image plane that is referred to as the "second image plane"

8 - The "Eyepiece". This optical group is like a jewelers' loupe. It is (or should be) a super fine magnifier. It's only job in the whole world, is to focus on the reticle. Let me repeat that for those that live in Rio Linda...

THE ONLY JOB FOR THE EYEPIECE IS TO FOCUS YOUR EYE ON THE RETICLE!!!!
It CANNOT adjust, or compensate for, or do anything else when things look bad in the scope, or when you can't hit the target.
You CANNOT use the eyepiece to try to correct for parallax. That is sheer folly at best and raw stupidity at worst.

OK... now that you know what the insides are like... let's move on.

We'll use the zoom scope for our examples because if you can understand the zoom scope, then the fixed scope is a walk in the park.
In the scope that is set for infinity range, the object forms an image (upside down, right/left reversed) behind the objective (the first image plane)... the erector cell "sees" that image, and flips it over and makes it right way around in a NEW image plane (the Second image plane). The zoom group adjusts the size of this image plane, and makes a NEW image plane (the Third image plane) that is the desired size. There is a reticle placed in this last image plane, and the eyepiece focuses on the reticle AND the image at the same time. When things are good, that's how the scope works!

But... IF the third image plane and the reticle are not exactly, (and I mean EX-ACT-LY) in the same place, then your eye cannot see them LOCKED together as one picture. It sees them as two separate pictures, and the eye will look at each separately, and the eye can also look AROUND one to see the other.

Lenses are measured in metrics (aka Millimeters).

Not because the Europeans wanted the metric system 25 years ago, but because optical strings and chains of lenses (like scopes) are really a string of numbers. There are constant ratios of "this divided by that's" that give image sizes "F-ratios" and image locations. It's so damn easy to do the engineering using a 10 based system that the optical guys were using the metric system way back in the 1800's.

The objective has a "Focal length".
This is the distance behind the lens that the first image plane falls when making an image of a subject that is at infinity (or very damn far away). If the objective has a focal length of 100mm, then the image of that 1000 yd target is 100mm behind the lens. The problem with geometric optics (which is what we are dealing with here) is that they follow the laws of geometry and optics make triangles like rabbits make babies. In an optical chain, when you change one thing, one angle, one ANYTHING - everything else follows along and the changes are BASED on the ratios involved at THAT stage.

If we take that same target, and move it to 100 yds, the image in the scope moves BACKWARDS, going further into the scope. Not by much, but it doesn't take much, because we are dealing with very small distances inside the scope and very high magnifications. How far the image moves back and what its new position is, is predictable by the mathematical ratios of the angles formed by the subject and the first image. OR, by the ratio of the distances to the Target and the focal length, multiplied by the focal length, then ADDED to the focal length.

The target is at 100 yds (91440mm).
The focal length of the objective is 100 so the displacement is 1/914 x 100, which means that the first image is now at ~100.1mm. Only .1mm, that doesn't seem like much.

Read the following paragraph twice.
In a 1x scope, 0.1mm would mean nothing, but this displacement is repeated throughout the chain, AND if any of the optical groups change the image ratio (aka image size), then the displacement (aka ERROR) is changed in direct proportion to the increase in magnification. So in a 3x scope, it would be .3mm, and in a 10x scope, it would be 1mm, and in a 30 power scope, the image would be 3mm behind the reticle. Now, you should have seen a pattern in this last paragraph.

READ THIS TWICE!!
With the same error in the objective (scope focused at 1000, and target at 100), the parallax INCREASES WITH MAGNIFICATION.
Got it? If not, READ IT TWO MORE TIMES!! If we do the same math for closer distances, like 50 yds, and 25 yds we will see that the error gets really big, so that with a target at 50 yards, and the scope set at 35 or 65 yds, the parallax makes the combination un-usable.

That's about it on rifle scopes.
There are thousands of "opinions" on scopes on the web, but this is the science from one that does optics for a living.
Now, you have a friend that says to set up a scope a different way? The guy at the next shooting bench at the range said to do it a different way? You got a friend that shoots bench rest and says something different? Before you take their advice ask them to explain how a scopes works from the inside out. This is the way to do it, because this is the way scopes work.

 

[JT505]

First Round Hit Percentage Source; ARL-TR-2065

In August of 1999 the Army Research Laboratory published a report entitled, Sniper Weapon Fire Control Error Budget Analysis (ARL-TR-2065).
This publication is available on the Internet - http://www.arl.army.mil/arlreports/1999/ARL-TR-2065.pdf

On page 32 of this report you will find a table entitled; Error Budget M118LR fired from a Knight SR25.

M118LR is 7.62mm military ammunition for use in military issued Sniper Weapon Systems.
The Knight (KAC, Knight Armament Company) SR25 is a Stoner Rifle 7.62mm Sniper Rifle issued to select U.S. forces.
The 10.5” X 17” Silhouette target is the standard size target used in training.

The probability of making a first round hit on an E-Silhouette target (19.5" X 40") is listed as follows:

100 yards = 100%
200 yards = 100%
300 yards = 93%
400 yards = 69%
500 yards = 46%
600 yards = 27%
700 yards = 15%
800 yards = 8%
900 yards = 5%

“In certain Sniper units who practice the art and science of sniping, a first round hit is not always as important as having the ability to see your shot, make the proper correction and follow it up with a second round hit, in under five seconds.

The report (ARL-TR-2065) fails to address two issues.
The first is to assume that ballistic calculators (software) are 100% correct. Such is not the case. Computer generated ballistic calculations are approximations only. The second is the importance of the follow-up shot. It’s the follow-up shot that corrects for error.

A good spotter will make the correct call and all subsequent shots should be on target. The primary variable will always be the wind.”

 

[JT505]

BARREL LENGTH AND THE PRECISION RIFLE - Why shorter barrels may often be better. By Eugene Nielsen

There’s a growing trend to shorter barrels on tactical precision rifles. In years past, a 24- to 26-inch barrel was practically a given. Accepted wisdom was that it was necessary to sacrifice a little maneuverability to gain a more complete powder burn and significantly reduced flash signature. Today, it’s not uncommon to see rifles with significantly shorter barrels.

Attitudes are changing.
The desire for more maneuverable rifles for the urban setting has led a growing number of manufacturer's to come out with shorter-barreled precision rifles. This brings up an obvious question - how short is too short? What sacrifices, if any, are made by going to a shorter barrel? To answer these questions, we must first start by taking a look at the subject of internal ballistics. Internal ballistics is a very complex subject. There are many factors which affect the internal performance of a given cartridge and bullet. Factors affecting internal performance include the powder chamber capacity; load density; amount and burning characteristics of the propellant powder; temperature of the propellant prior to ignition; uniformity and speed of ignition; diameter, weight and bearing length of the bullet; and the length of the barrel and its interior dimensions.

Longer barrels give the powder more time to work on propelling the bullet. For this reason longer barrels generally provide higher velocities, everything else being equal. However, the gas pressure behind the bullet diminishes as the bullet moves down the bore. Given a long enough barrel, there will eventually be a point in which the bore friction and air pressure in front of the bullet will equal the gas pressure behind it. At this point, the velocity of the bullet will start to decrease.

There isn't any clear-cut answer as to how much velocity will be lost per inch of barrel length reduction. The amount of loss is closely tied to the expansion ratio. As previously noted, the type and amount of powder, as well as the weight and bearing length of the bullet, also play a major part. Rifles with high expansion ratios (smaller calibers) tend to lose less velocity than rifles with low expansion ratios (larger calibers).

Tactical Operations article in the April 2000 issue of S.W.A.T. typifies the trend to rifles with shorter barrels. Tac Ops considers a barrel of length of 18 to 20 inches to be optimal for the urban environment, with 18 inches the preferred length. During the development of the Tango 51, Tac Ops took a standard 26-inch barrel and cut it down to 18 inches in one-inch increments. Between 10 to 20 rounds were fired at each increment. They found that a 20-inch barrel provides for a complete propellant burn and no velocity loss when using Federal Match 168-grain BTHP, a cartridge that has become something of a law enforcement standard. Going to an 18-inch barrel only resulted in a loss of 32 feet per second (fps).

Shorter barreled rifles are more versatile, being equally suitable for both urban and rural operations. According to Tac Ops, there isn't any need to go to the 26-inch barrel unless you want to go to a heavier bullet or push the round to higher velocity using more powder or use a slower burning powder. The Los Angeles County Sheriff's Department's Special Enforcement Bureau (SEB) performed tests similar to those conducted by Tac Ops and came to similar conclusions.

Tommy Lambrecht, SEB armorer and Special Weapons Team long rifle expert, recently chronographed the Federal Match 168-gr. BTHP rounds. Lambrecht said that the muzzle velocity was averaging around 2,660 to 2,670 feet per second (fps) from the 20-inch-barreled Tango 51 that Tac Ops delivered to him. The accuracy of the Tango 51 isn't hampered by the shorter barrel. While at the range with the Tango 51 we were consistently getting sub-1/4 MOA accuracy at longer ranges? Well, the shorter barrel doesn't hamper longer range accuracy either. As I mentioned in my article on the Tango 51, San Fernando (CA) PD Special Response Team long rifle marksman Chris Colelli fired a 3-shot group from the rifle at 700 yards that measured just under 2 inches center to center. The group, which was witnessed by several credible spotters, was shot off of a bipod with one small sandbag.

Colelli is a superb marksman, one of the best that I've seen, but he would be the first to admit that an element of luck played a role in this feat. Groups like these certainly aren't typical of what could be realistically expected under actual operational conditions. Still, they show that the rifle is capable of phenomenal accuracy provided that the operator does his or her part. Although the 20-inch barrel remains very popular with agencies purchasing the Tango 51, many agencies prefer an 18-inch barrel for its added maneuverability. With the 18-inch barrel, you're still shooting around 2,630 fps with Federal Match.
The target certainly isn't going to know if he's being hit with a bullet that leaves the muzzle at 2,660 fps or 2,630 fps. The terminal ballistics are identical.

Going to an 18-inch barrel doesn't adversely affect the accuracy of the rifle. Tac Ops has achieved incredible accuracy with the shorter barrels. The 18-inch barreled Tango 51 rifles will still shoot sub-1/4 MOA. The performance is just as good with the 18-inch barrel as it is with the 20-inch barrel out to a distance of 600 yards. After initially going with the 20-inch barrel for their Tango 51s, the Los Angeles County Sheriff's Department has decided to go with the 18-inch barrel and Tac Ops 30 suppressor on all new Tango 51s that they purchase.

Shorter barrels are actually often more accurate than their longer counterparts. A rifle barrel is a cantilevered beam and as such they sag.
More sag results in more whip and vibration as the bullet travels down the bore. Barrel sag induces longitudinal stress that can cause stringing of shots. Using a shorter, heavier barrel minimizes reduces stress and accuracy-robbing barrel vibration. A shorter barrel is stiffer and vibrates at a less. Barrel length and contour determines the relative "stiffness" of a barrel, i.e., how much a barrel will tend to vibrate. Shorter barrels generally have oscillations of smaller amplitude than longer barrels. Thicker barrels generally have fewer vibration nodes than slimmer barrels. The ringing frequency of a thicker barrel is higher and the oscillations are of a smaller amplitude and of a shorter duration. This equates to less barrel motion at the muzzle. The use of a shorter barrel also allows the use of a heavier contour without making the rifle unwieldy.

The use of a heavier contour tends to provide less variation between a cold shot and any subsequent follow-up shots. Barrels expand as they heat up. As the barrel expands any stress on or in the barrel will cause stringing of the shots. Bore expansion results in an increase in group size. Heavier barrels tend to be more consistent because they take longer to heat up.

An 18- to 20-inch barrel may be fine for a caliber like the .308 Win but what about calibers such as the .300 Winchester Magnum?
Many agencies are opting for this cartridge as a result of its long range ballistics. The .308 Win has a maximum effective range of about 800 yards. While this is certainly more than enough for most law enforcement scenarios (law enforcement snipers rarely have to engage targets at more than 100 yards), the .300 Win. Mag. does increase the maximum effective range but this comes with the price of additional recoil.

Many agencies purchasing a .300 Win Mag. will primarily be employing the rifle in an urban environment. The common reason for opting for the .300 Win. Mag. that it extends the capabilities of the rifle to longer ranges than the .308 Winchester is capable in those rare situations where longer range capability is necessary. This leads to an obvious question -- will going to a shorter barrel for added maneuverability in the urban environment adversely affect long range performance of a rifle in this caliber?

To find the answers, Tac Ops took a 26-inch barreled .300 Win. Mag. and chopped the barrel down in one-inch increments as they previously did with the .308 Winchester. Ten rounds of Federal Match 190-grain BTHP Gold Medal were fired from each increment. No velocity was lost from 26 inches to 22 inches. Velocity loss started to occur only after they went below 22 inches.

As a result of their tests, Tac Ops decided not to go below 22 inches on their .300 Win. Mag. tactical precision rifle, the Alpha 66.
According to Mike Rescigno, President of Tac Ops, the 22-inch barrel is ideal for the tactical shooters that are going to use the 190-grain Federal Match ammo. There isn't any loss of performance by going to the 22-inch barrel and this round. The Alpha 66 still provides 1/4-MOA or better accuracy.

For heavier bullets or hotter loads with slower burning powders, Rescigno recommends a 24- to 26-inch barrel. The longer barrel length is necessary for complete powder combustion with these loads. Rescigno adds that he has a 24-inch barrel on his personal .300 Win. Mag. just in case he wants "to shoot the heavier 220-grain bullets with a lot of powder."

At this point, I can hear readers asking, "What about muzzle blast and muzzle flash? Won't they be a problem with the shorter barrels?"
These are valid concerns. With both calibers, shorter barrels do increase the muzzle blast and muzzle flash somewhat. It's not as much as one might expect. From a practical standpoint, the differences between a 24- or 26-inch barrel and an 18- or 20-inch barrel are negligible, except when slow burning powders are used. Any concerns over the muzzle blast and sound/flash signature can easily be eliminated by the use of a sound suppressor (silencer). With today's compact, low-maintenance suppressors, such as the Tac Ops 30, there's no reason that all tactical precision rifles shouldn't be so equipped. More and more law enforcement agencies are coming to this conclusion.

The use of a sound suppressor provides a number of advantages to both the shooter and spotter. The suppressor greatly reduces any ground disturbance and eliminates any muzzle flash/sound signature that can identify the position or disturb vision and hearing. There isn't any necessity for the shooter or spotter to wear hearing protection. Many shooters find that their accuracy improves when a suppressor is employed due to the resulting reduction in the muzzle blast and recoil. The reduction in recoil also permits quicker follow-up shots. A sound suppressor can substantially reduce the recoil velocity and recoil energy of a rifle. Gas volume and gas pressure at the muzzle are major factors in the free recoil energy produced by a rifle. Shorter barrels generally result in increased gas volumes and higher gas pressures at the muzzle. All other factors being equal, increased gas volumes and higher gas pressures at the muzzle will increase the recoil velocity and free recoil energy.

Free recoil energy is proportional to the square of the recoil velocity of the rifle. Doubling the recoil velocity quadruples the free recoil energy. Sound suppressors reduce the free recoil energy by suppressing the effects of the expanding powder gasses. They also add weight, slowing the acceleration of the rifle.

In summary, the appropriate barrel length is closely tied to the caliber and the load or loads that will be employed. If a shorter barrel provides equivalent or better accuracy and little or no loss in velocity, why go to a longer barrel? Why sacrifice maneuverability and add excess weight? While old attitudes may die hard, chronographs and ballistics don't lie. Shorter barrels are often better. The proof is in the performance.

 

[JT505]

Randall's description of AR gas operation and how everything works in harmony- By Randall Rausch of www.AR15barrels.com

I have not written this up in a while and some day I really need to build a whole page dedicated to it. This is just off the top of my head. Much of it comes from Rick McDowel (competition specialties) when I was first learning AR's & from Tweak along the way and my own experiences mixed in along the path to enlightenment.

Ok, starting with a cartridge in the chamber, hammer back.
Trigger lets the hammer fall. Hammer hits the firing pin, driving it forward. Firing pin drives the primer (and attached cartridge case) forwards in the chamber until the shoulder in the chamber stops the shoulder on the cartridge case.The case will already be seated against the shoulder due to ejector tension, but the primer can sometimes move before the anvil legs on the primer stop against the primer pocket.

Headspace is the distance from the bolt face to the head of the cartridge when fully seated in the chamber.
Headspace gauges account for the length of the cartridge AND for the recommended amount of headspace, but what really matters is the amount of space, or lack there-of, of space between the case head and bolt face.

The firing pin continues forward to ignite the primer.
Primer flash ignites powder charge, instantly creating great pressure within the cartridge case. Cartridge case expands first outward towards chamber walls (path of least resistance) where pressure holds the case in place and then the case stretches backward until the case head is stopped against the bolt face.
Here is WHY long headspace makes cases fail!

Bullet begins movement down the barrel, first encountering the throat.
Here is why you want a throat DIAMETER closely matching the bullet. More about throat dimensions in this graphic;


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Loose throats do not control the bullet and keep it as straight while engraving into the rifling. Now the bullet has objurgated and engraved into the rifling and it's accelerating rapidly down the bore. As it passes the gas port, gas begins to flow into the gas block where it turns and heads towards the bolt carrier via the gas tube. The pressure is still high in the barrel, usually 15,000 PSI+ until the bullet leaves the muzzle. Just as the bullet leaves the muzzle, gas escapes around the base of the bullet.

Here is why a proper crown is important.
Gas is traveling about 5x faster than the bullet when it leaves the muzzle. An even crown releases gas all the way around the bullet at one time. An un-even crown lets gas go on one side first. This can tip the bullet just slightly sideways at the moment the bullet is released into the air. This is a very important time in the bullet's flight. Now, remember, high pressure gas always follows the path of least resistance, which is now out the front of the barrel instead of into the gas system. Barrel pressure drops immediately. During the bullet's travel down the bore between the gas port and the muzzle, we had a metered amount of gas fed to the action. This gas does the following:

Upon reaching the gas key bolted to the top of the carrier, it turns down into the bolt carrier where it is given a nice place to expand. This is the area inside the bolt carrier where the bolt lives. Gas expanding here forces the bolt carrier back AND the bolt forward. Note that the bolt is also being forced BACK by the gas pressure expanding the cartridge case on the other side of the bolt. For a short moment in time, these forces are about equal. Ideally, this is while the bolt lugs are unlocking and before the extractor starts pulling on the case. The bolt carrier starts to move backwards against the inertia of the carrier's weight, the buffer's weight and the operating spring. All of these effect timing, that's why we have different weights of carriers, standard, heavy (H), H2, H3 etc.

The next thing the carrier encounters are the cam surfaces against the cam pin.
Of course we know that the cam pin goes through the bolt. Rearward movement of the bolt carrier causes the bolt to rotate.
(pay attention here, this is the meaty part) Here is where timing comes into play. Let's make a couple assumptions here before we continue. Trust me that pressures in the case hold the case into the chamber, even though the chamber is slightly tapered. Also trust me that when you release all the pressure out the front of the barrel that the cartridge case will spring back down to size so it's no longer a tight fit in the chamber as it was with the gas pressure present.

Here's where timing comes into play.
We want the bullet to be out of the front of the barrel AND the pressure to have subsided enough that the case shrinks down BEFORE the bolt lugs are unlocked because when the pressure is high, the case WILL try to stay in the chamber. Now is the perfect time to point out that one sure sign of high pressures are the fact that the case extrudes into the ejector plunger hole on the bolt and the resulting pressure unlocks the bolt while pressures are still high.

This extruded brass gets wiped off the end of the case head, leaving a shiny spot and the brass usually makes it's way under the extractor, later causing extraction problems we will get to in a little bit. Here is a graphic illustrating what happens when pressures are too high and the gas system is getting too much gas;


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Now back to extraction, normal/correct version:
Pressure subsides, bolt unlocks, carrier momentum continues rearward, pulling the fired (and contracted) cartridge case from the chamber.
As the cartridge case reaches the ejection port, the case pivots on the extractor hook from pressure of the ejector until it is sent flying free of the rifle.
The bolt carrier continues backward while re-cocking the hammer until operating spring pressure or the buffer stops it. Operating spring returns the bolt carrier forward where it strips another round from the magazine up the feed ramps and into the chamber. Cartridge stops in the chamber, bolt continues forward, causing the extractor to snap over the rim of the cartridge case. Bolt finally stops against the case head, but the carrier continues forward. The cam surfaces in the carrier now cause the bolt to lock into battery again. Now we are back where we started.

Now for extraction, the WRONG ways. First, too much gas (most common):
The bullet has not left the barrel yet, but it's past the gas port. Too much high pressure gas is rushing into the carrier, causing it to move rearward faster then desired and unlock the bolt from the extension. Pressures are still high so the cartridge case is NOT ready to be extracted yet. The carrier's momentum continues to pull backward, but the pressures in the case actually hold in in the chamber. This causes a hiccup in the carrier's momentum.

Depending on the severity of the timing, several things can occur:
#1 The (weak) extractor spring allows the extractor to jump over the rim of the cartridge and the bolt carrier continues rearward, grabbing the next round and causing the classic "fired case in chamber, live round behind it" FTE.
The brass shavings under the extractor usually contribute to this one as well.
#2 The extractor does NOT slip off the case, but keeps pulling.
The extractor is strong enough to RIP the rim right off the case.
Same result as above, but MORE brass shavings everywhere from ripping case rims off.
#3 The extractor does NOT slip off the case, but keeps pulling.
During this pulling, the bullet has JUST left the bore, pressures recede and the case shrinks down, allowing extraction.

The rest of the cycle goes as normal, but you have strong pull marks on the case. Recoil will be higher than normal when the carrier is allowed to travel to the end of the buffer tube and bottom out swiftly against the end of the buffer tube. In normal operation, the buffer just kisses the end of the tube. Somewhere between here and the next section, we have proper operation.

Lastly, not enough gas (less common):
The bullet is out of the bore, pressure is subsided, case is extracted and on it's way to ejection. Depending on the severity of the lack of gas, the bolt carrier may not even get the case out of the chamber before the operating spring returns it forward. Adding more gas, the case just barely gets out of the ejection port, but the bolt grabs it on it's way forward, classic stovepipe. Add more gas and the cartridge clears the action, but the bolt does NOT get far enough back to strip a round from the mag. This is classic short stroking. You have a single shot action which extracts and ejects, then closes on an empty chamber after you fire it. In this condition, the bolt will also ride over an empty magazine and close on an empty chamber. Add some more gas and you will reach the point where it feeds from the magazine and ALMOST works properly, but it still closes over an empty mag. This is two things, first, poor mag springs are not pushing the follower up fast enough to catch the bolt and second, the bolt is not quite making it back far enough to catch on the magazine follower.
Add just a little more gas and you are back to proper function.

Now, take note, that a lack of gas in a rifle that was functioning fine before can be from several things:
Gas key screws poorly staked and they loosened up, allowing some gas to escape instead of doing it's job INSIDE the bolt carrier.
Gas ring gaps are aligned, gas rings missing or broken, allowing extra gas to flow past them.
Gas block/front sight base is loose, allowing gas to escape before it even gets down the gas tube.
Gas tube "mushroom" is severely worn, probably because it was not properly aligned with the gas key and gas is escaping there.


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Please note that failure to extract/eject is a symptom of EITHER too much or too little function of the action.

FTE alone is not enough information to decide what to change to fix the problem. You need to look for other signs such as the excessive recoil and case rim pulling of too much gas or the short stroking of too little gas. Unfortunately, many guys who don't understand the magic above always ASSUME that they have too little gas. What do they do? They open up the gas port. Following the examples above, you can see this only makes the problem worse. Lesson to be learned: Follow the published troubleshooting procedures. They are written that way for a reason.

Whew, that was longer than I expected to write. Hope it all sinks in and you can benefit from it.

 

[JT505]

Randall's description of AR gas operation and how everything works in harmony- By Randall Rausch of www.AR15barrels.com

Edited 2-3-06 to add more about gas port pressures:

We often hear about mid-length being smoother cycling or pistol being harsher cycling than the typical carbine length gas systems. Below is a plot of a 223 load. I have noted the locations of the various gas ports in blue. You can plainly see what pressures are introduced into the gas systems when the bullet JUST passes the gas port. This is the reason for the way the various gas system lengths function differently. Projectile travel at the bottom assumes that the bullet starts out about 1.5" from the breech, so add 1.5" if you want to compare velocities at different lengths.


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Dwell time:
There has been a lot of discussion lately about dwell time and how it relates to the function of certain barrels, particularly 18" rifle-gassed, 18" mid-length and 14.5" mid-lengths. I put together a new graphic to illustrate DWELL TIME. To use this graphic, find the gas port location above the pressure trace (Orange marks) and then locate the barrel lengths below the pressure trace (Blue marks). The DWELL TIME is the time between these two marks (indicated on the X axis of the graphic as well as in the data below the graphic. Optimum dwell time is right around 0.200 ms when you use the two most common gas system/barrel length configurations of 20" rifles and 14.5" carbines.


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Questions

Maybe I'm wrong, but it's my understanding that headspace is the distance from the chamber shoulder to the bolt face; that a properly head spaced rifle will have little or no "slop" of the cartridge in the chamber. IOW, the cartridge shouldn't move forward under the influence of the firing pin striking the primer because the cartridge is already fully seated, shoulder to bolt face, in the chamber.

Your understanding that there is zero clearance between the bolt face and cartridge case is incorrect.
Headspace is the amount of space between the cartridge case head and the bolt face. The dimensions of the cartridge have LOTS of influence on the final headspace. Because the dimensions of ammo are not consistent, steel gauges were developed to serve as a standard that can be repeated by multiple manufacturers. There are established dimensions within the industry that specify how long a cartridge OR chamber should be. These dimensions allow for manufacturing tolerances, so they are not an ABSOLUTE VALUE. In the case of a chamber, we have headspace gauges to see that we are within spec. The actual dimension of the chamber and ammo is NOT important as long as the two work together correctly. You want to avoid more than about 0.005" headspace with your ammo/chamber combination. For reliability's sake, you don't want to go much under about 0.002"

In the case of the AR-15 with its bolt mounted ejector, the case is already being pushed forward and all the headspace will appear at the case head. In something like an Uzi or Mauser, with a fixed ejector, the cartridge case does get pushed forward in the chamber as its being fired. There's actually a lot more going in in the chamber when the powder charge goes off, my version above was relatively simplified.

When the primer ignites the powder and the cartridge is already against the chamber shoulder, there is headspace at the case head. Pressure actually pushes the primer back against the bolt face. As the pressure gets higher, the whole case yields and stretches just in front of the case head to fill the chamber. The reason that high pressure loads show flattened primers is that when the primer is hanging out the end of the case, it slightly balloons out and then gets sized back down when the case head slides back. This is where the squared off primer shape comes from. The bolt carrier starts to move backwards against the inertia of the carrier's weight, the buffer's weight and the operating spring.

My understanding here is that the gas escapes from the gas tube, through the carrier key, and impacts both the "back" of the bolt face and the rings on the tail of the bolt. The only real gas effect on the carrier here appears to be radial, not longitudinal. The bolt can't travel forward (locked against the barrel extension), so it travels backwards, operating the cam, unlocking the lugs, and pulling the carrier along with it. The carrier really is just along for the ride on the extraction phase of the firing cycle; it's only real contribution (aside from structural) is to provide a camming surface to allow the bolt to unlock. Oh, and I guess it does keep the bolt travelling in a straight line .

You have a couple real important concepts backwards and I have underlined the important parts.
When in battery (locked) the bolt is captured both forwards (by the barrel/cartridge) and rearward (by the lugs on the barrel extension). It is the carrier that moves rearward and cams the bolt via the cam pin to make the bolt rotate 22.5 degrees to unlock from the barrel extension. By this time, the carrier's (and buffer's) inertia keeps it moving rearward and PULLS the bolt along with it via the cam pin. How would the bolt travel backwards and react with the cam cut in the carrier if it's already captured by the barrel extension? Does this make sense?

The gas does push forward against the bolt and rearward against the chamber within the carrier.
You are correct that the bolt has no way to move forward, so only the carrier can move and only rearward as it is at rest against the extension when in battery. The back of the bolt and the gas rings are just there to seal the hole in the front end of the carrier. The carrier is the piece that the direct gas impingement is having the effect on. The INERTIA of the carrier and buffer are what tries to hold it in place AS WELL AS THE SPRING. When pressure gets high enough (quickly) it overrides the inertia AND the spring and moves the carrier rearward. It's the bolt that is just along for the ride during this.

As for the gas impingement, it seems you are saying that carrier rearward movement is begun to be driven by the effect of gas on the carrier key,

No, the key just gets the gas inside the carrier, even before the effect of gas on the bolt rings. The gas rings on the bolt are important. They confine the gas from moving forwards up along the bolt. This way, the carrier takes all the energy that the expansion of the gas carries.

I guess the question that remains for me is this: does bolt unlocking occur primarily as a function of the carrier "pulling" the bolt rearward, or as a function of the bolt "pushing" the carrier rearward?

Neither, the gas pushes against the gas rings, the bolt and the carrier. The bolt and rings are mechanically confined by locking lug alignment, the carrier is not confined except by inertia and a spring, therefore the carrier moves back because of gas expansion inside. As the carrier slides back, the cam pin is forced over in the cam pin slot and this causes the bolt to rotate. It is the cam pin that unlocks the bolt. After the cam pin hits the end of its track, the bolt is already unlocked and it simply gets PULLED (along with the spent case) away from the chamber.

Upon further thought it has to be the carrier that performs primary unlocking function; as you said the bolt is locked fore and after by barrel extension and locking lugs. Only way for bolt to unlock is to rotate via the cam pin, and the only way for that to happen is for the carrier to do the "primary" moving.

Correct!
To carry this thought further, what would be the effect of gas being vented from the carrier key to a point outside the carrier as opposed to inside the carrier?The carrier would not be forced rearward without the sealing of the gas rings to create something to push against. The gas does indeed get vented out of the carrier through the two exhaust ports. Of course this does not occur until the gas rings pass the ports. By this time, there is sufficient inertia to finish the cycle.

Can the trigger fall against the firing pin if the bolt is full forward in the chamber, against the brass, but not rotated and locked, or just partially rotated, i.e. Bolt carrier slightly back off of the full forward position? If not, why not?

The carrier blocks forward firing pin travel.
If the bolt is partially locked, the bolt carrier is still at least 0.075" from the barrel extension. As the firing pin protrusion is somewhere around 0.040" when seated, the firing pin would not be able to reach the primer when being held back 0.075" by the carrier. What would happen is that the extra energy of the hammer would tend to drive the carrier closed. Maybe, BIG MAYBE, there is enough momentum to fire the primer, but not likely.

If the carrier is in motion, it will have finished locking before the hammer gets there if the hammer is released while the carrier is still back 0.0.075". When you check the auto sear timing on an M16, you are looking at the distance the carrier is from the bolt carrier when the hammer is released. Off the top of my head, this is about 0.100"or less. Any more and the hammer will beat the carrier and you get misfires.

I suppose another way to think of the head space clearance is that if you did not have clearance, you would have line to line contact between the bolt face and the brass or an interference fit between the same. The clearance allows an imperfect fit to still work over temperature ranges, over machining and assembly tolerance stack-ups, etc.

Exactly right, that's why we NEED some headspace, to allow for manufacturing tolerances.
Standards and Gauges are just an implementation method so that multiple makers in different areas can make products that are all compatible.

If the chamber pressure is high at beginning of the barrel and drops off as it travels down the barrel, then when the gas port is closer to the chamber, why do we need larger port size? Wouldn't the higher pressure provide more cycling force? If we need to match the pressure to longer barrel, won't we be using smaller hole? It is also demonstrated that short barrel can cause pre-mature extraction (high chamber pressure and with larger port size), people then use heavier buffer, longer gas tube to retard the timing. Why not just reduce the port size and reduce the pressure? (of course smaller port probably won't cycle)
Could it be that we need not only pressure but volume (mass) for the gas as well?

You need a certain VOLUME of gas to function the action, not a certain pressure.
You can get this volume with a large port and short duration or low pressure or with a small port and long duration or high pressure. With the SAME barrel length, you make the port smaller as it gets closer to the chamber. What you are doing here is two different things, first tapping into a higher PRESSURE gas supply, but ALSO increasing the DWELL TIME. Now, when you shorten a barrel, while keeping the same gas system, you are simply reducing the dwell time and it's appropriate to enlarge the gas port accordingly. If you are COMPARING two different barrels and both have the same amount of barrel length past the gas port, then the port will likely be smaller on the shorter barrel as that port is nearer to the chamber and therefore gets a higher pressure gas supply.

Running a proper size gas port or adjustable gas system is FAR BETTER than resorting to a heavy buffer (more reciprocating mass means more muzzle rise), but it's easier to just buy a heavy buffer and swap out parts than properly correct the gas flow. Adjustable gas tubes are $60 and offer one more thing to go wrong and heavy buffers are like $15, what would you choose?

MOST AR's come over-gassed from the factory so that they will run correctly right out of the box without waiting for the gas rings to seat into the carrier. Once the gas rings seat, the rifles are more likely to show signs of over gassing. That's one more reason heavy buffers and O-rings on extractors are so commonplace. For these very same reasons, you can take a 16" (carbine length gas system) barrel, cut it to 14.5" and they run perfect. This is not always the case though, I recently cut an LMT MRP 16" (mid-length gas system) down to 14.5" which DID require being opened up some. I was pleased that the MRP barrel gas port was more towards the small side of the range.

Copyright © 1996-2017 AR15.COM LLC. All Rights Reserved.

 

[JT505]

Carbine vs. Mid-Length Gas System on a 16" Barrel

Often times I receive emails and private messages asking why I prefer a mid-length gas system over a carbine gas system on a 16" barrel or asked which carbine I prefer and why. What is written below is a response to an email, the author was considering a Colt LE6920 and a Bravo Company Manufacturing (BCM) Mid-Length. In my response I explain why I prefer the mid-length gas system over a carbine gas system on a 16" barrel. The Colt 6920 has a 16" barrel with a carbine length gas system. The BCM Mid-Length has a 16" barrel with a mid-length gas system

I own (3) Colt LE6920's and (3) BCM Mid-Lengths and have used both at work, in training classes, run and gun rifle matches, etc. ALL (3) of my Colt LE6920's have had their barrels changed to Colt 14.5" SOCOM M4 barrels. I prefer to use a mid-length gas system on a 16" barrel. I do not like a 16" barrel with a carbine length gas system (same gas system that is on the military M4 carbine). If I have to use a carbine length gas system I prefer to use a 14.5" barrel (and I permanently attach a longer flash hider to make the overall length of the barrel 16").

The 3 most common type of gas systems are:
1) Carbine length gas system - (same length as the US Military M4 carbine) and takes a 7.0" rail or handguard.
2) Mid-length gas system - the mid-length gas system is 2" longer than the carbine length gas system (thus the front sight housing and gas hole are 2" forward of where they are on a carbine length gas system). The mid-length takes a 9.0" rail or handguard.
3) Rifle length gas system - (same gas system on the AR15 or M16A2 / M16A4 with 20" barrel). The rifle length gas system takes a 12.0" rail or handguard.

When talking about the different gas systems on a 16" barrel, think about the distance from the gas hole to the end of the barrel. The longer that the bullet is in the barrel after the bullet passes the gas hole, the more gas that is getting pushed back through the gas tube and back into the gas key. The end result is a sharper recoil impulse.

This is why on a 16" barrel, a mid-length gas system is slightly smoother than a carbine length gas system.

The distance from the gas hole to the end of the A2 flash hider on a 16" barrel with carbine-length gas system is approximately 9.5".
The distance from the gas hole to the end of the A2 flash hider on a 20" barrel with rifle length gas system is approximately 7.5"
The distance from the gas hole to the end of the A2 flash hider on a 16" barrel with mid-length gas system is approximately 7.5"
The distance from the gas hole to the end of the A2 flash hider on a 14.5" barrel with carbine length gas system is approximately 7.5"

Something to note.
The 20" barrel with rifle length gas system, 16" barrel with mid-length gas system, and 14.5" barrel with carbine length gas system all have the same amount of dwell time (distance from the gas hole to the end of the barrel).

The 16" barrel with the carbine length gas system has a dwell time that is approximately 1.5" longer. Thus it's pumping more gas into the bolt carrier key, forcing it back hard, etc. The 16" barrel with the carbine length gas system is harder on parts over the long term and you'll feel slightly more recoil impulse. Not a huge thing, but after shooting all 3 side by side on numerous occasions, I see no need to own a 16" barrel with a carbine length gas system. I own (18) AR15's. (2) are short barreled rifles, the rest of my AR15's are either 14.5" M4's with a carbine length gas system or 16" Mid-Lengths.


MK 262 VELOCITY DATA
__________________________________

7.5" 2053 FPS
10.5" 2363 FPS
DIFFERENCE 310 FPS
DIFFERENCE/INCH 103 FPS

10.5" 2363 FPS
14.5" 2576 FPS
DIFFERENCE 213 FPS
DIFFERENCE/INCH 53 FPS

14.5" 2576 FPS
16" 2669 FPS
DIFFERENCE 93 FPS
DIFFERENCE/INCH 62 FPS

16" 2669 FPS
18" 2769 FPS
DIFFERENCE 100 FPS
DIFFERENCE/INCH 50 FPS

18" 2769 FPS
20" 2820 FPS
DIFFERENCE 50 FPS
DIFFERENCE/INCH 25 FPS


M855 VELOCITY DATA
__________________________

7.5" 2244 FPS
10.5" 2639 FPS
DIFFERENCE 395 FPS
DIFFERENCE/INCH 132 FPS

10.5" 2639 FPS
14.5" 2861 FPS
DIFFERENCE 222 FPS
DIFFERENCE/INCH 56 FPS

14.5" 2861 FPS
16" 2938 FPS
DIFFERENCE 77 FPS
DIFFERENCE/INCH 51 FPS

16" 2938 FPS
18" 3046 FPS
DIFFERENCE 108 FPS
DIFFERENCE/INCH 54 FPS

18" 3046 FPS
20" 3061 FPS
DIFFERENCE 15 FPS
DIFFERENCE/INCH 7.5 FPS


XM193 VELOCITY DATA
_________________________________

7.5" 2364 FPS ... 10.5" 2755 FPS
DIFFERENCE 391 FPS
DIFFERENCE/INCH 130 FPS PER INCH

10.5" 2755 FPS ... 14.5" 2984 FPS
DIFFERENCE 229 FPS
DIFFERENCE/INCH 57 FPS PER INCH

14.5" 2984 FPS ... 16" 3075 FPS
DIFFERENCE 91 FPS
DIFFERENCE/INCH 61 FPS

16" 3075 FPS ... 18" 3245 FPS
DIFFERENCE 170 FPS
DIFFERENCE/INCH 85 FPS

18" 3245 ... 20" 3254 FPS
DIFFERENCE 9 FPS
DIFFERENCE/INCH 4.5 FPS

 

[JT505]

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