Opinions on AR 308 barrel length for my IRA build

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Sergeant
Full Member
Minuteman
Jul 24, 2006
592
144
Land of the cold Minneapolis MN, USA
#1
Well, here she is, almost have all the parts. Just trigger and BCG left hopefully by weeks end, then off to a smith.

So here is the question, what length barrel ?

img0083is.jpg


A surefire brake is going to be added.


IRA Upper and Lower
Magpul MIAD Grip
Magpul PRS Stock
Daniel Defense Rail
Rock 1:11 5R barrel blank AR10-T contour
DPMS barrel extension
PRI Gas Buster Charging handle
PRI low pro gas block
Surefire brake
Pmags
DPMS LPK and ejector port cover
The blingy chrome phosphate carrier and bolt

One thing I will point out, the take down spring in the rear is located vertical, not horizontal like AR15s towards the butstock. So the grip needs two captured holes, not just one. With the MIAD grip, you have to cut the miad wedge (not to be confused with wedge block -red thing) lower portion to fill a gap, else the spring is allowed to relax and take down pin moves freely. The IRA has to be the tightest fitting lower/upper I have ever had, and requires a little fitting, sanding of the take down pins also.


At this point, I am thinking 20 since the Surefire will add about an inch.
 
#5
Re: Opinions on AR 308 barrel length

To my understanding, its all about how far out you wanna reach, vs. at what length your powder burns out.

If its a benchrest gun, you wanna reach past 1000 yards, and your powder will burn for all 24", then go with 24"

To my understanding, many 308 powders burn out by 20" (i.e. past 20" you COULD start losing velocity, as the powder has burned out, and the friction of the bullett on the rifling continues. Keep in mind, this is a hypothetical discussion)

I was once told the M1A went with a 22" barrell cuz that was the optimal length for the military 308 cartridge.

But if this is a shoot and scoot gun, 24" is unweildy.

That said, I'd opt 20". Maybe 18" if weight and length is a important consideration, and I didn't mind losing a little velocity.
 
#7
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: garandman</div><div class="ubbcode-body">

To my understanding, many 308 powders burn out by 20" (i.e. past 20" you COULD start losing velocity, as the powder has burned out, and the friction of the bullett on the rifling continues. Keep in mind, this is a hypothetical discussion)

</div></div>

I dont mean to jack the thread but this is interesting, as I have obviously heard a few different lengths in which this occurs. I have heard as long as 28" and as short as 20" for optimum 308 velocity. Again dont want to disrupt the initial topic, just curious on why the roughly 8 inches of different theory (powder and bullet being variables).
 
#8
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: garandman</div><div class="ubbcode-body">To my understanding, many 308 powders burn out by 20" (i.e. past 20" you COULD start losing velocity, as the powder has burned out, and the friction of the bullett on the rifling continues. Keep in mind, this is a hypothetical discussion)

I was once told the M1A went with a 22" barrell cuz that was the optimal length for the military 308 cartridge.
</div></div>

Right, that makes a lot of sense. That's probably why Palma rifle have 30 inch barrels and I have a 32 inch barrel on my F-class rifle; I want to reduce the velocity of my bullets.

In actuality whatever powder is going to burn does so in the first few inches from the chamber. The expanding gasses do the work and as they keep expanding they keep accelerating the bullet. Until they reach the muzzle. If your barrel is short, the gasses will be very hot and still under quite a lot of pressure when they hit the comparatively cooler atmoshpere and you get a big-ass muzzle blast that announces to the world that you just wasted them. A longer barrel will make further use of the hot pressurized gasses and will keep accelerating your bullet and also reduce the muzzle blast as a side benefit.

It would require many feet of barrel before the friction of the bullet in the barrel is actually greater than the force of the expanding gasses and at that point the bullet starts to slow down. I suspect you might even use yards as a unit of measure for that.

In answer to the OP, 20 inches should be the minimum and the brake is going to be obnoxiously loud.
 
#10
Re: Opinions on AR 308 barrel length

somewhere on here lowlight has a video of himself hitting steel at 1000 yrds with an 18" 308.


I am getting a new 308 barrel made up for my DTA SRS. It is also a Rock Creek 1:11.25.
I am going with 20"
 
#12
Re: Opinions on AR 308 barrel length

How far do you intend to carry/move with it and shoot routinely?

With the PRS boat oar buttstock I'd cut the front down to 20 just to balance the weapon. 800 should be routine, every day acceptable.

I chose 22 to keep equivalent M14 velocities. 24 was just too long, and in many places 22 is right-on-the-edge marginal for 1,000 in high heat/high humidity shooting conditions (Benning, Bragg, Quantico, Camp Perry) even with a 175. Mine is fluted between the end of the chamber and the gas block and it's still heavy.

With a 22 you can swap out between an A1 or LMT SOPMOD stock with a happy medium between cheek position and gun balance while moving. The gun is still going to be heavy.

If the majority of the time you're going to be shooting prone with a suppressor then you'll want 20 and it probably won't matter where the rifle balances.

Stationary hobby/target grouping or plinking or coyote shooting is one thing -- dragging it through the woods trying to chase down deer, antelope, or people is another.

Did I mention it's heavy? I don't mean to be repetitive, but balance counts for a lot.

Larue's OBR is at 16, which is good for a shoot-and-scoot line rifleman or DM arm, definitely out to 600.

This is 22-inches with a Badger FTE brake:
21b0zes.jpg


This is factory (pencil, or "M14" profile) 16-inches on a carbine (easier to swing and carry, but nowhere near as accurate. Probably shorter than you're thinking if you're planning on keeping the PRS, but probably right if you're going to shoot a lot with a suppressor hanging off the end):
jphu09.jpg


20 inches with the JP Bennie Cooley brake (total overall about the same length as with Surefire):
ar_10_sniper_rifle_tube.jpg

 
#13
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sig685</div><div class="ubbcode-body">[
Right, that makes a lot of sense. That's probably why Palma rifle have 30 inch barrels and I have a 32 inch barrel on my F-class rifle; I want to reduce the velocity of my bullets.

In actuality whatever powder is going to burn does so in the first few inches from the chamber. The expanding gasses do the work and as they keep expanding they keep accelerating the bullet. Until they reach the muzzle. If your barrel is short, the gasses will be very hot and still under quite a lot of pressure when they hit the comparatively cooler atmoshpere and you get a big-ass muzzle blast that announces to the world that you just wasted them. A longer barrel will make further use of the hot pressurized gasses and will keep accelerating your bullet and also reduce the muzzle blast as a side benefit.

It would require many feet of barrel before the friction of the bullet in the barrel is actually greater than the force of the expanding gasses and at that point the bullet starts to slow down. I suspect you might even use yards as a unit of measure for that.

In answer to the OP, 20 inches should be the minimum and the brake is going to be obnoxiously loud. </div></div>


Do you understand what the word "hypothetical" means?
 
#14
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sinister</div><div class="ubbcode-body">

This is factory (pencil) 16-inches on a carbine (easier to swing and carry, but nowhere near as accurate. </div></div>

AS s I understand it, barrel length affects velocity, not accuracy. Reduced barrel length causes a velocity loss, which causes faster bullet drop at distance, but not accuracy, per se.

Now, shorter sight radius CAN affect accuracy.
 
#15
Re: Opinions on AR 308 barrel length

It could because the barrel will vibrate more that a heavier barrel. Also, as the barrel heats up, a "pencil" barrel will expand while a heavier barrel will stay the same for some time after the "pencil" barrel is making a pattern instead of a group.

-P.B.Walsh
 
#16
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: garandman</div><div class="ubbcode-body">Do you understand what the word "hypothetical" means? </div></div>

I was just pointing out the factual errors in your "hypothetical" post.
 
#17
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sig685</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: garandman</div><div class="ubbcode-body">Do you understand what the word "hypothetical" means? </div></div>

I was just pointing out the factual errors in your "hypothetical" post. </div></div>

You obviously don't know the meaning of the word "hypothetical."

I strongly recommend you forego ONE box of ammo, and buy a dictionary.
 
#18
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Walsh</div><div class="ubbcode-body">It could because the barrel will vibrate more that a heavier barrel. Also, as the barrel heats up, a "pencil" barrel will expand while a heavier barrel will stay the same for some time after the "pencil" barrel is making a pattern instead of a group.

-P.B.Walsh </div></div>

I was referring ONLY to barrel length, which is the subject matter of this thread.


I was NOT referring to barrel weight / contour, which is NOT the subject of this thread.


 
#19
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sig685</div><div class="ubbcode-body">
In actuality whatever powder is going to burn does so in the first few inches from the chamber. </div></div>


LOL

Speaking of factual errors, ABOVE is a classic one. The powder all burns up in the first few inches from the chamber, ehhh???


THEN WHERE DOES MUZZLE FLASH COME FROM GENIUS?

You are yet another illustration of my sig line. (below)
 
#22
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sumpter Steve</div><div class="ubbcode-body">Nice rifle. I would go 18-20" if it were mine. It makes for much easier handling and you should still be able to reach 800+ without a problem. JMO. </div></div>

Agree.

And if ever the OP is gonna mount a suppressor, 18" -20" will still make for a really long gun.

(Ops Ince cans require about 3" of barrel length beyond the gas block, as it is an over-barrel mount. Most other 308 cans that thread on the end of the barrel will add about 8-10" to OAL)

)
 
#24
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: garandman</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sig685</div><div class="ubbcode-body">
In actuality whatever powder is going to burn does so in the first few inches from the chamber. </div></div>


LOL

Speaking of factual errors, ABOVE is a classic one. The powder all burns up in the first few inches from the chamber, ehhh???


THEN WHERE DOES MUZZLE FLASH COME FROM GENIUS?

You are yet another illustration of my sig line. (below)


</div></div>

You're kidding, right?

You actually think that the muzzle flash is due to powder burning at the muzzle?

If you reread my original post, the answer was there for you to read:
"the gasses will be very hot and still under quite a lot of pressure when they hit the comparatively cooler atmoshpere and you get a big-ass muzzle blast that announces to the world that you just wasted them."

I have no clue what your signature says, I've long disabled them, but I suspect you may need to change yours.
 
#25
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sig685</div><div class="ubbcode-body">
You actually think that the muzzle flash is due to powder burning at the muzzle?

. </div></div>

Yes, genius.

Hot gasses DO NOT make a "big ass blast" to quote your insipid post.

Burning powder does.


A little 7th grade science - in order to have FIRE, you need three things - 1. Heat 2. Oxygen 3. Fuel. If all the powder is burned up 5" from the chamber THERE IS NO FUEL AND THERE IS NO POSSBILITY OF FIRE AT THE MUZZLE.


Ergo, NOT all the powder is burned up 5" from the chamber. Ergo, you prove the validity of my sig line.

"Better to keep your pie hole shut, and be thought a fool, than to open it and confirm to the world that you are."

Seriously, dude, quit while you are only WAY behind.
 
#26
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: PadronAniversary</div><div class="ubbcode-body">I got the surefire brake in anticipation one day I'll move to a CAN friendly state. No go here in MN </div></div>

Way to plan ahead. Sorry about MN.

I didn't say it before, but that's one sweet rifle.
 
#28
Re: Opinions on AR 308 barrel length

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: garandman</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sig685</div><div class="ubbcode-body">
You actually think that the muzzle flash is due to powder burning at the muzzle?
</div></div>

Yes, genius.

Hot gasses DO NOT make a "big ass blast" to quote your insipid post.

Burning powder does.</div></div>

You might want to refer to this page for some information on what constitute muzzle flash or blast:
http://www.navweaps.com/index_tech/tech-090.htm

There are endless sites describing similar information and even Wikipedia does a great job here: http://en.wikipedia.org/wiki/Muzzle_flash

But I find the first one I listed provides a more detailed explanation.

Here are some of the highlights from that page:

"This flash consists of at least the following five components:

1) Muzzle Glow
2) Primary Flash
3) Intermediate Flash
4) Secondary Flash
5) Sparks

Then the site goes on to detail the cause of each of the components."

Here is what they say about muzzle glow:
"1) Muzzle Glow is usually a reddish white glow or tongue of flame at the muzzle that appears just prior to shot ejection and persists after shot ejection until the chamber pressure drops significantly. The initial glow is usually the result of <span style="font-weight: bold">hot, highly compressed gases (unburned propellants) leaking past the projectile</span> driving band and is brightest in a worn gun. These gasses are hot enough to emit radiation in the visible light band. A "Cold Gun" with its lubricated barrel generally shows less muzzle glow than does a "Hot Gun" with its expanded barrel."

Here is what they say about the primary flash:
"2) Primary Flash occurs after the projectile has exited the muzzle and <span style="font-weight: bold">is caused by those propellant gases exiting the muzzle</span> behind the projectile. These are hot enough to emit large amounts of visible radiation but cool rapidly as they expand away from the muzzle."

Here is what they say about the intermediate flash:
"3) Intermediate Flash consists of a reddish disc, slightly dished towards the gun, which appears about three inches (7.5 cm) from the muzzle of a small-caliber weapon and about 20-25 calibers from the muzzle for larger caliber weapons. Intermediate Flash occurs at the time of shot ejection and persists until the chamber pressure drops. It is brightest at the edge nearest the gun and gradually fades as the distance from the muzzle increases. This flash is <span style="font-weight: bold">due to a Mach shock wave created by the escaping gasses</span> and projectile which, with its attendant pressure rise, causes the propellant gases to attain a temperature almost equal to the chamber temperature and so become self-luminescent."

Here is what they say about the secondary flash:
"4) Secondary Flash appears beyond the zone of the intermediate flash and is a rather ragged vortex of yellowish white flame. This is a result of the ignition of the combustible mixture of <span style="font-weight: bold"> propellant gases and atmospheric oxygen caused by the turbulent mixing occurring at the boundary of the gas jet as it leaves the muzzle</span>. The ignition of this mixture would appear to be initiated by its exposure to the high temperature of the intermediate flash."

There is even a picture. You also want to look at the Wikipedia page I listed.

Here are some highlights from it:

"Muzzle flash can be broken down into five distinct components.[9]

Muzzle glow is a reddish glow that is visible before the bullet leaves the barrel. Muzzle glow is created by superheated gases that have leaked past the projectile and have exited the barrel.

The primary flash is caused by propellant gases exiting the firearm behind the bullet. Although amongst the brightest of the flashes, the heat of the primary flash dissipates quickly and thus is no longer visible.

The intermediate flash is caused by shock waves created by the high speeds of the escaping gases and projectile, and appears as a reddish disc shape in front of the muzzle.

The secondary flash appears farthest from the muzzle as a large white or yellow flame. Secondary flash is caused by the mixture of fuel-rich gases and oxygen in the atmosphere surrounding the muzzle.[10]

Following the dissipation of the muzzle flash, partially unburnt powder or other heated materials can be ejected from the muzzle and appear as sparks."



<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">A little 7th grade science - in order to have FIRE, you need three things - 1. Heat 2. Oxygen 3. Fuel. If all the powder is burned up 5" from the chamber THERE IS NO FUEL AND THERE IS NO POSSBILITY OF FIRE AT THE MUZZLE.


Ergo, NOT all the powder is burned up 5" from the chamber. </div></div>

If you are a handloader, I suggest the following test. A powder charge for a .308 is about 45 grains of powder. Let's say that 10% is still not burned and comes out the muzzle and is ignited by something (you do not provide that information so, I'll leave it up to you. All you know is that it's certainly not the hot pressurized gasses. So, anyway put 4.5 grains of powder on the ground and ligth it with a match. I wonder if the result will be anywhere close to an actual muzzle blast.


<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Ergo, you prove the validity of my sig line.

"Better to keep your pie hole shut, and be thought a fool, than to open it and confirm to the world that you are."

Seriously, dude, quit while you are only WAY behind. </div></div>

Truly words to live by.
 
#30
Re: Opinions on AR 308 barrel length

Hey garandman, in case you are still out there.

I thought I would point you to this little blurb from an organization knows as SAAMI (Small Arms & Ammunition Manufacturers' Institute, Inc.)

http://www.saami.org/Publications/200.pdf

Here are some highlights from it:

"Oxygen from the air is not necessary for the combustion of smokeless powders since they contain sufficient built-in oxygen to burn completely, even in an enclosed space such as the chamber of a firearm."

"When smokeless powder burns, a great deal of gas at high temperature is formed. If the powder is confined, this gas will create pressure in the surrounding structure."

"When ignited in an unconfined state, smokeless powder burns inefficiently with an orange-colored flame. It may produce a considerable amount of light brown, noxious smelling smoke. It leaves a residue of ash and partially burned powder. The flame is hot enough to cause severe burns."

"When it burns under pressure, as in a cartridge fired in a gun,
smokeless powder produces very little smoke, a small glow and
leaves very little or no residue. The burning rate of smokeless powder increases with increased pressure."
 
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