I think I just had an epiphany (bolt vs gas gun)

[AzNooB]

I'll try to keep this short so that people will actually read my entire post.

From reading around and doing research, the only thing that makes a gas gun less accurate than a bolt gun (all else equal) is the hammer firing control system. Because the hammer takes longer to hit the firing pin, which then has to hit the primer, there is more time for user induced error (movement) during all this. The common misconception about gas guns being less accurate is that there is more moving mass during the recoil cycle. In actuality, the bolt doesn't begin to move until after the bullet exits the barrel.

So in theory, if both a gas gun and bolt gun were made to the same tolerances (lock up, chamber specs, etc) they would both be equally mechanically accurate.

With that in mind, how would a striker fired semi-auto system fair? Do you think it's something that precision shooters would seek? While it wouldn't increase the mechanical accuracy of the rifle, it would definitely reduce shooter error.

 

[LoneWolfUSMC]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AzNooB</div><div class="ubbcode-body">In actuality, the bolt doesn't begin to move until after the bullet exits the barrel.</div></div>

There are too many variables to be able to make this broad statement.

There is a reason it's called a "system".

I am still trying to find a reasonably priced way to do some high-speed filming to be able to visually document this. Unfortunately the only feasible method I have come up with to prove or disprove with a wide variety of systems (instead of one pressure meter equipped rifle) is with a Phantom Flex camera. I just don't have the coin to rent one for a couple days.

While I believe a striker fired system would aid in the accuracy potential of the system, you would have to do so in a manner that would not compromise reliability.

I say if you have the capability of designing and prototyping such a fire control system....GO FOR IT!! I would love to see it.

 

[Lowlight]

Re: I think I just had an epiphany (bolt vs gas gun)

What you can't handle $3000 a day to rent a high speed camera... aren't you a cop, rob a drug dealer like Denzel did ... LOL

 

[vmpglenn]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: LoneWolfUSMC</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AzNooB</div><div class="ubbcode-body">In actuality, the bolt doesn't begin to move until after the bullet exits the barrel.</div></div>

There are too many variables to be able to make this broad statement.

There is a reason it's called a "system".

I am still trying to find a reasonably priced way to do some high-speed filming to be able to visually document this. Unfortunately the only feasible method I have come up with to prove or disprove with a wide variety of systems (instead of one pressure meter equipped rifle) is with a Phantom Flex camera. I just don't have the coin to rent one for a couple days.

While I believe a striker fired system would aid in the accuracy potential of the system, you would have to do so in a manner that would not compromise reliability.

I say if you have the capability of designing and prototyping such a fire control system....GO FOR IT!! I would love to see it. </div></div>

The Casio Exilim ex-fh100 can do 1000fps, albeit at a much reduced resolution: http://www.youtube.com/watch?v=U3JfGw8YfeQ

 

[AzNooB]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: LoneWolfUSMC</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AzNooB</div><div class="ubbcode-body">In actuality, the bolt doesn't begin to move until after the bullet exits the barrel.</div></div>

There are too many variables to be able to make this broad statement.

There is a reason it's called a "system".

I am still trying to find a reasonably priced way to do some high-speed filming to be able to visually document this. Unfortunately the only feasible method I have come up with to prove or disprove with a wide variety of systems (instead of one pressure meter equipped rifle) is with a Phantom Flex camera. I just don't have the coin to rent one for a couple days.

While I believe a striker fired system would aid in the accuracy potential of the system, you would have to do so in a manner that would not compromise reliability.

I say if you have the capability of designing and prototyping such a fire control system....GO FOR IT!! I would love to see it. </div></div>

If you watch VuurwapenBlog's high speed videos, like the one, you'll see the gasses exiting the barrel right before the bolt carrier begins to move.

With a DI gun, you still have the rearward force of gas slightly above the barrel. On a short piston gun, you have the rearward motion of the piston. I'm assuming both of these are in motion as soon as the bullet passes the gas port. Either way, semi autos are capable of pretty insane accuracy. I would like to see a GAP AR-10 vs a GAP bolt gun or something similar on a ransom rest. That would "prove" the human component more than anything.

Lowlight, didn't you get some ridiculously small groups out of a GAP AR-10? Do you think a striker fired trigger system (similar to a Remington 700's) would help with consistency for people who didn't necessarily have the best gas gun shooting technique or are used to bolt guns?

 

[Takashi]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: LoneWolfUSMC</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AzNooB</div><div class="ubbcode-body">In actuality, the bolt doesn't begin to move until after the bullet exits the barrel.</div></div>

There are too many variables to be able to make this broad statement.

There is a reason it's called a "system".

I am still trying to find a reasonably priced way to do some high-speed filming to be able to visually document this. Unfortunately the only feasible method I have come up with to prove or disprove with a wide variety of systems (instead of one pressure meter equipped rifle) is with a Phantom Flex camera. I just don't have the coin to rent one for a couple days.

While I believe a striker fired system would aid in the accuracy potential of the system, you would have to do so in a manner that would not compromise reliability.

I say if you have the capability of designing and prototyping such a fire control system....GO FOR IT!! I would love to see it. </div></div>

On AR patterned rifles, the bolt is locked in until there is sufficient gas pressure to move the carrier back. (As in if you could push on the bolt from the muzzle end, it won't move). The carrier moving back causes the rotation. I believe the only time the gas pressure reaches this level is when the bullet is past the gas port.

 

[Mr Flannel]

Re: I think I just had an epiphany (bolt vs gas gun)

[/quote]If you watch VuurwapenBlog's high speed videos, like the one, you'll see the gasses exiting the barrel right before the bolt carrier begins to move.
[/quote]

That is a very interesting video, although I have to say I'm more interested in the muzzle/barrel movement on firing with that surefire brake. Does anyone have any experience or data which shows POI shifts when shooting the same barrels with top-ported brakes and then without?

On a side note, could a piston system induce that much downwards flex?

 

[WARDOG]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AzNooB</div><div class="ubbcode-body">
From reading around and doing research, the only thing that makes a gas gun less accurate than a bolt gun (all else equal) is the hammer firing control system....
...So in theory, if both a gas gun and bolt gun were made to the same tolerances (lock up, chamber specs, etc) they would both be equally mechanically accurate.
</div></div>
There has been much discussion about this topic.
I don't believe the firing control is the '<span style="font-style: italic">only</span>' thing that ..."makes a gas gun less accurate..."
Since cartridge prep carries a great percentage of consistent accuracy, I think the violent action of a semi bolt moving forward at great velocity, stripping a cartridge from the magazine, and ramming it into the chamber can easily alter the precision built ammunition. Also a looser chamber in semi's to allow for such a violent loading, and scarring to the case and bullet tip damage all affect consistent accuracy.
I think of a semi-auto as working like a inertia hammer-bullet puller. A simple test I have done in the past on two of my AR-10's (.308 & .300 SAUM) have shown that bullet seating depths change randomly when chambered. That right there affects accuracy (pressures, velocity, jump to the lands, etc).
While I am impressed with the accuracy of some of my semi's, a well built bolt action can duplicate the accuracy, and then some, for a smaller pricetag of weapon massaging to get it there.
I am intrigued by your firing control musing though.

 

[Young_Engineer_89]

Re: I think I just had an epiphany (bolt vs gas gun)

I am thinking abbot this, if when the pressure reaches the gas tube, it is at it's peak, then the carrier will start moving to the rear at that point in time. Am I correct in that understanding? If I am, the BCG would be moving before the boolet leaves the barrel, correct?

Not trying to say gassers can't drive like a laser, I own a larue stealth upper and it stacks em on top of each other when I do my part.

I would be interested in what the pros have to say about "why/if a gasser is less accurate than a bolt gun" and if there is a difference how much difference is there.

Might have to build a 223 bolt gun to do a test. Damn it now I have something else competing for my 243 wssm build monnies...

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: takashi</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: LoneWolfUSMC</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AzNooB</div><div class="ubbcode-body">In actuality, the bolt doesn't begin to move until after the bullet exits the barrel.</div></div>

There are too many variables to be able to make this broad statement.

There is a reason it's called a "system".

I am still trying to find a reasonably priced way to do some high-speed filming to be able to visually document this. Unfortunately the only feasible method I have come up with to prove or disprove with a wide variety of systems (instead of one pressure meter equipped rifle) is with a Phantom Flex camera. I just don't have the coin to rent one for a couple days.

While I believe a striker fired system would aid in the accuracy potential of the system, you would have to do so in a manner that would not compromise reliability.

I say if you have the capability of designing and prototyping such a fire control system....GO FOR IT!! I would love to see it. </div></div>

On AR patterned rifles, the bolt is locked in until there is sufficient gas pressure to move the carrier back. (As in if you could push on the bolt from the muzzle end, it won't move). The carrier moving back causes the rotation. I believe the only time the gas pressure reaches this level is when the bullet is past the gas port. </div></div>

 

[Dirty D]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">What you can't handle $3000 a day to rent a high speed camera... aren't you a cop, rob a drug dealer like Denzel did ... LOL </div></div>

Once upon a time we were turning a bale of cocaine over to the DEA for disposal and I turn to my chief in front of the DEA guys and say "hey chief, where is that other bale?" I thought that it was hilarious but my chief about shit himself and the DEA guys had no sense of humor whatsoever.

 

[gstaylorg]

Re: I think I just had an epiphany (bolt vs gas gun)

Whether the BCG starts to move before the projectile exits the barrel likely depends on the type gas system (piston vs DI), the length of the gas system, the overall barrel length, and the muzzle velocity of the projectile. The further the muzzle is from the gas port, the more likely the BCG would move before the projectile exited the barrel.

As an example of this, I had at one time a Sig 556 DMR with a 21" barrel. As compared to a Sig 556 SWAT with a 16" barrel but having the same length gas piston system, clearly the projectile would traverse the barrel of the DMR for a longer period of time after passing the gas port because the muzzle is 5" farther away from the gas port than on the 16" SWAT. I cannot say with any certainty whether this extra barrel length past the gas port made it so that the BCG was moving prior to the bullet's exit from the muzzle, but you get the idea.

The 556 DMR rifle also had an adjustable gas valve that could be turned to the "off" position so that the BCG would not cycle at all. I spent many hours at the range and many rounds of ammo trying to determine whether there was any difference in the precision of this weapon when fired in the normal semi-auto mode, or single shot/manual feed mode. FWIW in terms of this discussion, I could not distinguish any difference in the precision of this rifle between the two fire modes, using several different types of ammunition on the same day. However, I could understand how that observation might vary widely from one rifle/caliber to another.

 

[Takashi]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: gstaylorg</div><div class="ubbcode-body">Whether the BCG starts to move before the projectile exits the barrel likely depends on the type gas system (piston vs DI), the length of the gas system, the overall barrel length, and the muzzle velocity of the projectile. The further the muzzle is from the gas port, the more likely the BCG would move before the projectile exited the barrel.

As an example of this, I had at one time a Sig 556 DMR with a 21" barrel. As compared to a Sig 556 SWAT with a 16" barrel but having the same length gas piston system, clearly the projectile would traverse the barrel of the DMR for a longer period of time after passing the gas port because the muzzle is 5" farther away from the gas port than on the 16" SWAT. I cannot say with any certainty whether this extra barrel length past the gas port made it so that the BCG was moving prior to the bullet's exit from the muzzle, but you get the idea.

The 556 DMR rifle also had an adjustable gas valve that could be turned to the "off" position so that the BCG would not cycle at all. I spent many hours at the range and many rounds of ammo trying to determine whether there was any difference in the precision of this weapon when fired in the normal semi-auto mode, or single shot/manual feed mode. FWIW in terms of this discussion, I could not distinguish any difference in the precision of this rifle between the two fire modes, using several different types of ammunition on the same day. However, I could understand how that observation might vary widely from one rifle/caliber to another. </div></div>

That's pretty indicative of either the BCG not cycling prior to the bullet leaving or it has barely any effect. The round should be accelerated to muzzle velocity by the time it's around the gas tube and thats 2500 fps (for the sake of easy calculation) and another 2 inches to go or 0.0006 seconds more in the tube

 

[Lowlight]

Re: I think I just had an epiphany (bolt vs gas gun)

The Canadian Army did high speed footage and according to them, and I would have to agree, the BCG begins to move before the bullet leaves the bore.

If gas moves roughly 5X faster than the bullet, and there is about 5 to 7 inches of barrel after the gas block on most AR10 Type Rifles, with a much slower 308 bullet, the barrel length between the gas block and BCG would have to be at least 5X longer of the BCG to not be moving before it is completely gone... as well clearly after watching hundreds of people shoot gas guns, the driver causes way too many issues for the system to not be easily affected.

To answer the above, at the SHC Demo Day, with Scott Parks from Vortex on Glass and about 10 others around I did shoot a .25 MOA group out of a GAP10, on general I average about 3/8ths to a 1/2 MOA on a bad day with it.

As far as video, gas leaving the barrel before seeing the BCG appear to move is deceiving because gas does in fact pass the bullet.

 

[coldboremiracle]

Re: I think I just had an epiphany (bolt vs gas gun)

Just a thought, just because the BCG starts moving doesn't necessarily mean less accuracy because of movement. Being the precise machine that it is, the modern gas operated auto should in theory move the same amount, in the same direction every time, shouldn't it?

Just as a bolt rifle; trigger breaks,firing pin drops,pin impacts primer, recoil begins, bullet exits barrel.

With a gas gun, there is more stuff happening, but it should be the same every time and therefor should have repeatable results be they good or bad.

 

[Lowlight]

Re: I think I just had an epiphany (bolt vs gas gun)

Yes but how the shooter handles the movement is the key, you have multiple recoils, as well how they made the rifles is what matter.

if you talk to GAP the Heat Sink / Connection of the barrel to the receiver combined with the DI helps to keep it consistent.

A rifle is a huge tuning fork, any pressure on any part of the tuning fork will cause it to move away from the pressure. So if you have uneven pressures while the whole system is vibrating you have issues with barrel harmonic -- thus accuracy is effected. You have to manage the music playing when the rifle fires otherwise you get a sour note.

 

[gstaylorg]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">Yes but how the shooter handles the movement is the key, you have multiple recoils, as well how they made the rifles is what matter.

if you talk to GAP the Heat Sink / Connection of the barrel to the receiver combined with the DI helps to keep it consistent.
<span style="color: #FF0000">
A rifle is a huge tuning fork, any pressure on any part of the tuning fork will cause it to move away from the pressure. So if you have uneven pressures while the whole system is vibrating you have issues with barrel harmonic -- thus accuracy is effected. You have to manage the music playing when the rifle fires otherwise you get a sour note.</span> </div></div>

This makes sense and has always seemed to be a logical point of different between a gas gun and a bolt gun. Along the same line, how the actual process of the gas moving through the gas port once the projectile has passed, and how that might alter harmonics, acceleration of the projectile, etc., seems like it could have some effect. This was my primary reason for trying to determine whether there was a difference in precision between firing with the BCG cycling, or not cycling.

The other common difference that comes to mind is barrel contour. Your <span style="font-style: italic">run of the mill</span> gas gun often has a barrel contour that is much thinner than your <span style="font-style: italic">average</span> bolt gun barrel, particularly if it's a precision bolt gun. I wonder whether the perceived difference in precision between the two may have arisen in part because of this. Unless the barrel contours were similar, it seems as though it'd almost be comparing apples to oranges from a precision viewpoint.

 

[AzNooB]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: WARDOG</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AzNooB</div><div class="ubbcode-body">
From reading around and doing research, the only thing that makes a gas gun less accurate than a bolt gun (all else equal) is the hammer firing control system....
...So in theory, if both a gas gun and bolt gun were made to the same tolerances (lock up, chamber specs, etc) they would both be equally mechanically accurate.
</div></div>
There has been much discussion about this topic.
I don't believe the firing control is the '<span style="font-style: italic">only</span>' thing that ..."makes a gas gun less accurate..."
Since cartridge prep carries a great percentage of consistent accuracy, I think the violent action of a semi bolt moving forward at great velocity, stripping a cartridge from the magazine, and ramming it into the chamber can easily alter the precision built ammunition. Also a looser chamber in semi's to allow for such a violent loading, and scarring to the case and bullet tip damage all affect consistent accuracy.
I think of a semi-auto as working like a inertia hammer-bullet puller. A simple test I have done in the past on two of my AR-10's (.308 & .300 SAUM) have shown that bullet seating depths change randomly when chambered. That right there affects accuracy (pressures, velocity, jump to the lands, etc).
While I am impressed with the accuracy of some of my semi's, a well built bolt action can duplicate the accuracy, and then some, for a smaller pricetag of weapon massaging to get it there.
I am intrigued by your firing control musing though.</div></div>

If you're running your bolt fast, you would do the exact same thing to your ammo. I've seen videos of guys shooting half-minute 5-shot groups while doing rapid bolt manipulation.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: coldboremiracle</div><div class="ubbcode-body">Just a thought, just because the BCG starts moving doesn't necessarily mean less accuracy because of movement. Being the precise machine that it is, the modern gas operated auto should in theory move the same amount, in the same direction every time, shouldn't it?

Just as a bolt rifle; trigger breaks,firing pin drops,pin impacts primer, recoil begins, bullet exits barrel.

With a gas gun, there is more stuff happening, but it should be the same every time and therefor should have repeatable results be they good or bad.</div></div>

That's a good point. If we can keep every motion consistent, no matter how violent, it should be consistent down range as well.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">The Canadian Army did high speed footage and according to them, and I would have to agree, the BCG begins to move before the bullet leaves the bore.

If gas moves roughly 5X faster than the bullet, and there is about 5 to 7 inches of barrel after the gas block on most AR10 Type Rifles, with a much slower 308 bullet, the barrel length between the gas block and BCG would have to be at least 5X longer of the BCG to not be moving before it is completely gone... as well clearly after watching hundreds of people shoot gas guns, the driver causes way too many issues for the system to not be easily affected.

To answer the above, at the SHC Demo Day, with Scott Parks from Vortex on Glass and about 10 others around I did shoot a .25 MOA group out of a GAP10, on general I average about 3/8ths to a 1/2 MOA on a bad day with it.

As far as video, gas leaving the barrel before seeing the BCG appear to move is deceiving because gas does in fact pass the bullet. </div></div>

Some good points. Either way, we know a well built gas gun is already capable of accuracy above most peoples' skill levels.

What I'm wondering is if a more forgiving trigger system with less opportunity for shooter induced movement would make you more consistent with it and reduce your groups on your bad days to 3/8 MOA. I know we're talking a quarter MOA improvement here. The simple fact is that technology has gone such a far way that only incremental improvements can be made until we start shooting lasers.

 

[cooleyjc]

Re: I think I just had an epiphany (bolt vs gas gun)

I don't have a gas gun so I have no data. I am curious as to whether or not the variation in muzzle velocity for an AR is generally larger or smaller than a bolt action in the same caliber with as similar a configuration as possible. If the variation in muzzle velocity is very close to the same then I think you have to go look at other differences to get a handle on the original question in the thread.

Responding to the second post about high speed filming. It might not be necessary to film the process to answer the question. An optical sensor at the end of the barrel to detect the passage of the bullet, and an electrical contact on the bolt that opens when the bolt moves could show the same thing. You would need something like a digital oscilloscope to capture the signals, but it wouldn't have to be a high end scope. It's the kind of equipment you could find in a good high school or college electronics lab.

Please excuse my ignorance, but what exactly is meant by a "striker fired system" and how would it differ from the existing firing mechanism on an AR?

 

[rideHPD]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AzNooB</div><div class="ubbcode-body"> In actuality, the bolt doesn't begin to move until after the bullet exits the barrel.
</div></div>

A big thing I would say here is that you won't really see the recoil from the round affect the movement of the gun because of the relative masses of the projectile, gas, and rifle and the momentum and impulse transfers. The majority of the recoil of a rifle if I ran the simulator correctly comes from the free expansion of the gas as the bullet leaves the muzzle, where the gas accelerates very rapidly from the MV of the bullet to several times faster. This process occurs over the duration of a fraction of a second. Lowlight has precisely the right idea here in that any unbalanced force on the gun that is masked by static friction with be exploited by the rifle. Because of this human error you will almost certainly never see any real improvement in trying to bring a semi-auto closer to a bolt gun, no matter what you do, because of the sheer fact that it is vastly outweighed by discrepancies in technique from shot to shot. Reducing that impulse time during which you can affect the shot is actually probably the most effective thing you can do, assuming you have a good barrel and smooth trigger(+things that help you do your part right) and have reasonably tight tolerances for every other part of the gun. I wonder if adding the resistance to the firing pin from having to depress a primer instead of falling on a dry chamber during dry fire changes how we shoot by influencing trigger follow-through and therefore introducing play in the system during vibration.

 

[AzNooB]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JCC135</div><div class="ubbcode-body">I don't have a gas gun so I have no data. I am curious as to whether or not the variation in muzzle velocity for an AR is generally larger or smaller than a bolt action in the same caliber with as similar a configuration as possible. If the variation in muzzle velocity is very close to the same then I think you have to go look at other differences to get a handle on the original question in the thread.

Responding to the second post about high speed filming. It might not be necessary to film the process to answer the question. An optical sensor at the end of the barrel to detect the passage of the bullet, and an electrical contact on the bolt that opens when the bolt moves could show the same thing. You would need something like a digital oscilloscope to capture the signals, but it wouldn't have to be a high end scope. It's the kind of equipment you could find in a good high school or college electronics lab.

Please excuse my ignorance, but what exactly is meant by a "striker fired system" and how would it differ from the existing firing mechanism on an AR?</div></div>

Molon did some ammo tests on AR15s. I initially thought M4 feedramps would cause inconsistencies (due to the rounds hitting a steeper surface) in OAL, thus causing changes in pressure. Now that claim seems invalid.

However, the inverse is seen with MK318 Mod 0 ammo. This is not match grade ammo, but is frangible ammo with a larger opening.

And it's also interesting to see the consistency of 77gr.223 FGMM over the years.

Here is some data gathered from a pair of .308 bolt guns on Sniper Central.

Based off of this, I wouldn't say consistency in muzzle velocity is a function of precision. Some of the ammo with the greatest variances have the best accuracy, notably, HSM 168gr Range Match.

To answer your question about the fire control systems, bolt guns use a striker fired system. The firing pin gets is cocked and held back via spring tension and the trigger sear releases it so that it directly strikes the primer on a cartridge. ARs and most semi-autos on the other hand use hammer fired trigger systems. The hammer gets cocked back and swings a good distance to hit the firing pin, which then strikes the primer on the cartridge. Many believe that the extra time that it takes for all of these actions to occur allows shooters to move more before the bullet exits the barrel. So basically, if you're not able to hold your entire body still for the extended duration of the firing process on a gas gun, your groups will be wider than they would be on a bolt gun of equal mechanical accuracy.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rideHPD</div><div class="ubbcode-body">A big thing I would say here is that you won't really see the recoil from the round affect the movement of the gun because of the relative masses of the projectile, gas, and rifle and the momentum and impulse transfers. The majority of the recoil of a rifle if I ran the simulator correctly comes from the free expansion of the gas as the bullet leaves the muzzle, where the gas accelerates very rapidly from the MV of the bullet to several times faster. This process occurs over the duration of a fraction of a second. <span style="color: #FF0000">Lowlight has precisely the right idea here in that any unbalanced force on the gun that is masked by static friction with be exploited by the rifle.</span> Because of this human error you will almost certainly never see any real improvement in trying to bring a semi-auto closer to a bolt gun, no matter what you do, because of the sheer fact that it is vastly outweighed by discrepancies in technique from shot to shot. Reducing that impulse time during which you can affect the shot is actually probably the most effective thing you can do, assuming you have a good barrel and smooth trigger(+things that help you do your part right) and have reasonably tight tolerances for every other part of the gun. I wonder if adding the resistance to the firing pin from having to depress a primer instead of falling on a dry chamber during dry fire changes how we shoot by influencing trigger follow-through and therefore introducing play in the system during vibration.</div></div>

Can you rephrase that sentence? I don't quite follow what you're saying.

It seems like you agree with me that the shooter is more to blame for poor gas gun precision and that something to mitigate shooter error will yield better results than increasing the inherent accuracy of a gas gun.

 

[Chanonry]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AzNooB</div><div class="ubbcode-body"> In actuality, the bolt doesn't begin to move until after the bullet exits the barrel.
</div></div>

Newton was wrong then?

As soon as a force is applied on the bolt it will 'begin' to move. I guess the force is an impulse applied for as long as there is pressure in the barrel i.e. until the bullet exits the muzzle.

Why would the bolt be static during this pressure pulse then magically move when it is no longer there?

 

[oneshot onekill]

Re: I think I just had an epiphany (bolt vs gas gun)

I've chewed on this quite a bit in recent years. When you rule out all external variables, it can only be the number of moving parts and the vibration caused by that movement that reduces accuracy in gas-guns. I believe it's ALL of the parts. From the trigger group parts, to the upper and lower receiver wiggle, to the BCG movement... All of it. Gas-guns are just loose (at least most are). You can argue timing all day long... You can say it's the shooter... You can blame it on ammo. But I don't think any of that holds water. I know some of you guys are rock-solid shooters no matter what the platform is. I've ruled out ammo because I've found that as long as you reload with consistency you're golden, and slamming the round into the chamber doesn't change anything. And I believe once the projectile is better than halfway down the barrel, the pressure behind it no longer has any effect. Or at least not much. Certainly not enough to change accuracy. My $0.02...FWIW.

 

[CoryT]

Re: I think I just had an epiphany (bolt vs gas gun)

Chanory, the bolt does not move instantly because it is locked. In point of fact, it does 'move' inthat is presses rearward against the locking recesses, whatever fraction of an inch there may be prior to firing. The case moves forward when struck by the firing pin until contact with the chamber or extractor, then rearward as the bullet starts to move in the case. When gas comes in contact with the BCG or piston, the carrier starts to the rear, camming the bolt to the unlock position. Once unlocked, the bolt itself is free to move to the rear with the BCG.

Really, the only limitations in semi auto accuracy is the number of moving parts and the vibrations they induce coupled with the tolerances required to make a reliable system. The shooter will always be the weakest link in the chain, and a good gas gun will have more intrinsic accuracy than most of the drivers can use on any given day.

The downsides to a semi auto are weight, length, cost and maintenence. For a given cartridge, an equally accurate bolt gun with be shorter, lighter, cost less and require far less user/armorer maintenence. For that, you give up rate and volume of fire. This really only matters at relativly short ranges, say 400 yards or so. Much past that, the time to make a good aimed shot are about equal for a skilled bolt user and a semi auto. The semi will have a larger magazine, and therefore keep lead in the air longer having fewer reloads.

Nail, use hammer.
Nut, use wrench.

 

[LoneWolfUSMC]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Chanonry</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AzNooB</div><div class="ubbcode-body"> In actuality, the bolt doesn't begin to move until after the bullet exits the barrel.
</div></div>

Newton was wrong then?

As soon as a force is applied on the bolt it will 'begin' to move. I guess the force is an impulse applied for as long as there is pressure in the barrel i.e. until the bullet exits the muzzle.

Why would the bolt be static during this pressure pulse then magically move when it is no longer there? </div></div>

Because Newton was not talking about direct impingement gas operated rifles.

Newton's law when applied to rifles simply states that when you fire a rifle the action of the bullet moving away from you will cause a recoil impulse moving back at you. It has nothing to do with the operating system of the weapon. Newton's Law still applies to bolt action rifles, but the bolt remains locked until I unlock it.

 

[LoneWolfUSMC]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">What you can't handle $3000 a day to rent a high speed camera... </div></div>

One of these days I am just going to suck it up and fork over the cash.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: vmpgsc</div><div class="ubbcode-body">
The Casio Exilim ex-fh100 can do 1000fps, albeit at a much reduced resolution: http://www.youtube.com/watch?v=U3JfGw8YfeQ </div></div>

I did the math awhile back and based on muzzle velocity of a rifle 1000 fps isn't going to get it.

 

[Lowlight]

Re: I think I just had an epiphany (bolt vs gas gun)

Some people have to realize there is a difference between the BCG moving and the "bolt unlocking"

There is almost an 1/2" of movement to the rear before it turns the bolt and unlocks the lugs... this is the delay.

There is air in the barrel, air in the gas tube, gasses both in front and behind the bullet, all this can cause pressures to begin to move things. In confined spaces it's worse and stepping it down speeds it up. So while max pressure is behind the bullet there is still pressure building before it passes. Some of it that passes via the lands will inherently head up the gas tube.

So when we say the BCG is moving before the bullet exits that doesn't mean Unlocked, just means moving.

I don't use fancy triggers, I use pretty much plan old ones, I just concentrate on follow through. Uneven pressures against the rifle will affect those harmonics I spoke of, letting off the trigger is even worse. So the trigger control necessary to drive the rifle accurately is issue number one for the shooter. Clearly there are difference between a well built system and a poorly built one on a thousandths of inch scale, but shooting is a game of micro seconds and micro inches, tiny changes we can't fathom matter, and you'd be surprise who much we move in that mystical realm.

 

[LoneWolfUSMC]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">
There is almost an 1/2" of movement to the rear before it turns the bolt and unlocks the lugs... this is the delay. </div></div>

To make sure I wasn't mis-remembering I just pulled an AR10 out of the safe. The way the cam pin slot is cut in the carrier there appears to be less than 0.1" of movement before the bolt is rotating. Since it is a increasing angle and there is some slop in the system, it's difficult to say exactly how far the carrier can move before unlocking without actually de-greasing the whole deal and setting it up on the bench.

Now this is with a grand sample of ONE AR10. I don't know if the cam pin slot in a LR308, POF, GAP10, etc. are different. They could be.

I am OK with math, but I am not smart enough to figure out if the air column in the barrel and in the gas tube can rise to a high enough pressure to cause the carrier to move to the rear more than 0.1" before the bullet has passes the gas port.

It DOES seem logical that the gas vented to the carrier would begin the bolt's rearward motion prior to the bullet exiting the muzzle. This is simply due to the fact that once the bullet has exited the barrel the pressure drops sharply and there is less energy to operate the action.

I guess it doesn't really matter because I am almost 100% sure that the bolt carrier group BEGINS moving before the bullet has left the barrel. Thus it requires a strict adherence to the fundamentals to keep the barrel pointed at the target through the chaos.

This is one of the reasons I am obsessed with a high speed video shoot. I really want to see what the state of the weapon system is when the bullet exits the muzzle. I also believe it will help ME to be a better shooter if I can see what I am doing at that precise moment.

 

[sinister]

Re: I think I just had an epiphany (bolt vs gas gun)

Concur with Frank.

You will have precursor air (the air in the tube in front of the bullet) pushed out ahead of the bullet; gas blow-by that escapes from behind the bullet in any gaps between the lands and grooves and the bullet jacket that aren't sealed; stagnant and minimal precursor air in the gas tube; and the bleed-off of the main column gas in the gas tube as the bullet passes the gas port.

If you look at the cam pin slot in the bolt carrier the carrier (once gas pressure has overcome the stationary inertia required to get the carrier moving aft) can or is moving to the rear before the bolt starts its twist and unlock and extract stages.

The shooter can increase stationary carrier time by increasing both carrier and buffer weights, retarding movement until gas pressure overcomes the combined stationary inertia of the carrier group, carrier weight, and buffer. The carrier may not start moving until after the bullet leaves the muzzle, pressure drops, and the brass case is no longer obturated against the chamber walls (increasing your case life and relieving pressure pushing against the inside of the primer cup).

A fast AR trigger still requires the hammer to travel in an arc before contacting the firing pin to start the ignition cycle (as opposed to the direct in-line travel of a bolt-action's striker and firing pin in an example like a Remington 700).

If you look at most ARs (excepting the LMT) there is minimal barrel support in the receiver (the barrel extension is coaxially-aligned and locked for an inch or just a little longer) as opposed to a full 6 inches of barrel shank on a DTA SRS -- extending the length from bolt face to first harmonic node point.

 

[rideHPD]

Re: I think I just had an epiphany (bolt vs gas gun)

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

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rideHPD</div><div class="ubbcode-body">A big thing I would say here is that you won't really see the recoil from the round affect the movement of the gun because of the relative masses of the projectile, gas, and rifle and the momentum and impulse transfers. The majority of the recoil of a rifle if I ran the simulator correctly comes from the free expansion of the gas as the bullet leaves the muzzle, where the gas accelerates very rapidly from the MV of the bullet to several times faster. This process occurs over the duration of a fraction of a second. <span style="color: #FF0000">Lowlight has precisely the right idea here in that any unbalanced force on the gun that is masked by static friction with be exploited by the rifle.</span> Because of this human error you will almost certainly never see any real improvement in trying to bring a semi-auto closer to a bolt gun, no matter what you do, because of the sheer fact that it is vastly outweighed by discrepancies in technique from shot to shot. Reducing that impulse time during which you can affect the shot is actually probably the most effective thing you can do, assuming you have a good barrel and smooth trigger(+things that help you do your part right) and have reasonably tight tolerances for every other part of the gun. I wonder if adding the resistance to the firing pin from having to depress a primer instead of falling on a dry chamber during dry fire changes how we shoot by influencing trigger follow-through and therefore introducing play in the system during vibration.</div></div>

Can you rephrase that sentence? I don't quite follow what you're saying.

It seems like you agree with me that the shooter is more to blame for poor gas gun precision and that something to mitigate shooter error will yield better results than increasing the inherent accuracy of a gas gun.</div></div>

Lowlight pretty much said it again, but what I meant is that before the shot is broken the rifle is stationary, and there are forces placed on it, from the loaded bipod, your cheek, your shoulder your firing hand and your non-firing hand and maybe a bag. What I'm saying is that if there is any friction involved in keeping the weapon static to that force will in a sense disappear when the weapon starts to vibrate, just the same as how an electric sander placed on an incline will move when turned on because to friction coefficient will decrease (for those who know the difference between static and kinetic friction, kinetic during vibration actually is a good bit different because of the wave phases of the energy). Basically if there's any frictional force involved before firing that force will decrease in magnitude and the rifle will accelerate because of an unbalanced force. Make sense?

 

[oneshot onekill]

Re: I think I just had an epiphany (bolt vs gas gun)

Let me throw this into the equation... It's my understanding that when gas enters the bolt carrier from the gas tube, it actually pressurizes the space behind the bolt and holds the bolt forward and locked until the pressure is diminished (which is not until the bullet is out of the barrel). This indicates that the BCG is actually NOT moving rearward until the bullet is out.
This is how those that don't like piston driven AR's try to say that a mechanically driven BCG will damage the bolt lugs and ultimately cause catastrophic failure. Personally I think it's BS but it does change things here... I think....
Or am I smoking crack? It's late. I had a rough day and my brain hurts...

 

[rideHPD]

Re: I think I just had an epiphany (bolt vs gas gun)

I'd say they are, there are so many pores from which the gas can escape through in-between the lower and upper, ejection port, etc. that it wouldn't stay pressurized.

 

[Hellbender]

Re: I think I just had an epiphany (bolt vs gas gun)

About 20 years ago (when I was heavily into AR's) I looked at building a striker fired AR to combat the slow locktime of the trigger system.

It can be done, but there is not a simple solution.

Just adding up the spring pressure balances of the system show that changing the hammer/striker cocking time from rearward (existing) to forward will require major redesign of the system.

 

[LoneWolfUSMC]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: oneshot onekill</div><div class="ubbcode-body">Let me throw this into the equation... It's my understanding that when gas enters the bolt carrier from the gas tube, it actually pressurizes the space behind the bolt and holds the bolt forward and locked until the pressure is diminished (which is not until the bullet is out of the barrel).</div></div>

You are partially correct in the fact that the gas tube pressurizes the void behind the bolt in the bolt carrier group.

However this does NOT keep the bolt locked. It is directly the opposite. When the gas expands it puts equal pressure on the carrier and the bolt. The bolt is stationary against the barrel extension and cannot go forward. The carrier is stationary against the buffer and CAN move. So the carrier begins it's rearward travel. When it goes back, the cam pin slot causes the bolt to cam and unlock.

The gas doesn't help the bolt stay locked. The gas is what unlocks the bolt.

 

[oneshot onekill]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: LoneWolfUSMC</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: oneshot onekill</div><div class="ubbcode-body">Let me throw this into the equation... It's my understanding that when gas enters the bolt carrier from the gas tube, it actually pressurizes the space behind the bolt and holds the bolt forward and locked until the pressure is diminished (which is not until the bullet is out of the barrel).</div></div>

You are partially correct in the fact that the gas tube pressurizes the void behind the bolt in the bolt carrier group.

However this does NOT keep the bolt locked. It is directly the opposite. When the gas expands it puts equal pressure on the carrier and the bolt. The bolt is stationary against the barrel extension and cannot go forward. The carrier is stationary against the buffer and CAN move. So the carrier begins it's rearward travel. When it goes back, the cam pin slot causes the bolt to cam and unlock.

The gas doesn't help the bolt stay locked. The gas is what unlocks the bolt. </div></div>
Ah... Yes, that makes sense. Also explains why those against piston AR's claim damage from the mechanics...
Told you I was tired... LOL!

 

[Victor Co Heather]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: LoneWolfUSMC</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">What you can't handle $3000 a day to rent a high speed camera... </div></div>

One of these days I am just going to suck it up and fork over the cash.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: vmpgsc</div><div class="ubbcode-body">
The Casio Exilim ex-fh100 can do 1000fps, albeit at a much reduced resolution: http://www.youtube.com/watch?v=U3JfGw8YfeQ </div></div>


I did the math awhile back and based on muzzle velocity of a rifle 1000 fps isn't going to get it. </div></div>


I'll do it for $2k a day! : )

 

[BigJohn141]

Re: I think I just had an epiphany (bolt vs gas gun)

Quick, everyone go to the Discovery Channel's website and post about this on the Mythbuster's forum. They have all the cool high speed cameras and love doing gun myths so this would solve it for free.

 

[Wannashootit]

Re: I think I just had an epiphany (bolt vs gas gun)

This is an interesting read. I'm still trying to wrap my head around it.
But, the premise seems to make sense- whether or not the bullet has left the barrel before the BCG moves (which I always thought was the reason for the inherent inaccuracy due to harmonics) is load dependent.

For what I can understand at this point, it makes sense...

http://ar15barrels.com/prod/operation.shtml

 

[coldboremiracle]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BigJohn141</div><div class="ubbcode-body">Quick, everyone go to the Discovery Channel's website and post about this on the Mythbuster's forum. They have all the cool high speed cameras and love doing gun myths so this would solve it for free. </div></div>


Those stupid F@ck's would screw this up beyond recognition, and in the end they would determine that a musket firing tuna fish would be the most accurate firearm for hunting the zombie chickens of Antarctica.

 

[LoneWolfUSMC]

Re: I think I just had an epiphany (bolt vs gas gun)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: M.45</div><div class="ubbcode-body">
I'll do it for $2k a day! : ) </div></div>

If that was a whole package for 8 hours of shooting on scene, then I would take you up on it.

Looks like about $2500 for a Phantom Gold and another $80 for a PL mount Zeiss Prime because everything I have is EF mount. Just a tiny expense for a project that I don't expect to make any money off of.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BigJohn141</div><div class="ubbcode-body">Quick, everyone go to the Discovery Channel's website and post about this on the Mythbuster's forum. They have all the cool high speed cameras and love doing gun myths so this would solve it for free. </div></div>

I would venture that they don't "have" anything. More than likely they rent it just like any of us would have to. However their production budget is a little larger than mine.

Most of Mythbuster's firearms stuff is exactly what I expect from folks who have little experience pulling the trigger. I get all excited every time a see a "gun" or "bomb" episode. I am usually disappointed at the end.