Advanced Marksmanship Differences shooting gas versus bolt guns?

[Chris F]

Re: Differences shooting gas versus bolt guns?

...Here's the .308 AR firing sequence;

...very similar to the .223 AR sequence.

Dissecting this video illustrated some other things going on that might give an alternative explanation to the "last round out of group" phenomenon observed by Lowlight that doesn't involve a bullet still in the barrel.

I'll process the video and float the theory when I have a bit more time.

 

[rkgsmith]

Re: Differences shooting gas versus bolt guns?

I have an LWRC I'm shooting right now for T&E. I can turn the gas system off and hand cycle the bolt under spring tension. I may try some groups and see if there is any difference.

 

[WesP]

Re: Differences shooting gas versus bolt guns?

If you cant see the muzzle blast until the bullet clears the barrel therefore allowing it to escape... for the carrier to move while the bullet was still in the barrel, wouldnt the blast from the carrier have to come first? In the video posted by chris f it is quite clear the gasses are exiting the barrel and brake LONG before the carrier moves. No doubt follow through is important no matter what you shoot. But in this circumstance i dont think its important because of carrier movement. One way to test if its because the bolt is held to the rear or something else at work effecting the last shot would be to simply remove the bolt hold open.

One thing to note for all...after the last round is chambered, there is no longer ANY upward spring tension exerted on the BCG by the magazine. The bolt hold open is actually holding the follower down so the carrier is "relaxed" and sitting more freely on its lower bearing surfaces. When ammo is in the mag there is an upward force on the BCG. This condition change could be looked at as the semi auto version of inconsistant bedding. I suspect this may contribute to the issue noticed by Frank.

 

[BallCoeff_606]

Re: Differences shooting gas versus bolt guns?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sterling Shooter</div><div class="ubbcode-body">There is no difference when shooting a bolt gun or a gas gun. Any gun requires the shooter to complete the firing task in a manner which supports undisturbed aim.
[...]</div></div>
That right there is a contradiction. "Supporting undisturbed aim" in an accurate well-fitted small-bore autoloader is quite a bit different from doing the same in a high-accuracy large-bore non-braked bolt rifle. (At least in mine).

With the well-fitted target-grade small-bore you have the luxury of keeping a nearly unfaltering aim from start to finish on every shot, for a whole magazine.

With the big bolt action, the rifle compresses the shooter's body starting at the shoulder with a shock wave which inevitably snaps his head (and other body parts including eyeball) one way or other, usually kicking a glancing blow to the bottom of the cheekbone for good measure. If you're lucky, you'll just see a white cloud then a blur through the scope before being able to refocus somewhere many mils off to the side somewhere.

So, the trick becomes maintaining initial aim long enough during that body shock wave until the bullet leaves the barrel. That's a substantially different problem than with the small-bore autoloader, rhetoric aside.

 

[Jim_D303]

Re: Differences shooting gas versus bolt guns?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">The bolt carrier absolutely starts to move before the bullet leaves the bore, it moves back to unlock the lugs, and that movement still takes place before the round has completely left the bore.

You can argue it, and say nothing moves, but the fact the Point of Impact changes when things like the last round is fired clearly shows the movement has happened.

Speaking with the guys who test it using high speed film, the carrier is moving.

There is plenty of distance to move the bolt carrier before the locking lugs unlock... that movement is why follow through is important, especially in a 308, because the bolt is so big and heavy compared to a 5.56.

The gases reach the bolt carrier in plenty of time, starting the movement back, which then unlocks the bolt after the bolt carrier "travels' the prescribed distance in order to turn the bolt to unlock the lugs.

I think this needed to be clarified -- the bolt versus the carrier </div></div>

I realize this may be a dead horse for some, but I've found this thread pretty interesting. (and it's not my first time thinking about this stuff)

My experience with semi's is with AR's in 5.56mm. I've studied it as much as I can, but I'm sure there is a lot I have left to learn on it.

RE: bolt carrier movement and the last shot having a different POI....

I think we've agreed by now that the carrier has NOT been caught by the bolt catch before the bullet leaves the bore, right?

So, while I really believe that you're observing a difference in POI with your semi shooters...I am beginning to think that there is a different variable causing this.

My theory is the upward pressure a cartridge has on the bolt carrier (drag) could be the root cause. A difference in friction on the BCG COULD theoritically change how the BCG moves while the bullet is still in the bore.

This is only my theory, but I'd be interested in seeing it get investigated.

What if you shoot, call it 9 rounds, from a magazine normally, then on the last round, you removed the magazine and let the bolt cycle closed. Would you still witness a change in POI?

Here is another theory for a controled test. Load 1 round in a magazine, then a dummy round (or live round that does not get fired) below it in the mag. Chamber the live round, and leave the mag with the dummy round in to cycle. After the shot, unload the dummy and set it back up, until you have a 10 shot group. (replicates normal firing, but keeps the feeding consistent with how the rest of the rounds will have to be chambered)

Contrast that with another group shot by single loading 1 round at a time, then removing the magazine. (makes the bolt cycle fully with no friction from feeding rounds on the BCG)

Contrast both of those, with 1 round loaded at a time, with the bolt locking to the rear on an empty mag every shot. (gives you the "hand chambered round" group with the bolt locking to the rear)

I believe that there is a difference causing what you're observing, I just think there might be another variable causing it, that isn't what the BCG does after the bullet is out of the tube.

My theory is that the groups fired with the magazine locking back on empty, and cycling with nothing in the magwell will be the same.

Just my observation, but I'm interested to hear more experienced people debate the possibilty.

 

[Sterling Shooter]

Re: Differences shooting gas versus bolt guns?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BallCoeff.606</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sterling Shooter</div><div class="ubbcode-body">There is no difference when shooting a bolt gun or a gas gun. Any gun requires the shooter to complete the firing task in a manner which supports undisturbed aim.
[...]</div></div>
That right there is a contradiction. "Supporting undisturbed aim" in an accurate well-fitted small-bore auto-loader is quite a bit different from doing the same in a high-accuracy large-bore non-braked bolt rifle. (At least in mine).

With the well-fitted target-grade small-bore you have the luxury of keeping a nearly unfaltering aim from start to finish on every shot, for a whole magazine.

With the big bolt action, the rifle compresses the shooter's body starting at the shoulder with a shock wave which inevitably snaps his head (and other body parts including eyeball) one way or other, usually kicking a glancing blow to the bottom of the cheekbone for good measure. If you're lucky, you'll just see a white cloud then a blur through the scope before being able to refocus somewhere many mils off to the side somewhere.

So, the trick becomes maintaining initial aim long enough during that body shock wave until the bullet leaves the barrel. That's a substantially different problem than with the small-bore autoloader, rhetoric aside.
</div></div>

No contradiction, all shooting is the same, properly point the firearm at the target with consistent sight alignment and pull the trigger without disturbing aim using smooth trigger control. And, it does not matter whether you're shooting smallbore, or high power, executing the 2 firing tasks, with either firearm, demands consistent support/control of the firearm until recoil has subsided.

My statement does not mean that consistent control is obtainable for heavy recoiling firearms, or for rifles which defy remaining consistent themselves from the time the trigger is pulled to the time the bullet clears the bore, perhaps, such as when the gun is not bedded properly.

 

[edward r homeyer]

Re: Differences shooting gas versus bolt guns?

I have found bolt guns to be more forgiving in shooter error. Gas guns require a good solid follow through everytime. Any error seems to be magnified. With the gas guns I am familiar with SR25, M110, OBR a good preload on the bipods are required. Also you have to drive the gun, hold onto tight, just shy of white knuckle and really lock into it. Concentration on follow through will usually show you error in your position. If you aren't still on target after the shot you are probably doing something wrong. These are my observations. good luck.