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.