Here's my thinking on the last shot conundrum. It's a fact that the first shot and the last shot fire under the exact same conditions and bolt lockup- you wouldn't like the hole in the side of your gun if the bolt was opening before the bullet was long gone.

But what about feeding? ARs feed violently, as evidenced by scratches on the bullet and case after being stripped from the magazine. There is a slight difference between how the last round is handled during feeding as it may be held in a different position by the follower, and it'll feed with less friction. So maybe the last round isn't so beat up during feeding, and doesn't suffer from scratches or a slight change in COAL.

So that's one explanation for what can be different between the first and last rounds. My take? It's all in our heads, and this is just a case of confirmation bias where we look past all the times that the last round groups perfectly.

(I started a new thread so I didn't hijack this thread)

https://texashuntingforum.com/forum/ubbthreads.php/topics/7976727#Post7976727


Alright, let's talk about this last round, bolt lock back on an AR-15 rifle. I just so happened to have a rifle in for load development with an AR-15 in 6.5 Grendel, about a 18" barrel shooting suppressed. The lower is a Palmetto State lower, and I'm not sure what the barrel is. Overall, and very nice AR. I loaded up the test loads with a 123 SST in Hornady brass and CCI #450 primer. The main picture is the entire target with test loads from A to F. I shot loads A through D with a 6th round follower to avoid the bolt lock back. Just for poops and giggles, I went ahead and shot test loads E and F with only 5 rounds in the mag so the bolt would lock to the rear on the 5th shot.

All shot at 100 yards.
Load A shot really well. A .764", 5 shot group, with 6th round loaded as a follower.
Load B and C are stringing and we are out of the accuracy node.
Load D had 3 nice tight shots, but they didn't come in sequence and the group was a little open. I figured I would come into another node on E or F.
Load E, I shot 5 rounds, with the 5th round locking the bolt to the rear. Load E had 4 shots in a .854" group and the 5th round with the bolt locking to the rear went high left. The group opened up to 1.355", 5 shot group. During normal testing, I would NOT load the 5 rounds and shoot 5 rounds. My first round in the mag would be my last follower round, and I would load my 5 test loads on top of that.
Load F shot a 4 shot .733" group and the 5th round bolt lock to the rear opened it up to 1.197", and the round went left.
Conclusion= both Load E and F shot very well 4 shot groups, but the 5th round bolt lock to the rear caused a POI shift. This rifle appears to go left, and maybe slightly high, but certainly left. I gave all groups an honest group to shoot them my best. I was shooting off a front sand bag and rear sand bag, and "driving" the rifle nice and tight.

So what caused this? The recoil impulse is totally different on the last round bolt lock back. The rifle is upset with this change in recoil impulse. What specifically causes the POI shift, I'm not 100% sure. But what I can tell you is, that it is 100% legit and you will NEVER find me shooting a precision shot with the last round locking back, if I can plan ahead on my round counts.

More shooting with this new AR over the last few days seems to be telling me that I need to run it wet and the bore needs to be relatively clean. What I don’t know yet is how dirty I can let it get before accuracy deteriorates. Might be as low as 25 rounds. That’s not good, or at least not good enough. I wonder what a new very accurate barrel will cost me to buy and get installed.