AR rifles, the last round, bolt lock back POI shift

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 with about an 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 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.

I think of it similar to a pitcher in baseball. Once the pitcher releases the ball, the ball is gone. But if the pitcher has bad follow through, it will still effect the pitch. The AR rifle is the same way. It's all about consistency and keeping things the same from shot to shot. Consistency is the key for accuracy.

I'm not saying all AR's do this, but there are MANY AR's that will exhibit some form of a POI shift on that last round, bolt lock back. It is a common enough occurrence that I account for it in my shooting.

I’ve been loosely following this and I don’t see how the bolt locking back could be a factor. I do see how the lack of drag on the bolt from the next round in the magazine could change the recoil and effectively change poi. In fact does the bullet being in the mag put pressure on the bottom of the bolt when it’s closed and ready to fire causing the bolt to lock up differently. I don’t know jack about ARs, just thinking about what will cause a poi shift. I think it has to do with bullet vs no bullet in the mag and nothing to do with the bolt locking back.

I'm on the fence with this. Could it just be stringing due to the barrel getting hot? Or less friction on the bolt carrier by the mag follower vs a cartridge still in the mag? I think I'm in agreement that that the bullet is gone before the overall recoil of the rifle can affect the bullet trajectory...but I'm always open to learning something new.

Edit: wp and I thinking along the same lines...

?I missed that you had started a new thread, and replied on the other one, but for continuity sake, Ive copied that response here

But the recoil impulse is different due to the bolt not coming forward, the first half (roughly) of the impulse is the same, meaning the recoil impulse is only different AFTER the bullet is gone. That is inarguable. That means its not having any effect on ballistics.

I understand what you are trying to show with your exercise above, but if Im understanding that you were using different recipes per group, then It doesnt give us a real conclusion because you dont have a control group and you changed two variables, not one.

As for the pitcher analogy- Followthrough for a pitcher (or shotgun shooter) matters be cause it encourages good fundamentals earlier in the process. Meaning- If a pitcher doesnt follow through, he IS actually starting to slow his arm speed before the ball is gone, vs following through leading to full velocity until AFTER the ball is gone. this goes back to my suggestion that its human error

Two notes

First off, I appreciate being able to have a genuine conversation that hasn't resorts to personal attacks or condescension about this. Thats rare online these days.

Second, you guys have inspired me to try a controlled environment exercise to see what I find out about this. 100 yds, KAC SR-15 MOD 0, some Hornady 73 gr ELDM and a NF NX8 2.5-20. (not what usually sits on that rifle, but for this I want the extra mag). Im going to head out tomorrow and give this a whirl and see what I can see. Im thinking 4x 5 shot groups w/ a "leader" and "follower" round inserted in the mag, and another 4x 5 shot groups w/ mags loaded to 5 rounds. Alternating between 7 round mags and 5 round mags to account for temp changes in the rifle as best as possible. Ill be able to keep track in the order of my shots as well and see if there is any consistency to the change in the first or last bullet in the magazine. if I dont have any discernable difference, I have a cheap [censored] diamonback I can borrow from work and repeat the exercise with

This is an interesting idea- a bullets pressure on the bottom of the BCG could slow it vs a lack of pressure from the follower. But again, the bullet is gone before the bolt's rearward movement begins ( except in an out of battery discharge), so I sort of doubt it.

What about the second theory of pressure on the bcg when a bullet is in the mag. I’m talking about with the bolt closed ready to fire. The last shot would not have that pressure. Essentially causing the bolt to lock up differently.

Same thing happens with my AR, suppressed or not. Last round always strings away. I’ve heard so many ideas as to why, I stopped listening

That's a good thought. There is a fair amount of play in AR bolt carriers, so that could cause some shot-to-shot differences.

If this is the case, you could pretty much test it with two shot groups since it should be repeatable. First shot should go to POA, the second shot will be a flier in a predictable direction.

As soon as the company leaves, I’m trying it.

If, in fact, the last round does fly outside the expected group repetitively, and I know I’m not the problem, obviously there is a reason. I believe that the simplest answer is best, and that would likely be the ‘last bullet in the mag’, however that actually affects the firing process.

It isn’t the barrel
It isn’t the bullet
It isn’t the scope
It isn’t the recoil spring
It isn’t the bolt

Everything is constant and consistent till the last round in the mag. People I respect say this is happening, so it’s happening. Considering what factors could cause the last round to shoot low or wide, I’m left with with one reasonable reason (not that there isn’t another reason we have not considered), and that’s a bullet leaving the muzzle at a different velocity than previous rounds. Probably slower. That could cause the bullet to fly low or cause a change in barrel harmonics which would cause a change in POI.

Has anybody looked at this with a chronograph? A velocity diff should be apparent.

So your saying perhaps the pressure from the bullets in the mag creates a specific bolt-lock position, then the empty mag creates a different lock up position essentially? That could explain why nicer rifles do it less-better bolt/barrel lockups in the first place, so less susceptible to changes due to pressure.

not necessarily. different lockup could produce different POI with similar velocity

I am in the “different bolt lock-up” camp. I have noticed similar results (first shot instead of last) with semi-auto handguns when the first round is fed into the chamber by easing the slide into battery as opposed to letting it slam shut as designed.

As hunts beat me to it. I was just about to ask if anyone had noticed similar results with pistols. I couldn’t shoot a tight enough pistol group to tell a difference. Heading out this weekend and Im going to test one of my ARs

I think that this is the most sound of all the theories as to what is actually causing it.

I spoke to one of my custom AR builders who has a well known custom AR shop about this last night. He said 2 things cause it. The lack of pressure from a following round on the BCG, and the difference in gas pressure with the BCG locked open. He said he doesn't see it very often, but if it does, that's generally why.

Not saying he is wrong, but I don't understand how the pressure could be different up to the point that they bullet leaves the barrel. The bullet has exited the barrel prior to the bolt beginning it's cycle, correct?

I'm running a Rock River, all factory, with the 18 inch bull barrel. On mine it's both the first and last round in the mag that don't hold. The first round is always 3/4 to 1 inch high and the last round is out 3/4 to 1 inch in random directions, but usually low right. And it's 100% repeatable with both factory ammo and my handloads. No idea what causes it, I just know it's so.

I'm late to this discussion but I actually have experienced nearly the same phenomena as Chad, oddly enough my personal testing was with an 18" grendel. Details below. I now own 6.5g and 223/556 snap caps. I conducted this test with the snap caps. The test wasn't "thorough enough" for me to share but it's enough make me believe its effects are real.

Rifle: Wilson Combat recevier set with an Odin works complete barrel/bcg kit.
Load: 123 eldm with Hornady brass, cci 450 and RL15.

As much as i thought about it, couldn't figure out the exact mechanism. Example, whether the bullet leaves the barrel before the bolt head begins to move is not one answer. it depends highly on the barrel/gas port details so i don't think that's exclusively it. In fact, i couldn't figure out one explicit thing so. However, the best thing I can think about to conceptualize it is follow through. Be it for golf, swinging a baseball bat or shooting shotguns (and to also I'm learning distance rifle shooting), follow through affects the performance even though the shot or hit has been initiated. If the "follow through" of the rifle is at minimum inconsistent, then of course you'll see it down range.

This and the fact that it beat up my brass are the reasons why I wanted to change my ar grendel in to a bolt grendel.

Obviously not saying this isn't a real problem, but I've personally never seen this issue or at least didn't notice it. I do tons of accuracy testing for our barrel production and initial load development for our ammo company and always shoot the gun to bolt stop when shooting groups. But as we all know there are LOTS of variables when trying to shoot good groups.

Also when shooting groups and working up a gas port size at the same time I've never noticed the last shot when the bolt didn't stay to the rear and smacked closed on an empty chamber effecting the group either.

Bill, you might have missed it in the other discussion, but Chad did say that this phenomenon typically does not occur or is not prevalent in the higher end rifles. Maybe you just shoot high quality rifles?

Guilty as charged

I've never noticed it. I shoot an AR-15 form the S&W custom shop with their 5R barrel, and a Wilson Combat AR-10 in 6.5 Creedmoor.

Started my ‘last round in the mag’ accuracy testing, but immediately found the first round was the one that was “out”. The rest shot a nice group, which is certainly good for coyote hunting. Could be the reload. Wasn’t me. Again, this is real early in the process and I don’t have enough data to say anything remotely knowledgeable. What I do know though, is that’s I’d much rather have the last round accuracy problem than the first round accuracy problem.

From here I’ll test every morning. If the first round is consistently out of the group, I suppose I’ll have to find out what to do about it. Tomorrow I’ll shoot with the bolt being wet with lube, the logic being that it might lead to more consistent bolt lock-up.

Couldn’t wait till tomorrow. Wet the bolt and carrier group with regular gun oil. Loaded a couple of mags. No first round flyers. Excellent.

Can’t help but wonder if everything will smooth out after hundreds of rounds.

I'm having trouble with the first and, to a lesser extent, last round falling outside the group with my Patriot Arm AR-15 350 Legend. I don't care about the last round but the first round is generally 2-3 MOA higher than the rest of the group. I tried everything I know to fix it including swapping the CMMG barrel for a Faxon, lapping the receiver face and bedding the barrel extension, numerous different loads... Now before I go hunting I have to fire one round first so I'm confident my next round is on target.

I'll try 603's fix and oil the BCG but shouldn't have to do that.

Any other ideas how I can fix this??

I finally quit fighting my AR and had it rebarreled. I was never going to get that original barrel to shoot worth a darn. Shoots great now. I still keep the action wet, to a degree anyway.

Have some experience with M-60...
Agree with the statement in blue...
Biggest variable over looked with, True gun control tis the jerk behind the trigger...

One of the Notsee grammer police will be along shortly ta explain...


on the groups Chad... @ what range, (distance), might have over looked it...

This poster will self destruct in 5sec...

I keep the action wet, too. It’s for the best.

With equal lube, ammo and maintenance, you can somewhat predict effect of the last bolt hold open based on rifle configuration.
A light weight carbine will tend to string the last shot more than a heavy rifle with long bull barrel, rifle length gas tube, rifle buffer and FA bolt.
It's not as pronounced shooting off of a bench where you are holding the rifle for accuracy and have it anchored better.

Try it with a 9mm or 45 pistol at 20 yards.

This issue has been discussed for years. I am a fan of Remington semi autos (oddly enough). I still hunt a lot with a Model 750 Carbine, also have a 742 1st year production carbine, and have dealt with a ton of 742's and 7400's over the years. This is an ongoing debate about those folks too, and I think it is worse in those that in AR's....

I think it is worse as it is a solid stop, not incorporating the recoil buffer tube prior to locking up.

It also doesn't occur in HK 91/93/94/PSG series guns..... because they do not lock open, by design and choice.

I think it's a slightly different lock up position, and no drag on the underside of the bolt,....

It also is the same effect whether they are shot from a bench, sand bags, Stewart Sight Vise, or LeadSled....

I personally don't think it can be the stop mechanisms, but Like many of you, I have given up worrying about it.