Bolt not locking back... need your help!

AR/M4 AR Technical Discussion
1

I’ve searched and found several symptoms of bolts not locking back, but none seem to apply. I probably have only 250 rounds of 223/5.56 through my AR, and probably 1200 through my CMMG 22 conversion kit (I’d shoot some 223 through it after the kit to keep the gas tube clean.) Here’s what’s happening…

Yesterday I was shooting American Eagle 223, and the first magazine locked back ok. From then on, nothing would lock back… I thought maybe the magazine had a weak spring or bad follower, but none of the other magazines would lock back either. I thought maybe it was weak ammo, so I tried some Winchester 5.56 (which I’ve never had an issue with) and it wouldn’t lock back either. Whenever I’d pull the charging handle back on an empty magazine, it would lock back.

This made me think maybe it was a gas issue, so I cleaned the gas tube the best I could in case it was short stroking. I took it out today and still had issues. If I manually pushed the bolt catch up when shooting, that would lock it back (so it wasn’t the bolt catch, and it wasn’t short stroking). I tried a friend’s magazine, and a couple others of mine (all are pretty new PMAGS), and none fixed the problem. I then put my upper on his lower (to make sure it wasn’t my bolt catch), and it still wouldn’t lock back. However, I noticed if I put a little upward pressure on the magazine, that would seem to engage the bolt catch enough to lock it back on an empty magazine.

Does anyone have any idea what is happening here and why? I’ve done as much troubleshooting as I know how, but I’m at a loss. Your help is appreciated!

2

What make/model is the rifle in question?

3

Failure to lock back can basically four things.

  1. Bad mag (weak mag spring or bad follower).
  2. Short cycling (not enough gas or weak ammo).
  3. Bad bolt catch.
  4. Overcycling where even a good mag can’t keep up with bolt velocity.
4

Just to be clear, youve had 250 rounds of 223/556 prior without any problems locking back?
Have you tried another upper on your lower?

However, I noticed if I put a little upward pressure on the magazine, that would seem to engage the bolt catch enough to lock it back on an empty magazine.

Was this on your lower or his?

5

The first thing I want to know is who makes the weapon. If it was pieced together then specifically the upper and lower as well as the sum of the parts used to build it.

6

Double Star lower with DPMS LPK, CMMG 16" midlength upper.

Correct, no previous problems with it locking back. I did not try his upper on my lower (realized later I should have tried it.) When I put upward pressure on the magazine, I had gone back to my lower and hadn’t tried it with his.

  1. Tried 4 different magazines total. Two of mine have few rounds through it, one was my friends literally brand new one.
  2. I used hotter 5.56 and still happened… not to mention when I manually engaged the bolt catch it would work, so the bolt carrier was in fact getting back far enough (which seems like it’s got enough gas).
  3. Friend’s lower/bolt catch wasn’t locking back either
  4. This sounds possible. I wish I would have tried his upper on my lower.
7

It’s most likely number 2. Could be a lose fitting or lose gas block and or carrier key. Some carrier keys and gas blocks feel tight but in fact leak.

8

Hmm… any way to check that? I may just have to get ahold of my friend’s upper and swap BCGs and see if that affects it, or try my BCG is his upper and see if there’s any issue there and try to narrow it down.

9

A few thoughts.

  1. Ask CMMG what size the port is.

  2. Look for leakage around the FSB and where the tube meets it.

  3. Try and see if the carrier keys will move when you try and tighten them.

  4. Try another BCG and also see if you can locate an H or H2 buffer.

Does it fail to lock back on all magazines? Including one that your friend has? Do they lock back in his weapon?

10

Recently i was changing the buffer to a heavier one in my LMT lower, just to play with the cycle rate and so on. Testing to see if it would indeed lock back on only one round fired at a time, it would not. I had 20 plus new P-mags that i was also testing. It would not lock back. suspected the buffer i was trying, but actually turned out to be the P-mags. I wound up having to file a little excess material off the rear of the mags on about 18 out of 20 i had. They work just fine now. So dont discount the magazines as the problem. Just have to check all things and work thru it…

11

If is was locking to the rear, then stopped locking to the rear the carrier key is immediately suspect.

Try a known good carrier.

12

.

That is about as low as it gets.

If you get the rifle fixed, I suggest selling it and buying something else.

You will continue to have problems with this gun after you get this issue solved.

13

Here’s an update (still haven’t resolved the issue)

I borrowed my friend’s AR (Double Star lower, CMMG LPK, BCM midlength upper), and his would not lock back either with American Eagle. I got some Federal XM193 5.56, and this would not lock back either gun. Again, applying slight upward pressure to the magazine would lock it back every time on both guns. I tried 10 different PMAGS and all have the same results.

I really have no clue what it is at this point… either two (nearly brand new) guns are having issues, or 10 PMAGS have gone bad… both are hard to believe.

Note to self: never ask scottryan anything because he’s clearly an elitist who contributes nothing to the actual question asked.

14

This sounds exactly like what I’m going through… I will take a close look. Thanks!

15

Here is what I want to know. Did you put his upper on your lower? If you did and it still didn’t work, then I suspect your lower is out of spec.

I also recommend that you try some different mags just in case. In order to isolate the problem you need to get an AR that is known to function. Then you can swap items over, like the BCG, lower, etc…

In addition you really need to try and make your post(s) clearer.

For example- You could say;

I took the upper from my friends AR and placed it onto my lower. Using xxx ammo and xxx magazines weapon would still not lock back.

I then took my upper and placed it onto xxxx lower and using xxx ammo and mags the following happened.

The same would go for using the BC from each weapon. This will help to isolate the problem.

It makes it easier for use to follow what’s going on.

I know that I am going to hurt some feelings here, but I suspect that the problem is a combination of parts that make up the Franken AR.

I know that will sound “elitist” but I have put together alot of AR’s and have never experienced the kind of stuff I read here on an almost daily basis.

16

I had the same thing happen on 4 of 5 Pmags this weekend on a 6920. The last round wouldn’t lock the BCG to the rear. I planned on shooting 10 mags that range session. The 5 USGI type D&H mags I brought along, all locked to the rear. Guess it’s time to retire those 4 mags, which are about 3 years old.

17

I struggled with the same problem. Todd has the most likely answer. But if swapping out the bolt carrier with one known to function properly doesn’t solve the problem, check the gas block for proper sealing and especially proper alignment. Misalignment due to improperly drilled FSB taper pin holes (by an incompetent third party) in an otherwise perfectly good Noveske barrel turned out to be my problem.

18

I might have missed it, but have you tried mags that were NOT PMAGs?

I no longer own any because I’ve had numerous that would not hold the bolt open on the last round (new mags), as well as other issues that this thread isn’t about.

If you have not yet done so, try a standard USGI mag that is known to work in other guns. Eliminate the mags as a variable, if you haven’t done so already.

19

The magazines are the only variable I have not eliminated. I will try to pick up other mags today and see the results. After hearing other people have had this issue with PMAGS, it’s starting to sound like they are in fact the issue. I’ll post results after I can pick up a couple different mags (hopefully I can track down some USGI ones locally today).

20

FYI- I have about 100 PMAG’s and they work in all of my AR’s. So I am not so sure it’s the magazine per se.