Bcg not holding back on last round.

[JohnnyEgo]

Just to be sure, you should swap bolts / carriers and see if it still happens.
I had a similar style of malfunction with a bad carrier key once.

[coloradocad]

I had an overly tight hammer spring that I had to replace because it wouldn't allow the bolt carrier to go back far enough. You could try switching that out and see if it helps. I have also heard about cheap Chinese buffer tubes that are too short and don't allow enough travel or too much play. Have you changed any of your Springs or buffer tubes?

[colorider]

Have not changed anything. Seems to only happen on the 20 round pmags though. It cycles flawlessly, and once in a while it will hold back on the last round with the 20 round pmags. Not a big enough deal for me to start buying another tube or springs. If I get time and the ambition, I will give it a shot.

[ben4372]

This happen to me once on a bushmaster M17s. Same mag setup. The bolt was just barely catching. The catch wasn't able to fully pivot to catch the bolt, needed a bit of filling on the bolt catch. Is it also possible the upper is looser than before and the catch isn't engaging fully. Insert a mag and see how much engagement you get.

[colorider]

Upper and lower have a fantastic fit. The catch engages fully and solid when it's not,the last round of. 20 rd pmag. I am going to test some 30s next week.

[ben4372]

You might look at the followers in a 20 verses a 30 and see how high the followers come up. I had to trim a Desert Eagle follower once so it could come up a little further. Sounds like you figured its a magazine problem, thats easier to fix than some things.

[Ed@DarkStorm]

I had a similiar problem on a batch of new PMAG 20's I got a few months ago. Interestingly, as I was troubleshooting, it went away. I thnik something might just have needed to be broken in on them, even though that makes no sense. I've shot thousands of rounds through them since and not one failure to latch since that first week.

[YammyMonkey]

What are you using for a buffer? I tried running a 9mm buffer in a mid length gun & that combo would barely engage the bolt stop. The same thing could happen with a stronger spring as well. I think the LMTs have a gas port size on the small end the spectrum which would compound things too.