Bcg not holding back on last round.

[colorider]

All of a sudden it won't hold back on last round. Have to use charge handle. Same mag. Magpul 20rnd. LMT bcg. Psa mid upper.
Lubed like normal. Clean. . Cycles like a dream. Same ammo used. AE 55 gr.
Any tips. Did not try a dif mag. This one only has 40 r ds thru it.
Just started happening this trip out

[TheBelly]

can you force this to happen? Try loading just one round into the rifle and seeing if it wasn't just a weak round.

If the ammo is weak, and mix that with a middy upper, it might just be that it's short stroking enough to pull a round, but not to lock it back.

[colorider]

Does it with just 1 round as well as with a full mag. Just started happening and did on every mag finish.
Same ammo I always use and never had a problem before. Hummmmmm

[DeusExMachina]

If NOTHING has changed from when it was working before, it's either bad gas rings, loose/damaged gas key, or weak ammo.

Older LMTs had MIM gas keys, so inspect and make sure it's not damaged.

[dan512]

I am a simple things first kind of guy, try a different mag.

[kwando]

I am a simple things first kind of guy, try a different mag.

I agree with this... Take the lower off and see if with an empty mag it presses the bolt catch upwards

[DeusExMachina]

Just as simple is to remove the BCG from the upper, extend the bolt, and set it down on a table/counter. If the BCG drops onto the bolt, bad rings.

[colorider]

Thanks fellers. Will do that when I get home and inspect.

[Sharpienads]

I've read about some Magpul 20 rounders not pushing the bolt catch:

http://www.ar15.com/archive/topic.html?b=2&f=124&t=213466

[colorider]

I got a chance to run a different 20 rnd pmag in my gun. Had the same problem, but only 2 out of 3 times. So a bit better then the other. Did not work with my LMT upper either. Did not catch on the last shot. Checked everything mentioned above. Bolt is nice and snug in the carrier, but not too snug. Will not slip when set on table. Can move it back and forth with little effort by hand. Feels just like when I got the gun. Cleaned every bit of the bolt, carrier, fireing pin, upper, lower. Even took out the buffer spring and buffer. Cleaned in there and lightly lubed in the tube. There was some gunk in there, so maybe that has something to do with it. Also noticed some crud in the bcg where there bolt gas rings go back and forth.. Cleaned that up really nice and lubed using clp on a patch. No excess in there. The follower is cleanly lifting up the bolt catch. No binding or anything. Goes up really fast and smooth.
After reading that long thread on ar15.com, I am starting to think it may be a mag problem with the 20 rounders. Not much I can do about that now. Not like Magpul is going to send me replacements at the moment. Not really a big deal as I am not using this as a life saving device. Simply plinking around and shooting from the bench with the 20s.
Thanks for all the suggestions and tips. I will report back if I find the solution or see if the clean and lube job fixed it.

[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.