AR is locking back but leaves a single round in the magazine. Ideas why?

[M4Fundi]

My AR is locking back as if it is empty, but it is leaving a single round in the mag. Mag's are two new GI. First I have seen this. I am going to range tomorrow and test with PMags Gen2 and Gen 3 and a few other manufacturers of GI mags to test if it is just those two mags. Thought I'd ask here if someone could nail down the cause before I get there. Thanks!

[omegajb]

Is this a new rifle? Did you build it or was it built at the factory?

I'm guessing here but maybe the magazine is inserting a little higher than it should be. It could be out of spec or the magazine catch needs one more rotation to tighten.

Try other magazines first but if you're still having the same problem use something to press the mag release so the tab sticks out enough on the other side of the receiver and rotate the tab one rotation clockwise.

Also look at the bolt catch, make sure its moving freely and it might need a drop of oil if it's biding up.



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[26 Inf]

My AR is locking back as if it is empty, but it is leaving a single round in the mag. Mag's are two new GI. First I have seen this. I am going to range tomorrow and test with PMags Gen2 and Gen 3 and a few other manufacturers of GI mags to test if it is just those two mags. Thought I'd ask here if someone could nail down the cause before I get there. Thanks!

Is this something that just started happening, or are these the first mags fired through the rifle?

You may be slightly out of spec in the lower, or, less likely IMO, the mag catch itself

On my rifles, with one round in the magazine in my rifles the follower is either touching the bolt-catch, or just a hair from touching.

So, I'd suggest shotgunning your rifle and using a magazine with one round to check if the follower is raising the bolt-catch.


So a

[tlw613sp]

Is this something that just started happening, or are these the first mags fired through the rifle?

You may be slightly out of spec in the lower, or, less likely IMO, the mag catch itself

On my rifles, with one round in the magazine in my rifles the follower is either touching the bolt-catch, or just a hair from touching.

So, I'd suggest shotgunning your rifle and using a magazine with one round to check if the follower is raising the bolt-catch.


So a

This ^^^. Although ideally with a dummy round. You know, for safety and all.

[556BlackRifle]

I agree with the others. Check the bolt catch engagement. Possibly a weak, broken or jammed up spring.