Fixed it

If it happens with many different types of magazines, it sounds like the magazine follower is activating the bolt release early. You can verify this by taking the upper off and checking how the bolt release functions as loaded / unloaded magazines are inserted in the lower.

It could be caused by an out-of-spec bolt release, magazine release, lower, or a combination of all three.
 
I do believe that others have previously reported out of spec bolt releases that had a little too much material, resulting in the follower prematurely pushing it into a position to lock the bolt back.

If you're feeling adventurous you could try swapping out your bolt release for a generic one, such as DPMS, or you could try modifying your current one. Maybe 5 minutes with calipers and a file could fix it.
 
Don't know if this is what is causing your problem but I have seen it before in a buddies AR.

The hole for the bolt catch plunger somehow got peened over (holding the plunger below flush), so the bolt release was no longer spring loaded, and would just flip back and forth under recoil. When it got to the last round, I guess the follower is high enough that the bolt catch bounces of of it and catches he bolt.

Try checking that.
 
If it happens with many different types of magazines, it sounds like the magazine follower is activating the bolt release early. You can verify this by taking the upper off and checking how the bolt release functions as loaded / unloaded magazines are inserted in the lower.

It could be caused by an out-of-spec bolt release, magazine release, lower, or a combination of all three.

ya that check mag release first since its always the last round.
 
Do you have a BAD lever on it? Did you just add anything to the rifle?
When it locks back with a round still in the mag does it stay locked back if you remove the magazine? or does it slam closed as soon as you remove the mag.

Need more info to be able to troubleshoot without seeing the rifle.
 
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