Question for AR experts

[capp325]

Has anyone encountered the following malfunction:

With one or more rounds remaining in the magazine, the bolt locks back. The round that was supposed to be fed gets caught underneath the feedramp. The round is still partially in the magazine but has moved forward about halfway. The bullet is pushed back into the cartridge.

What could be causing this?

 

[bear.23]

A weak mag spring.. The mag doesn't push the last round up high enough to clear the mag.. the bolt pushes the round into the mag and crushes it but and the bolt bounces.. I can't see how the bolt locks back unless it's a really lousy mag

 

[foxbat]

What kind of ammo are you using?
Hollow point ammo has a habit of hanging up on the feedramp, pushing the bullet into the case, and then either hanging up and causing a jam, or worse,
feeding into the chamber and causing a KABOOM because of an overpressure situation.

 

[capp325]

I was using Winchester (45 and 65gr), Remington UMC, and some other Remington round.