Originally Posted by
jonconsiglio
I'd put my money on the angle. When the bullet passes the gas port, the pressure builds. When the bullet exits the barrel, the pressure is released. The carrier does not start to move backwards until the bullet has left the barrel, so it's inertia (I think that's the word I'm looking for) at that point. I've fired my rifle in some odd places and never had this issue. I have heard of others having failures to lock back when firing down at extreme angles.
The buffer and ammo will play a large part in this as well. 223 with an H2 will be more prone to this than 5.56 with an H buffer, for example.
I could be wrong, but I can't see this being related to the hole itself. I've taped off my muzzle and used caps before without issue. Everything that needs to happen is happening before the bullet exits the barrel.
BUT, the OP stated it shoots fine at that angle otherwise, so maybe I'm just plain wrong here....