Originally Posted by
Joe Mamma
Even though it sounds overgased, I'm just not convinced it is yet. Of course being "overgased" is really just a word and relative to other things with a gun . . .
I think you may have a mag follower/spring issue. Maybe the follower is binding slightly on the way up for the last round and/or the spring is weak or something like that. Even if your PMAG does not have any defects, using any gen 3 PMAG could be an issue with your gun as montrala discussed above. I don't use PMAGs, so I can't be more helpful/specific on this issue.
I may have missed it, but have you used more than one PMAG? It sounds like you haven't. I would try another (same generation) just in case. That should be cheap and easy to do. You might just have a bad PMAG.
As you and Iraqgunz mentioned, the bolt carrier swap should have no effect.
By the way, I never pay too much attention to ejection patterns. But that's just me.
Joe Mamma