I recently came across some faulty ammo that wouldn't fit all the way into the chamber, and would jam! The diameter of the casing got slightly bigger down towards the shoulder of the casing... the round was only going about 2/3 - 3/4 way into chamber and getting stuck!!
It was Armscor FMJ ammo!
I'm not suggesting that your problem is the same... only sharing a recent experience of mine.