I had this same problem for a bit on and off with my t97. The rounds would push the bullet back into the brass. It happened mostly with the factory mag but still sometimes with gen2 pmag and STANAG mags of all types, usgi, steel, Aluminium etc.. Also 4 different types of ammo. I narrowed it down to the feed ramps and emery clothed the hell out of where the bullet vears off the ramp when feeding. Have not had any problems since except very seldom still with the mag it came with, its too sloppy and wiggles too much changing the angle at which the bullet feeds and jamming it above your chamber instead of feeding properly in it. You will see marks/wear on your feed ramps where the rounds are hitting to be chambered, emery cloth in those spots. I guess like someone else said if you don't want to F with it you can always return it in the end, up to you.
