Ok... bought an AR pistol. Multi cal. Brand new rainier arms complete. 10" barrel. It has a neveske flaming pig on it. Racking the first round, it fires. Does not chamber a follow up round. So I chambered a round, drop the mag and it is short stroking. Not locking back on the last round. Can anyone point me in a starting direction to solve the issue. I've tried 3 different types of ammo, and different mags. Any help would be greatly appreciated.