Most likely the cheep UMC 180 gr 10mm ammo. Started the test with 50 rounds. First 20 were in my new Arredondo +5, next 30 were 2 stock mags. First failure, did not fire. Dropped the mag, and cleared the weapon. No dent in the primer. 8 rounds later, another FTF. Nice dent in the primer. Finished the mag. Changed mags, 30 rounds flawless...WTF???