I did get a chance to test a little. Not enough ammo to test as much as I would like. 4, 30 rd Promags fired. 1 failure to feed. Soon as I can afford more ammo I will test more. And I was right about one thing, the plastic mags have a smaller inside length requiring a shorter overall cartridge length. Factory 55 gr and 62 gr 5.56 FMJ seemed to work best.