If the question is will a 40SW mag load more than 10 rounds of 9mm and feed properly: I accidentally loaded my 10 round 40SW mag with 9mm (only 10 of course), and accidentally loaded it in my P99-9. I racked the slide and fired the pistol. The first round fired, but the second round didn't feed properly. I pulled the mag and realized that I had loaded the wrong one. I have since marked my 9mm mags with green on the base to go with my military P99-9mm to avoid this issue. I specifically purchased the 9 in green and the 40 in black to help me with my usual ineptitudness.