I bought a nearly new M&P 9 in 2014. It has been a good gun, but I have a question. I bought about 20 spare factory mags for it not too long after I got it. I usually keep three loaded. The same three have been loaded to capacity for the last four years or so. Last night I switched them out, and took the three that I had been using (empty) and put each of them in the gun and pulled the slide back. When doing it fast, I noticed that they all locked back, but one of them just barely lifted the slide stop into the recess in the frame (not as far up as it should go.) When I took that same magazine and pulled the slide back slowly, the follower wouldn't even come up and lock the slide back. Am I leaving these things loaded too long? Never have experienced this with other handguns. At any rate, I downloaded the three mags that I switched out from 17 to 15 to give the spring a little less stress. I routinely download 30 round rifle mags to 28 to make them easier to insert under a closed bolt, but never have had to do it to pistol magazines. Anybody else encounted a similar problem?