My M&P 40 Pro does it. At first I thought there was something wrong with it. Called Smith they said to send it in. I then decided to load some mags up with practice rounds and try in in my shop. I found that when you slammed the mags in at slide lock they usually go forward, both in a Glock and an M&P. I tried several different guns they all did it. It is my believe that the frames flex on the the polimer guns and the slides goes forward. Best to avoid slide lock.