I just purchased my SA XD-40 SC which has two (pasisve) safeties, the grip safety which is released when you grip the weapon and the trigger safety which is released as the first portion of trigger pull. I'm not familiar with the M&P but if I only had the one safey on the trigger I would probably not keep one in the chamber. I feel ok since both safeties have to be released before the weapon goes bang and I don't put my finger on the bang switch until I'm ready to fire. If you have both the grip safety and the trigger safety you should be ok to keep one in the pipe. Don't know if this will help or not?