[m&p45acp10+1]

Are you using a decapping pin die, or the sizing/decap die?

I have had no problems with decapping it with a standard sizing/decap die. Some are crimped primer pockets. You will notice when priming if they are. It will feel like you are trying to push a bowling ball through the tail pipe of your car if it is crimped. If so put the brass to the side for removing the primer pocket crimp. Lyman makes a hand tool for it that cost less than $20 and takes a few seconds. Or RCBS makes primer pocket swage that goes on the press and presses a rod to remove the crimp.