We were on the range the other day doing some T&E work, and my partner was shooting his HK 416 from prone from 50 yards. I decided to be a smarty, and stepped up on the line beside him, and shot his target with my M&P .40. I was trying to hit where he was attempting to shoot a group at in an attempt to mess with him. On my second shot, I noted a drastic change in recoil. It wasn’t the feeling of a “hot” or over charged round. It was as if the recoil spring failed. I stopped, stepped to the side, and kicked the round out of the chamber. The center round was the one that came from the chamber. I then inspected the remaining rounds in the magazines, and inspected the recoil spring. The recoil spring had very little resistance to it, and I could pull the slide back nearly 1/4 inch before the spring would even think about engaging. I reassembled the gun, and hand cycled the two rounds on the left and right. I got minor case deformation just hand cycling it.
The spring has about 4500 rounds on it. I ordered a handful of new ones from Brownell’s and took the gun out of service they arrive. Not sure what caused the failure, but it is interesting nonetheless.