Son and I ran out to Birchwood tonight to do some shooting. Grabbed my S&W M&P .45 and some factory ammo that was laying around so we could entertain ourself as our rifle barrels cooled down.
After about 50 rounds through the .45 I heard a dull "pop". This alarmed me so I pulled back the slide and noticed the case was still stuck in the chamber. Luckily I always carry a .22 cleaning rod so I inserted the rod in the muzzle and popped out the case. What I found was a 40 S&W case. The bottom third of the case looked normal but the top 2/3rds expanded and split to fit the .45 chamber. To be honest I was shocked it even fired.
Have no idea where the .40 round came from. I do have a Glock .40 but it hadnt been to the range in years. Guess I will look a little closer when I am loading the magazines in the future. First time in my life I ever fired incorrect ammo in any weapon.
Might add I have run about 500 rounds thru this pistol in the last couple of months. All bullet weights, shapes and sizes. Not one failure to feed or eject until the .40 round. Super reliable pistol and very accurate to boot.