It is 100% 9mm. Shot at a remote location where there was no other brass. I picked up all of my brass. 40 S&W would not fit on. 9mm shell plate, which this one was processed on - Lee LM 5-station progressive press. Lots of opinions on OOB, which I believe is a very likely scenario. I have confirmed my CZ P10c will drop the striker about 1/8” out of battery. Either way, also suspect it was an overcharged case, based on what happened to the case and the felt recoil, which was tremendous.