I've been reloading for many, many years: numerous calibers, bullet casting, the whole bit. Every once in a while I run across something that makes me scratch my head and say "Huh?!"
To be clear- I didn't do this; it isn't mine. I've been finding these on the ground at the range recently. I've seen the "Glock smilie" before, but this is a 10mm case that is bulged, blown out like a balloon all the way around the base. The only thing I can think of is the possibility that someone is shooting 10mm ammo in a blow-back .40 of some kind? That doesn't seem logical, but I can't understand what happened here.