The bulge around this case seems to be mostly symetrical. Pistol chambers are not, with a barrel hood on top and a feed ramp on the bottom. The only cases I've seen bulges this way are out of a blow-back operated subgun, Uzi and Mac to be specific.
Based entirely on this one pic, and the handguns you were shooting, I don't think this one is yours.
Well, there's no way it isn't. Like I said, private land, fellow shooters had same type firearms but I was the only one shooting this headstamp. No one with access to land has any of the mentioned blow-back style guns, except for my sub2000. Looking at it's chamber it's most likely the culprit.