I live in Colorado and have antelope hunted in area 68 in WY for 20+ years. It is about halfway between Rawlins and Casper, near Pathfinder Res. The "old timers" in the area have always said that the occasional steel .50 cal cases found on the ground were from fighter pilots, out of Cheyenne, doing machine gun practice during WWII and into the early Korea years, by chasing coyotes and antelope and strafing them. The headstamps I am aware of are all from the early 40's which would seem to support that. And a couple of years ago I had my brother out there hunting and he even found a few links that held the rounds together in the belt. It was a different time, and the way those ranchers feel about antelope and coyotes I never have had any reason to disbelieve them.