I knew the cause the instant I pulled the trigger, which, btw, felt like somebody had smashed my trigger finger with a hammer. I was afraid to look down, for fear that I'd have half-a-finger. When I did look down, my finger was black. I was afraid to clean the black off, as I just knew I'd see bone. Truth was, my finger wasn't even bruised and it didn't hurt at all.
I made several mistakes, both in the load/bullet weight and just by what was going on while I was shooting. FWIW, I could NEVER get .400 Cor-Bon to feed reliably from double-stack Glock mags. The rounds would nose-dive into the feedramp, usually causing a serious setback. It was hot, I was frustrated, and just let one of those feedramp-jam-setbacks get by me. To exacerbate the problem, I was using a powder that was too fast for .400 Cor-Bon , along with a 180gr. bullet.
After consulting with Peter Pi, Prez of Cor-Bon, I, of course, used a slower powder and cannelured all my bullets for .400 Cor-Bon reloading, roll-crimping into the cannelure.