You very well might be right about the bolt. After this many years and the shock it is kind of a blur but your explanation is the most logical that I have read.
The cartridge was in multiple pieces. A piece of the rim ended up embedded in my breastbone and had to be surgically removed.
Many years ago I had to investigate the out-of-battery firing of a Browning .50 caliber machinegun aboard a Navy minesweeper. The Gunner's Mate didn't know squat about the fifty and misheadspaced the puppy (by about 1/2 inch!). A junior seaman was the first up to fire it from a pedestal mount - you'll note that the Gunner's Mate in question didn't test fire it himself - and it blew up, blasting brass fragments all through the poor sailor's groin and upper thighs. He would have bled to death if they hadn't been near a Soviet spy ship out there in the middle Atlantic and the Soviets had a decent surgeon aboard at the time. I had a lot of fun with that investigation!
Firearms are inherently dangerous at both ends - I am very glad you made it..
Semper Fi, Chainmail