Well, I now know that 50 primers going off at once sounds like. Running a Dillon 650 for 45 acp, and midway into my run of 100 the press went off in my face, launching the low primer sensor rod into the ceiling, and blowing the whole steel blast shield completely out of the primer station mount and across the room. I caught a tiny bit of shrapnel in my right arm, but otherwise injury free - not even ringing in my ears surprisingly.
I completely disassembled the priming station and could not find an obvious cause, just alot of shrapnel and black soot. Cleaned everything up, and no parts were damaged on the press itself. Cant say as much for the primer tube though, I had to drive it out of the blast shield with a drift punch - looks like every primer in the tube went off. I'm thinking a primer got caught half way into the index wheel and got pinched as it rotated, setting it off. The blast shield did its job though, and I pressed it back into the station mount. The aluminum primer tube is split the whole bottom 6 inches.