This reminds me of a couple of guys I knew back in the late 1980's/early-1990's.... They bought a Lee 1000 progressive reloader, and it had a BAD habit of either double, or triple charging cases! Actually, it ONLY did this once. ...and that one batch had a double (or triple) charge in it. These were .357 Mags they were loading, and they blew up a REALLY nice Colt Trooper! It was a classic back strap blowup! Fortunately, the shooter wasn't injured (just a sore hand).
I am guessing you really meant "topstrap". If the backstrap blew, then I can see why his hand was sore.