the gunpowder in the blank cartridge ignited a bullet fragment that became embedded in the barrel
From other reports, somehow there was a real bullet left in the barrel (from missfire or something??). Then when they put in the blank, the real bullet (or fragment) was pushed out of the barrel by the gases and wad from the blank.
I have heard of this when folks use bad ammo and the round doesn’t have enough energy to push the bullet all the way out of the barrel. Although I always thought then what happens is you blow out your barrel or the receiver?? (Although perhaps that is because you are shooting a bullet into a stuck bullet, and not just gases and a wad???)
(SMH)