Would almost had to have been caused by a squib load - or a reload without powder plugging the barrel - followed by firing another cartridge. That has nearly happened to me several times while firing revolvers, and is exactly why I never shoot rapid DA fire with a revolver. I have read that the old Thompson SMGs were very prone to getting bulged barrels. A shooter would get a squib load that lodged a bullet in the barrel and perform immediate action, causing a barrel bulge.