Article in (I think) American Rifleman about 10 years ago. It was determined that it is not the gas or flame cutting the top strap, it is the powder grains. They act much like the beads in bead blasting.
Easy cure. Take a carpenter's pencil, the ones with the rectangular lead, and rub the topstrap above the bbl. cylinder gap. This will prevent top strap cutting. Works for me. Walt