Solder melts at 360F. Something is 'too hot to touch' at something like 120F. Nasty burns result at or near boiling point of water (212F). I've not heard of anyone actually being burned by hot barrels, but I'd imagine long strings of BP would get things too hot to be comfortable even with gloves.

Summary: I think the shooter would stop before the barrels would unsolder.

"The price of good shotgunnery is constant practice" - Fred Kimble