Just my HO, but I think Mike is correct in this! Giving the barrels are thinner metal, they would contract faster than the breach, thus breaking the grip of the rust and gunk, using the freeze method.
Heat cuases metal to expand, thus closing cracks. If the barrels where off of the breach, then heat would show the cracks, but they are currently constricted buy the breach, they have no place to expand too, so any cracks would close!
When welding engine blocks, each pass with the welder required cool down time before the next pass, or the block would cool and split the weld.