If you should use the boiling water rust process I described,plug the bore and chamber.
If you plug BOTH the bore and chamber, the plugs may blow out when the barrel gets hot and the trapped air expands.
From Brownells' instructions for Diropan IM:
"3) MAKE A CAREFULLY FITTED, LONG WOODEN PLUG - to fit one
of the barrel, or design some other type of holding mechanism that
is just as effective to avoid touching the barrels during the bluing operation.
We use two plugs on double barrels with one plug in each barrel,
at opposite ends, so neither barrel is plugged at both ends
.We also use
a soft, black iron wire loop through parts and barrels so we can hang
them easily in the tanks." [emphasis added]