my dad was gifted an old army and navy sxs 12 bore a few year back. the inside of the barrels were a little pitted but not to bad. after a couple of shots he tipped the gun up and it sounded like a rain stick from all the rust that had shaken loose inbetween the barrels. was a little hesitant to use it after that
Clean the rust of down to the bare metal and then measure the depth of the pits.
Post the depth of the pits and the thickness of the barrel at that point (a luthier or engineering workshop might be able to help), then ask us if it is safe.