I seriously doubt you should have any concerns about safety. Most metal expands over 40 times its size in rust. even If the barrel was fully clogged in rust and rebored the actual amount of metal lost is very thin. I know of several people who have taken a rusty old barrel, rebored and rifled it to the next larger caliber, and continued on without a hitch. And Ive even owned some muzzleloaders (bought cheap after whoever owned them and didnt know about black powder, probably used them for the first season and never cleaned them pulled them back out of the closet and saw it was all rusty) Many guys pick them up cheap, and with a little TLC bring back a decent gun. Or even a fine entry gun for a child or a new to the sport friend. I still have a sidelock or 2 that has definite pitting in the bore, especially near the breech, where a load may have sat. you can feel it while loading. BUT they still shoot true, and I consider them safe. One even comes to mind, purchased at a local flea market that was still carrying a load. It took the most time and effort to dislodge the rusted in load than the gun cleanup almost. I went through several different methods and techniques to finally clean out the chamber, but have another shooter for less than a fraction of its possible worth.