I shot a SKB 100 12ga IC/M with #4 steel for many years. Lead buck shouldn't hurt it. I shoot NP BBs out of a 150 yr old Damascus barreled 10ga I don't think I'd shoot large buckshot out of it, I do load my own and keep pressures around 6000 psi.
A lot depends on barrel wall thickness and chamber length, lots of post war European guns still had 2.5" chambers. So what I'm saying is a lot depends on the shotgun itself. Quality of manufacture, quality of steel, type of lockup, have the barrels been honed to clean up pits thinning the barrel walls, chambers lengthened to use modern ammo and the list goes on. Even hot blueing sxs barrels can cause deterioration in the solder holding the barrels together. Unbeknown to me one of my favorite duck guns was hot blued before I got it and is now 20 yrs later starting to have the barrel ribs separate.
Good luck with your shotgun