The gunsmith will need to examine the bores for pitting with a bore scope, and measure the wall thickness from breech to muzzle. If he can't do that, you need another smith.
Unfortunately, as part of 'cleaning up' the barrels for blueing the barrels are often honed to remove pits, and the exterior surfaces draw filed to remove corrosion, leaving unsafe wall thickness.

Please see
https://www.shotgunworld.com/threads/is-my-gun-safe-what-load-was-my-gun-designed-to-shoot.366087/