Since you have not removed the breech plug, can we assume these are external pits? If so, what is the depth of the one you are most concerned about? It would be nice to at least use a bore scope to examine if there is internal pitting in the same neighborhood. I don't know of any wall thickness gauge that could give you an accurate measurement that far from the muzzles.
I suppose you could make a judgement call then, and do a little "American Proofing" with the gun tied in a tire, very stout load, and long string on the triggers while hiding behind a large tree. But as with any proofing, failure won't do anything for the resale value.
