Just guessing, but I'd think slag from welding or inclusions in the original material might show as circumferential elongated forms as that's the direction that the steel was drawn while being forge and welded.

If those roundish spots are pitting, I'd think they'd be easy to see. Maybe if the barrels look smooth, they could end up being possible hidden defects. Might be tough to tell though exactly where those spots are, and the bulge picture doesn't seem to have any of those funny looking spots around it.