If that is indeed inline with visible exterior ribband welds then perhaps it was that way from day one but I would guess it more likely that there was an unwelded inclusion in the original weld which subsequently corroded and eroded to the present form. You likely did the safest thing.
The experiments ive done with helical welding usually look just like that in places on the outside of the barrel but when ground down the unwelded portion is removed. The welds in that barrel could yet be sound but its hard to say.
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