Here is a seller who uses the phrase "swamped rib" in connection with a Darne. You can decide whether he meant swamped left to right or muzzle to breech.

My Darne is swamped and concave in both directions. A handy pair of Ithaca/SKB 100s shows one of them to be strongly swamped from muzzle to breech, but the other flat. My Cashmore is also swamped, bow to stern as well as port to starboard.

Interestingly, all but one of the guns that were handy this morning have swamped barrels. That includes the two Ithaca/SKBs, OShatz, and Darne. The Cashmore barrel was not swamped at all.

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BrentD, (Professor - just for Stan)
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[Linked Image from i.imgur.com]