It isnt that simple.

The rib on the bottom is usually flat, parallel, and consistent along its entire length. Almost every swamped rib Ive ever seen curves down and then back up as it approaches the muzzle. They usually end in a ramp with a short flat section that mounts the bead. There is usually no bottom rib on a gun with a swamped rib, and, peculiar to Darne guns, seldom a bottom rib on a gun with a game rib, either.

Best,
Ted