Most barrels that I have measured for myself and a few for others seemed to be thinner near the ribs and I believe that is from setting barrels for POI (point of impact)

I have a W.W. Greener circa 1892 that at the muzzle, inside left and right had to be filed and there is no land.

Have to find another site for uploading pictures, Photobucket now wants me to pay for 3rd party posting.
Photob

David