In early Lefever Arms Co catalogs it was recommended for black powder use to load wads two sizes larger than the gauge. As best as I can determone this was speaking of actual wad diameter. By the time i started reloading just prior to the plastic age a card wad in a box marked 12 gauge actually measured to about an 11 ga. While those marked 11 for use in the brass shells measured to 10ga dia's. This would have been the wads efever referred to as "Two" sizes larger than the bore gauge. Many 12ga Lefevers of that era do have bores which run large in the 11 ga area. These same catalogs advised if getting the gun for smokeless to order it "bored" for standard wads only one size larger than gauge.
I can only conclude the term "Bored for Smokeless" has reference to the bore diameter & not the wall thickness. I think by this time regardless of the "Bore" most US makers were using suitable wall thickness over the chambers to handle the smokeless loads of the era. You won't find many guns from the 1890's or later having very light breeches to their bbls.
