Drew;
I may be all wet here, but I have "My" interpretation of this "Bored for Black and Nitro Powders". First off this same statement was used by several different makers. I believe the time it is most often seen was actually prior to the general "Condemnation" of Twist/Damascus barrels so do not think it truly was addressing the strength, it was assumed either type of barrels would be used with both types of powders.
What I believe it was referring to, & this is somewhat borne out in Lefever Arms Co catalogs via their recommendation of wads. Black Powder was quite often loaded into brass shells, hence bores were often oversize for the larger diameter wads.
Shells loaded with Smokeless were generally loaded in paper cases which used regular sized wads. I believe what Ithaca, Lefever & others were saying was they were now boting their barrels much closer to nominal size for use with the paper shells loaded with smokeless powders, but they were still suitable for use with Black.
The actual size of the bore had little to do with the barrel's strength.
