Existing proof law as I understand used the same criteria for proofing smokeless as Black, IE a given % of overload was required. This resulted i unnecessarily high pressures for smokeless. Many of those which were Nitro Proofed specified a very light load of smokeless. I do have an 8x58/16/16 drilling proofed in 1907 which has all 3 barrels With the N mark.
Early 9.3x72R guns were black powder proofed only, The load was increased at a later date & they were then smokeless proofed. I have a hammer drilling in 9.3x72R/16 by Miller & Val Greiss which was proofed in 1913. All three of these barrels were also Nitro proofed. Supposedly the rims of the smokeless cartridges were thicker so they would not chamber in the black powder proofed guns. I cannot confirm this as I only have a few smokeless cases.
