Drew;
A couple of comments.
I believe that at least some companies when they said "Bored For Nitro powder" were speaking of the bore diameter. It appears many guns in the Black Powder era were made for use with brass shells & had oversize bores for the larger wads. Smokeless loads were put up in paper shells thus the Bored for nitro, meant the bores were more standard for the nominal gauge to fit the smaller wads in paper shells. The fact they were boring Twist & Damascus "For Nitro Powder" dos show they were condoning it's use.
The 2nd comment is that in relation to bore size the 2 3/4-1 in 16ga & 2˝-7/8 in 20ga are both heavier loads than the
3-1 1/8 12ga load. This would account at least in part for the higher pressures recorded in these two gauges.
