I've added quite a bit of information to this document regarding turn-of-the-century (and a bit later) loads
https://docs.google.com/document/d/1...UOZEFU/preview
And this pressure data might be helpful in choosing a load ballistically similar to that for which your gun was intended
12g
1 1/8 oz. 3 Drams (83 gr) Curtis & Harvey’s No. 4, T.S. Black Powder (similar to FFg) was about 6500 psi.
For comparison, 1 1/8 oz. 3 Drams (1200 fps) of DuPont FFFg Black Powder (82 grains) is about 5000 psi.
1 1/8 oz. 3 Dram Equivalent of BULK Smokeless was 6500 - 7500 psi.
1 1/8 oz. 3 Dr. Eq. of DENSE Smokeless was 9000 - 10,000 psi.
1 1/8 oz. 3 1/4 Dr. Eq. BULK Smokeless was about 8500 psi.
1 1/8 oz. 3 1/4 Dr. Eq. DENSE Smokeless was 9500 - 10,500 psi.
16g 1 oz. 2 3/4 Dr. Eq. BULK Smokeless was about 7000 psi.
20g 7/8 oz. 2 1/2 Dr. Eq. BULK Smokeless was 8000-9000 psi.
7/8 oz. 2 1/2 Dr. Eq. DENSE Smokeless was about 11,000 psi.
Note that the pressures for Dense powder are very similar to today's loads, which is not to say that pressure is safe in YOUR gun (sorry for the disclaimer).
