At the risk of repeating myself...I will wink because this is probably the most important summary to answer the question "What load should I use for my vintage gun (which has been evaluated by a double gun specialist smith and found to have adequate wall thickness and mechanical & stock integrity)?"

c. 1900: The “standard” U.S. 12 gauge field and inanimate target load was 1 1/4 oz. shot with 3 1/4 Dram Equivalent (1220 fps) of Bulk Smokeless with a modern transducer pressure of 8000 - 9500 psi.
Just before WWI: The “standard” U.S. 12g field and inanimate target load was 1 1/8 oz. shot with 3 Dr. Eq. (1200 fps) Dense Smokeless with a transducer pressure of 8,500 - 10,000 psi.

12g pressures
1 1/8 oz. 3 Drams (82 gr) Curtis & Harvey’s No. 4, T.S. Black Powder (somewhat similar to medium grain 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 8,500 - 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.

1 1/4 oz. 3 1/2 Dr. Eq. BULK Smokeless was about 11,750 psi
1 1/4 oz. 3 1/2 Dr. Eq. DENSE Smokeless was 12,600 psi
Pressure is beyond the modern SAAMI recommendation of 11,500 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.
Up until about WWI the standard 20g load in the U.S. was 7/8 oz. with 2 1/4 Dr. Eq. Bulk or Dense Smokeless powder.
7/8 oz. with 2 1/2 Dr. Eq. had been the standard British 20g load even before the 20g was offered by U.S. makers.

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