John Brindle, The Double Gun Journal, Black Powder & Smokeless, Damascus & Steel; Volume 5, Issue 3, 1994, Some Modern Fallacies Part 5:
The pressures produced in the breechloader by black powder, as black powder was actually loaded at the end of the 19th century, were not significantly lower than those given by smokeless (then often termed semi-smokeless) powders developed as a substitute for, and an improvement on, black powder.
From a 1895 study published in The Field for 1 1/8 oz. 3 Dr. Eq. @ 1222 fps, converted to psi with Burrard's formula
72 gr. Curtis & Harvey's No. 2 T.S. (somewhat similar to FFFg) = 3.2 Tons = 9,632 psi
82 Gr C&H No. 4 T.S. (similar to FFg) = 2.26 Tons = 6,474 psi
84 gr. C&H No. 6 T.S. (similar to Fg) = 2.1 Tons = 5,936 psi
1 1/8 oz. 3 Drams (1200 fps) of DuPont FFFg (82 grains) is about 5000 psi
Black Powder substitute pressures from the 2001 Lyman Black Powder Handbook & Loading Manual
1 1/8 oz. with 100 gr. GOEX FFg at 1200 fps = 4,800 psi
1 1/8 oz. with 100 gr. Pyrodex RS at 1200 fps = 5,600 psi
