None until the barrels have been carefully evaluated; which takes the time, interest, expertise, and tools to be done properly. Really.
I have witnessed a dealer running a wall thickness gauge quickly down the bottom (usually the thickest part) of a barrel, declaring the MWT is ___ and the barrels are "fine for light loads". That is meaningless.
As a guide, 1 1/8 oz. 3 Drams (83 gr) Curtis & Harvey’s No. 4, T.S. Black Powder (similar to FFg) was about 6500 psi.
1 1/8 oz. 3 Drams (1200 fps) of DuPont FFFg Black Powder (82 grains) is about 5000 psi.
