ORIGINAL: cayugad
Having Damascus twist barrels I would be real careful about loading it. Personally I would not shoot it. It would be a wall decoration. Granted I am guessing you are sure of the integrity of the firearm. So you want to start with light loads and work up from there. The old rule of thumb used to be equal powder and shot loads. I am still testing that theory. I shoot 80 grains of APP 2f in my 12 gauge cap lock. I use an over the powder card, then a fiber wad, then 90 grains of #5 and finally an over the shot card.
The only way to find a good tight load is to experiment with different loads and different set ups. I have shot the plastic wad cups in mine and they also worked good. I am still testing different things with mine, but feel it good out to 30 yards.
Is your gun one with Damascus barrels or solid? Are you referring to 90 gr.
by volume of #5 shot? Dixie Gun Works catalog gives information about using both 1 1/8 and 1 1/4 oz. of shot in the 12 ga., without reference to shot size or volume. How does that shot weight relate to volume? I suppose I can answer that last question by measuring and then weighing the shot or vice versa. I have plenty of over the powder cards, fiber wads and shot cards from the prior owner. The gun seems quite sound and has been shot with up to 70 gr. of FFg black powder that I know of. Thanks.