1st of all, gun weight has nothing to do with power.
; If you are referring to the 12 bore with the badly pitted breech faces and firing pin holes, I'd pass.
; empirevr - Both are handloading propositions - are you going to now learn how to handload?
: If in reasonable shape, the .577 would be the one to get. If it's bores are no better than the 12, I'd pass on both.
: You need a bore condition that can be rectified to reasonably smoth and shiny it's full length to shoot lead bullets. Bores with rings of pitting, as some you've sent me are toast. Pitted bore that won't shoot lead, will sometimes still shoot jacketed bullets, however there are no jacketed bullets for 12 bores and none that I'm aware of for the BP .577's.
; You may be able to shoot undersized balls or bullets in the 12 bore, if using shot wads to protect the bullet from the rough bore, but the bore will plastic foul form abrasion with the bore. Also, the BP charge will melt plastic wads ans spray the insides with melted plastic. Been there, done that. Fibre wads can insulate plastic wads from the BP, but not stop abrasion fouling.
: As to loading data, use as much BP as will fill the case, minus the bullet or ball and wads.
; Black powder + the wads makes up 100% of the case capacity. It must be a slighlty compressed charge for the BP to burn properly and safely.