You are probably looking at a long tedious job in getting it to shoot well. My experience with old Marlins is that the bores tend to be oversize and worn to boot. I would slug the bore and measure it, then try to buy cast bullets sized to fit. Accurate 5744 powder is a good place to start as a smokless replacement in blackpowder cartridges. The bullets will be 255 grain flat nose anywhere from .375 to .381. Don't even consider going toward hot loads. Your objective is to find some combination that will stay on the paper, then decide which way you want to go from there.
