3f powder in almost all cases will produce a little more velocity then 2f powder. The way to use 3f is cut the load down to compensate for the additional velocity. In a .45 caliber I would shoot 3f powder. The smaller the bore the faster they foul. you can get around this a little by using a faster burning powder. To speed the burning rate, you need to normally have a hotter ignition source, or a small grain powder. So 3f would be the choice for a .45. Although I do shoot 2f in some of my .45 caliber inline rifles.
As for the max charge, in the T/C manual for .45 caliber round ball shooting, it shows 90 grains as the max charge in FFg powder. This is for the Cherokee and the Seneca model rifles. So if I were shooting 3f powder I would limit my loads with a round ball to 80 grains. With conicals the max load is 80 grains of FFg. My friend has hunted with a .45 caliber shooting roundball for many years. He shoots 80 grains of 3f powder and claims the ball will get a complete pass through on most the white tail he has shot with it. (which is quite a few).
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