So the reason they use the larger bullets is to make up for the slower speeds that muzzleloaders are kind of limited to (with a few exceptions - like those that can use smokeless powders). A .300 Win Mag pushes that 180 at around 3000 fps - that translates into a bunch of ft lbs of energy downrange. If it was only going out at 1400-1700 fps, it would be far less than is normally desired for a clean kill on a deer-sized animal. I don't have charts at my fingertips, but energy is a product of speed and weight; change either one downward, and energy goes down - sometimes way down.
Having said that, I know ofsome who are getting around 2000+fps with a 200 gr 10mm bullet in a sabot and finding it adequate for hunting (roughly approaching 30-30 specs.).But that 200 gr size seems to be about the minimum that we can go right now. Stick around - technology will change that soon (my guess).