For snowshoe hares I use everything from 32 caliber to 58 caliber with reduced loads. Heck, I'm likely even to try reduced loads with RB in my 75 caliber smoothbore Bess rather than shot.
Every muzzleloader I've ever owned shot incredibly well with reduced powder charges. Plenty of accuracy for head smacking to at least 25 yards and usually further. And if you're popping heads, there's no such thing as "overkill" with the larger calibers. I probably put in 10 days in the field on snowshoe hares for every day in the field for big game, so you can imagine the extra field practice I get with my "big game" rifles. Heck, I probably kill more pounds of snowshoe hare in a season than big game, so I'm a rabbit hunter through and through, no matter the size of the hole in the bore.