I have no experience with military carbines, but have a lot of experience with shorter guns in the field. Since most muzzleloader hunting is done inside 100 yards and often inside 50 yards, there's no liability in terms of power. I prefer 58 caliber, but have done just dandy with 62, 54 and 50 as well.
The one thing you will notice is the shift in balance. Shorter barrels can be very light in the muzzle, which in shotgun parlance might even be called "whippy." They're certainly not something you'd take to the range and use to compete with long barrels having lots of muzzle weight. But for quick shots, especially at running game, they beat the pants off a muzzle-heavy gun.
If I added just one more gun (yeah, right) to my short gun array, it would very likely be a 58 or 54 caliber smoothbore. Plenty accurate with patched round ball, but terrific with shot, too. I have a 62 smoothie, but it's a little heavy for my tastes. A lighter version would be just right for my tastes and needs I think.