Closer is always better, but there is no reason at all that deer-sized game cannot be cleanly dropped at 200 yards or so with a muzzleloader-- providing you practice a reasonable amount with a reasonable load.

http://www.prbullet.com/DCenter.wmv

At 200 yards, the exact load I used to drop this 350 lb. Bull Caribou at 157 yards has 1200 ft. / lbs. of kinetic energy left-- and that is much more than needed to cleanly harvest a deer.