Muzzleloading patched round ball loads in .50 caliber typically have about 55-60% of their muzzle velocity left at 100yds. That leaves only about 33% of the muzzle energy value to do the job at 100yds.
If you can get the velocity up to about 2000fps at the muzzle, then there is about 500 ft.lbs. left at 100yds. That is a generally accepted energy figure for use on whitetail deer. (Even though the general figure is 1000 ft.lbs. for centerfire cartridges--go figure!?)
BUT, if the gun (or shooter) can't hold 5 consecutive (off-hand) shots on a paper plate target at WHATEVER range they feel comfortable with, then I don't feel the shot should be taken. Exceptions would be shooting from a position with a good rest.
I'm comfortable with taking a 100yd shot ONLY IF; I have a good position--even using the side of a tree to help steady the gun, AND, I know for certain (from chronograph proofing) that my load is 2000fps + or - 75 fps. Typically my shots come a lot quicker, and off-hand are under 40yds. Where I hunt, 50yds is a LONG shot and 25-30yds is more average.
Always know you and your gun's limitations, and try not to exceed them. It's more fair to the deer and to yourself.
Shoot Safely!
WV_Hillbilly