I think if you take the suggestions above, and limit your range to 100 yards, you won't have any problem with a .50 or .54 round ball taking a deer.
IMO, there is no reason for going to a conical if your shooting with open sights, and the round balls cost a lot less.
The lower cost allows you to practice more and the practice will end up paying off on the hunt.
I have long said the archery hunter that adds a muzzleloader to his hunting will have the best of two worlds. Archery and big bore firearms. Combined, these will make the best hunting possible.
IMO If your state has a Muzzleloader season only, it's even better. There are no people who take the 1/4 mile shot at "something", and I have seen happen in the regular rifle season.
Muzzleloading shooters IMO take much greater care with their shots than cartridge shooters. Maybe that is because they know they only have one shot. Maybe it's just because their nicer people.
IMO, If your hunting in an area where the folage limits the range to 75 yards or less, a .45 cal gun can also be very effective on a deer.