Modern muzzle-loaders are pretty amazing. Anything within 150 yards is a gimme shot if you are a good marksman. After that the wind and distance have a huge affect on the bullet. If the deer was standing still on a calm day I might shoot a couple hundred yards. After that you need to be very accurate on distance and wind. The farther you get out the more accurate you need to be. 400 yards is probably very realistic for range shooting on a stationary target, but not for hunting. I am sure some will disagree but I would say 200 yards is about as far as you would want to go without jeopardizing a clean kill on the animal.
I agree! It all depends on the gun and shooter. I'd never shoot 400 yds with my .50 cal, but with the .45 cal? If the wind was constant, or it was calm, I would. Variable or swirling winds? No way. It all depends on the animal too. If the deer are calmly grazing, yes. If they are on high alert? Nope.
But then again, when hunting I always have my laser rangefinder with me, and if shots could be past 200, I have a portable wind meter (Kestrel 3500). I've had enough range time so I KNOW what the gun will do in different wind conditions, and have a cheat sheet taped to the stock.
I always use some sort of a rest, a tree will suffice for broadside shots within 200 yds in calm conditions. But a bipod/tripod and any support I can find for the butt is what I try for. Bipod and backpack, bipod shooting sticks and sitting in a chair, etc.