I pondered this question fairly recently because I was in the process of buying a blackpowder cartridge rifle. I started reading about the buffalo hunters, the guys shooting at Creedmor and all that. These guys were shooting groups out at ranges up to 1000 yards that would rivil anything being shot today.
It occured to me that there shouldn't be any reason why a muzzleloader shouldn't be able to hold it's own also. On these forums, every once in awhile, someone asks this very question and the answer is pretty much always given as something like 75-100 yards. No discussion ever takes place as to why this would be the limit and everyone goes on about their business. I agree and disagree with that range estimation. For general hunting type shots that are encountered randomly in the field, you are limited to a range of somewhere around 100 yards primarily because of two factors:
The sights.
And accurate range estimation.
Up until fairly recently, most muzzleloaders didn't have sights that were designed for precision long range shooting. But with the low velocity and heavy bullets typically shot with muzzleloaders you get a very steep trajectory and IMO this limits your range (of ACCURATE shooting) more than do the sights. This doesn't mean you can't hit a target at 300 yards or more, it just means that you have to know almost exactly what the range is, and know pretty much exactly what sight setting is needed to hit at that range. I guess it goes without saying that this means you need to have sights that you can adjust readily for the range (elevation). For target shooting or known distance shooting, there is no reason why this range couldn't extend out a lot further, possibly out to the maximum range you could see the target with the sights your rifle is equipped with. BUT, knowing the distance is critical.
If you want to read about the absolute fringe limits of blackpowder shooting, read about the famous shot made by Billy Dixon at Adobe Walls. Billy fired a shot at some assembled Indians at over 1500 yards using a Sharps buffalo rifle using blackpowder and cast lead bullets. I read an article on the shot in Precision Shooting magazine where they broke down the numbers on that shot. I don't remember the exact numbers but at that range, the bullet was dropping something like five inches for every nine inches of forward movement. Long story short, to pull off a shot like that, he needed to know the range almost exactly or he could have easily missed by so far that the Indians would have never even known he was shooting at them.
Again, I realize he wasn't using a muzzleloading rifle, but I think you get the idea.
One other minor point, if your goal is long range shooting, the in-line guns are NOT the way to go. You want a big, heavy bullet that is at least somewhat aerodynamic: not a jacketed pistol bullet. You also need a bullet rather than a round ball. You also need a bullet with proper lubrication and proper grease grooves to enable you to do more shooting than cleaning.
I have a T/C Renegade in .50 that I used to have equipped with a Lyman peep sight. The peep sight was a huge improvement over the factory T/C sights, which in turn were a huge improvement over traditional muzzleloader sights. I never did any long distance shooting with the rifle, but looking back on it, I doubt that the sight had enough built in elevation to take you out very far. If you look at rifles such as the old Sharps rifles or the Remington rolling blocks or even the old lever actions like the '73 Winchesters, you will often find them equipped with large tang sights. The height of that tang sight determines how much elevation you can get out of the rear sight, which is essential for long range shooting with black powder.