Some folks may not like what I am about to say but here goes.
I have a Parker Hale Whitworth. I've taken a lot of deer with it. My longest shots on them has been 100 yards. I didn't use a rangefinder but paced off the distance after the shot. That's how I know with a reasonable degree of accuracy how far the shot was. I am also fairly decent at estimating range out to about 500 to 600 yards.
When you've been a forward observer for Field Artillery you get to do that sort of stuff.
Anyway, back to my deer hunting escapades with the Whitworth. I didn't carry a rangefinder with me at the time as I knew that the woods were thick enough that 100 yards would be the limit and the trajectory didn't vary but a few inches from up close to 100 yards.
Do I think that my Whitworth, or any other muzzle-loader is capable of taking game at longer distances? Of course I do; given the right conditions, a good position or rest and the ability of the shooter.
Now here is where I'm going to really upset the traditionalists.
If a person was going to harvest an animal past 100 yards with a good muzzle-loader they should do the following:
1) Shoot at long range targets at the distances that you would be expecting to shoot a game animal and see what the accuracy is at those distances. Obviously, all the shots should fall within the vital area of the animal.
2) Also place the target a few yards closer and farther back to see what sort of elevation error that you would be getting. Remember that muzzle-loaders have rainbow like trajectories. A few yards difference in range estimation can make a difference between a lethal hit, miss or crippled animal.
3) If possible, chornograph the load that you will be using. Use ballistic software to get a good idea of what your load will do along with shooting at actual distances. Carry a ballistic chart with you in the field. If it is a really long range shot you will probably have time to figure out your elevation and windage corrections anyway.
4) Get a quality range rangefinder. Yes, I am referring to one of the really good laser rangefinding marvels of 21st century technology. I'm just as much as a traditionalist when it comes to muzzle-loading but the main reason that I'm advocating the use of a non-traditional range finder is that we owe to the animal that we are harvesting.
5) Along with the modern range finder, a long range hunter may want to use the range card that the military uses. Yes it is a modern aid but remember we are talking about trying to harvest a game animal at long distances. The principal behind using the range card are the same as with a modern high velocity centerfire cartridge rifle but at reduced distances. You can use it in a blind or tree stand. Besides if you are sitting there all day with nothing to do completing a range card will keep you occupied.
If anyone thinks they are another Davy Crockett, Simon Kenton or Billy Dixon go ahead and eschew the use of a modern range finder. That's up to each individual.
On the other hand when it comes to taking a big game animal when you only have one tag and you want to dispatch it was quickly and as humanely as possible then use a range finder and ballistic table. And by all means practice. We owe it to the animal that we are harvesting.
Now please excuse me for a few minutes while I slip into some comfortable nomex.