I think they're called "Punt" guns and are modern day descendants of the target rifles carried by Co. E & I of 1 U.S.S.S. (Berdan's Sharpshooters). 30 plus pounds, they have barrels that are about 2" thick and are shot from a rest. Of course, they're impractical for anything other than range work. Even the Sharpshooters had them boxed and carried by wagons though in one early battle the Second Andrew Sharpshooters were made to march with them - they straggled onto the battlefield along with all the other stragglers.
If you really want a muzzle loader that is accurate (and relatively mess free when it comes to all the meticulous work required), go with a Civil War era minie rifle. The best of the bunch are the first generation Parker Hale Enfields and they'll cost you about $600.
Now, turning back to your gun, it may be inherently accurate but you, like the rest of us (including me) may be too lazy to find out what is optimal. Weigh your balls and put them in separate bags. Next, find material of varying thicknesses for the patch. Try out different powder charges/patches/ball weights until you find the optimal one for your gun. Then it's a matter of KY windage for drop.
The frontier riflemen did this and could hit a man's head out at 200 yards and the figure of a man at 300. Beyond three hundred though, they rested it on a log.