Twist has some effect on muzzle velocities, in that a given powder charge will give higher muzzle velocities in slow twist than in fast ones, to some degree. A smoothbore will give higher velocities thamn a rifled bore. The reason for this is that rifling gives some drag on the bullet, so some of the available energy in a load is used up overcoming this friction. But a 1/20" twist .45 caliber rifle will NOT give you higher muzzle velocities than a 1/28" barrel, using the same bullet and powder charge. Don't try to make a muzzleloader into something it is not!! NONE of these things will give you the flat trajectory of your .243!! Far from it. Why? Because, as a rule, the B.C. of the ML bullet is quite a bit lower, so they shed velocity much faster!! There are a few exceptions to this rule, but in .45 caliber, to get a high B.C., you must use a pretty heavy bullet, and these can't reach the high (2650 FPS) muzzle speeds these folks are touting!! You have to use a 175-grain or so pistol-type bullet in a .45 with a sabot to get 2650 FPS, and such a bullet has no staying power!!

Keep yore powder dry!!