Well on the boxes of 22 shells it says that they will travel 2 miles and I remember in the Hunter Ed book it having a chart that showed a centerfire going well over 2 miles and a magnum going something close to 3 miles. So a muzzleloader going 1.5 miles is very plausible to me. I also remember reading an article about a 45-120 blackpowder cartridge sharps being able to shoot around 1.5 miles with the proper angle and I would bet that would be close to the velocity of a muzzleloader out of an inline.