Sorry, but I'm not buying this. Do they honestly expect us to believe that a muzzle loader is capable of propelling a round at a killing velocity to a distance of over 1.5 miles? I don't think so. Maybe if this were a high-powered round like a .308 or .300 Magnum, but not a black powder rifle.
Call me skeptical, but I'm not convinced that a black powder rifle could do this.
I recall seeing a show on a great Civil War shot, something like 1 mile, if so then perhaps a mile and a half is believable. Very sad, terrible story.