I think a reamer would work, but the cost of making it for this kind of work would be prohibitive. You are basically tapering a bore so that the muzzle is at Groove diameter, and 2-3 inches back of the muzzle the lands are at the bore diameter. In between, the lands have been reduced by abrasion to taper down to the groove diameter at the muzzle. There is relief behind the muzzle that is just enough to allow a PRB to be pushed into the barrel, flush with the muzzle, by thumb pressure, so that the patch material can be cut off the traditional way with a patch knife at the muzzle. Then the ball is run down the barrel with either: 1. First a short starter, and then the ramrod; or. 2. only using the ramrod. The ball is centered in the bore, and the land slowly take a firmer "bite " of the patch and ball as they are pushed down through the tapered section behind the muzzle. The rest of the way, the PRB is loaded down the barrel to the powder charge the conventional way, using a Ramrod that has a mark on it to indicate when the PRB is seating on the powder, and another mark that indicates when the barrel is EMPTY.
I hope this helps. There are several ways that coning tools are made. As a machinist, I am sure you understand metal removal techniques.
