I prefer to use a tapered grinding bit to produce the crown on a muzzleloader.
Something like what is shown in this link
http://www.sourcingmap.com/grindin...gn=usfroogle&gclid=CJz0ofHc27wCFc5hfgodLzUA9A
A spherical stone/bit will leave a razor sharp edge where the bore and rifling meets the concave cavity and this IMO is no better than leaving the face of the muzzle flat.
Either shape will need to have the sharp edge removed if a patched ball is going to be loaded.
Using a tapered stone a tapered enlargement is created that helps in guiding and compressing the balls patch into the bore and I feel this greatly reduces the possibility of the patch being torn.
Although these tapered grinding bits are made to be used in power equipment I would
never try to use it that way.
The slightest bit of sideways pressure would remove more material on one side of the bore than the other resulting in a poor shooting barrel.
Rather than using power equipment, I use my fingers to rotate the bit while keeping the shank parallel with the center of the bore.
The bore guides and centers the bit while this is being done.
After a sufficient amount of barrel material has been removed to compress a thick patch (.020-.030 per side) I still need to use wet/dry sandpaper to round off and blend the sharp edges left by the bit both in the bore and on the face of the muzzle.
To do this I tear a 1" X 1" piece and press it down into the bore with my thumb. Then by rotating my hand/thumb back and forth as I slowly rotate the barrel, all of the sharp edges are removed and the resulting radius is polished.
Notice, I always let the bore and muzzle face do the guiding of both the grinding bit and the wet/dry sandpaper to prevent uneven removal of the metal.