320 grit emery or wet/dry paper. Pushing it into the muzzle with the end of your thumb, rotate your wrist and turn the barrel 180 degrees every 20 seconds or so. Turning the barrel will keep your crowning job square with the world.
No business I know of, sells a rifle with a shootable smooth crown. They are all machine cut and will cut a proper patch, that is a patch that is thick enough to do the job of keeping the barrel clean and deliver good accuracy. Most 'factory' crowns are too sharp to load even weak (thin patch) combinations.
All of the 'crowns' shown will allow loading bore sized balls with .022" denim patches, without tearing.
Hint- shove a piece of cloth down the barrel about an inch, to catch the "grindings".