I used a dremmel tool attachment-cone shaped stone/grinder and wet/dry 400 grit sand paper, all BY HAND. NO power tools at all. The crown is critical to accuracy because it is the very end of the rifeling that the bullet is in contact with as it exits the barrel. There are different profiles at the end of a barrel, some are flat, some are flat and recessed and some are round. (the recessed style is to protect the muzzle crown) Regardless of the profile, the concern is where the lands and grooves stop and a slight taper opens up from the bore. This is easy to see. What is critical is that the crown is symetrical, with no imperfections at all. It must allow the bullet to exit without being upset by a burr or being sent wobbling because the crown is uneven.
First I noted the deepest part of the rifeling and worked the rest of it down to that point, making very sure it was square. This was a very small distance, just enough to bee seen with the naked eye. I hand turned the cone grinder bit and checked often. Then the 400 grit with oil to hone and smooth everything.
Be careful, you can ruin a barrel, but if you are careful and patient this can be done and if you have ridiculous sized groups, It wont hurt unless you are sloppy. This worked for me.