You have to keep turning the barrel as you thumb polish the crown - about every 15 seconds or so, I turn the barrel 90 degrees. Twisting your wrist pushing straight in with the tip of our thumb is the only method needed, but a lathe simplifies this as well- running forward and reversing it from time to time.
Usually, there is about a 45 degree angle cut with a tool by the barrel mfg'r. when you receive the barrel. Those sharp edges are merely rounded with the thumb or on a lathe, or bench held using a tool in the hand drill. This form of crowning does not seem to hurt accuracy, but allows much tighter combinations (without difficult loading - no hammers, no mallets, no steel rods needed), which improves accuracy.
A tapered tool like a stone grinding cone will hold the paper well. I usually use 320 grit. I have finished off with finer crocus cloth but it is not necessary, especially if using power.
I coned a barrel (1" deep cone) once for testing and could not find a load that would shoot well in it. I will never cone another. I had to cut off the coned portion and re-crown to the radiused crown you see above in my pictures. These, what I call radiused crowns, are only about 1/8" deep from the top surface to the full land height. This short radii is much closer to the radii that is used by the bullet swaging die companies (Corbin) for their dies. I also found a long tapered cone creates more drag with really snug combinations and more force is needed to thus swage the patch and ball into a long cone, than into the short radii we use.
