I use a rotary file from Nicholson and slow speed with a piece of thin cotton between it and the muzzle to eliminate chatter. Works like a dream! Hugh Toenjes
I used to use a stone for years, then switched to just using the stone with emery wrapped around it. The stone was only for the angle - tapered, of course. I found the resiliency of the cloth would slightly round the corners of the grind, making a finish easy and quick by hand or with the same tool and finer grit & crocus cloth to a perfectly polished crown. It doesn't take much on a small bore - be careful. It's very easy to be a bit ham-fisted and make a huge funnel at the muzzle.