Philip,
John Binins showed me how to do it when I was in his carving class at NMLRA class in 1997. He showed me how on a barrel that was not coned.
I use a regular chain saw file. Put some electrical tape on the very end of the file so that it will not cut. About one inch will do, about three layers. Then I put the file all the way into the muzzle to within two inches of the handle on the file. Now I put the pressure on the muzzle end of the file and the other end of the file with the tape on it will come up against the opposite side of the bore. This will keep the angle of the cut exactly the same. This is important! Now line up the file with the groves in the rifling and give it several strokes, using only the last two inches of the file, using the handle of the file for a stop. Be sure to get the grove in the center of the rifling. Then go on to the next grove and do the same thing. After going all the way around the bore I examine it closely and make sure that all the file groves are the same depth. I usually go around the bore twice. If the groves are not the same , give the ones that are not quite enough a couple more strokes so that they are the same. Once you are satisfied then use a three cornered file to cut the little v mark in between the circles, with this file you will have to be the judge to keep the angle all the same.
All the ones that I do now I cone with Joe Woods coning tool first. I have done them with and without coning but I believe if the barrel is coned first there is less chance of making a mistake with the design. When coned, I believe the ball has already started to leave the barrel before it gets to the design. I have never had one that hurt the accuracy .
If you are not comfortable with hand tools or if you are not confident I would suggest that you don't do it. If you have a piece cut off barrel that would be a good place to try your first one.
I am attaching a picture of the first one that I ever did, and one that I just did on a set of swivel breach barrels.
