Actually, your grandfather did the right thing. I looked at round head bolts with a circle template one time and found they were mostly elliptical, so they have to go straight in or you can get an asymmetrical crown.
I've had great success using ball bearings I groove with a Dremel tool and superglue to a piece of tubing for a handle. I have a lathe, but this lets me use it just to cut off the muzzle square and dress the OD, which doesn't require indicating out a perfect bore center. The lap then takes about 10 minutes and gets a sharper result.
I wrote it up long ago, including how to pick the ball size to get a certain angle at the edge of the grooves. I've attached it in PDF format.