Use a round headed 1/4 inch brass machine screw. Get one about 1 1/2 inches to 2 inches long. Most hardware stores carry them.
A 1/4 inch round headed machine screw will have a head about 1/2 inch in dia. Be sure and get one made for a slotted screwdriver. The slot in the screw will help hold the lapping compound on the screw head.
I have used regular valve grinding compound available at auto parts stores. I would use a medium grit, something like 220.
As mentioned above, wallow the screw head around on the end of the barrel as you run the drill. It doesn't take much.
Before you start, run a patch from the breech end of the barrel, stopping about 1/2 inch before the muzzle. Leave the patch in place until you finish, then push the patch on through. This will clean the compound out of the barrel. The patch will keep the compound from getting down inside the barrel.
