There's at least 4 ways to do this. The hole, the groove, or both can be tapered, and the groove can be tapered along the whole length, or mostly widened at the muzzle.
1) Make a non-tapered ramrod groove, drill the hole with the same sized straight drill, then progressively widen the groove all along the length. You'll need custom thimbles.
2) Same but mostly widen the groove mostly at the muzzle end and possibly also make the groove deeper at the muzzle end to make a slimmer forend and nosecap. First thimble is bigger.
3) Make a non-tapered ramrod groove, drill with a straight bit a smaller size, and taper the ramrod in the hole. Thimbles all the same size.
4) Make a non-tapered ramrod groove, drill the hole a smaller size and then ream it out with a tapered bit so the hole is tapered. Thimbles all the same size.
I prefer #2, a unifornly made ramrod groove the same diameter as the ramrod drill, to make the drilling go right, and then change things later, usually widening the ramrod groove and making it a little deeper at the muzzle end so the ramrod swells mostly at the end. Then I taper the ramrod in the hole somewhat so it can't bind.
