Marc,I bought a metal from Brownells called Cerosafe that melts at a very low temperature. I used this to slug the bore by stuffing a patch about 1 inch from the muzzle and pouring the cerosafe in. The 7 point rifling makes finding the bore size difficult by just trying to mike the slug. You need to turn the slug in the jaws of the micrometer to get a true reading. I would use that measurement to order a bullet mold ( paper patch). Patching will then increase the diameter to seal the bore at the rear tapered end of the bore. I would guess miking the rifling just after the chamber ends would give you the correct size for a grooved bullet.
