Colonel, using a clean wooden dowel, I would drive a slug just into the muzzle, then knock it back out; then drive another up into the bore at the throat, and knock it back out. This will tell you whether or not the bore is tapered. Hopefully it gets tighter toward the muzzle, the other way round is really not ideal.

If you have a five groove barrel, send the slugs down to me, and I can measure them for you. I have a gizmo for five groovers. An even number of grooves, and you should be OK with a good micrometer.