Take a conical such as a Maxi-ball or a Lee REAL, lube it up good and then run it through your bore. Run it down to just above a normal seating level - then run it back out with the breech plug removed. Slow and careful the whole way. Then measure the narrowest points across the upper band of either bullet (where it is engraved), trying to get a full diameter reading. That will be your land to land. I doubt you'll be able to get anything on groove to groove - but that isn't the major problem anyway.

T/C has a fairly wide tolerance on bores.