Mike:
It might be best to 'slug' the bore. Use a big lead fishing-sinker if nothing else is available, peen it to just oversize with a mallet (drop-fit into chamber), lube it, then drive it about 3 or 4 inches into the rifling with a stout dowel and the mallet. Then drive it back out, and measure. You can drive it all the way through, but some early rifles have a few points of constriction over the length of the barrels.
I don't understand what you mean by "Land + .580 groove = .598" On a bore-cast, simply measure across the 'lands', which are the grooves in reverse, so to speak. That gives you the correct projectile diameter.
I know you know this, I'm just not getting it for some reason. Apologies.
