I always feel that slugging the bore is a very valuable step. I oil the bore well and use pure lead (muzzleloader swaged balls) to do this. I feel for tight spots as the ball passes through the bore (like where the barrel passes through the frame.

Next I take the lead slugs and check them in the cylinder throats. If they are larger than the throats and you can't press them through with light finger pressure, you are going to have problems. It means your throats are too small for the groove diameter. If they do pass through, use your pin gages to determine the throat diameter and use that for your bullet selection.

I have never read where anyone was successful in trying to use pin gages in the barrel and then by using a land height to determine the groove diameter. But you might be the first. When I played around with it, I just couldn't get an accurate measurement.

And if this doesn't discourage you, send them to Matt Dardas. He offers a free service to measure the groove diameter of your slugs for FREE! How can you beat that?