In a revolver the fit of the bullet to the cylinder throats is more important than knowing the groove diameter. Simply slug the throats and size to that diameter.

If the cylinder throats are smaller than groove diameter, and this is not uncommon, it doesn't matter what size you make your bullets they will be undersized before they even get to the barrel!

Do this. Slug the barrel and then try to pass the slug through the throats. If it passes easily, or with slight pressure, size your bullets to fit the throats. If it will not pass relatively easily through the throats (Use a new bore slug for each charge hole.) then the cylinder throats need to be enlarged until the bore slugs do pass with light pressure. It doesn't do you one d****d bit of good to know the barrel groove diameter if you don't know the other dimensions too!

Note: Never in the process do you have to measure the barrel slug as how it fits the cylinder is all that matters!

__________________
Gunsmithing since 1961