Howdy
I agree with Fox Creek Kid. They vary. Forget what Cimarron says about the groove diameter, they are just quoting the general spec that Uberti puts out. It does not account for tool wear. My Uberti '73 slugs out at .427, my Henry is .429. The comments about chambers are also correct. With a tight chamber you may not be able to fully chamber a round with a bullet as large as your groove diameter requires. I can shoot .427, .428, and .429 bullets through my .427 groove '73 just fine, but the chamber does not like to accept rounds with .429 bullets in them.
With my two different groove diameters I have compromised on .428 dead soft lead Mav-Dutchman bullets. Everything is fine.