Jack, not sure whether you're being serious, but it's relatively important to set to a ring gauge, assuming you want an accurate reading of the bore diameter. Sure, you can read choke constriction without using the ring. Simple matter of the difference between bore diameter and choke. But if you're buying a Brit 12ga that's being represented as "in proof", and if you come up with a BD well into the .740's, then you know it's not--which may influence your buying decision, or enable you to negotiate a more favorable price.