What is the correct diameter for a 12ga. chamber gauge? A 16?

I once had a well known barrel man tell me that the reason he measured my 12ga. at 2-3/4" when myself, my gunsmith and the Birmingham Proof House all measured it at 2-1/2", was that his chamber gauge was a different size. He said it all depends on the gauge you use, they are all different.

What horse sh*t you hear in this business...

My problem lies in reconciling my gross habits with my net income.
- Errol Flynn