Like 8bore, I set the gauge first, then insert into the bore.
Greg, I think I answered your 16ga question on the other thread. Yes, your gauge will work for both 12's and 16's--except, as noted above, when you have a very underbored and/or tightly choked 16. Then it may not go in. But just start at 0 and if the bore's .662, that's what the gauge will read. Numbers will get smaller as you get into the choke area. Same thing as with the 12: Just subtract choke diameter from bore diameter and it tells you how much constriction you really have.
