You answered your own question in para 3. If you set the bezel of your dial indicator to zero with the mike in the bore and withdraw toward the muzzle, you get the constriction in thousandths as the mike leaves the choke by counting the multiples of 10, 5, and 1 thousandths that the needle moves All you have to do is watch the needle. No remembering the bore ID number and subtracting the choke exit number. Doesn't matter if one bore is .723" and the other .725" you still know the constriction in each. If you want to do the arithmetic, you use the gauge ring (which I believe is .700" for 12 ga.) to calibrate the dial bezel, and then actually measure bore and choke and do the subtraction to get points of constriction. So yes, you absolutely can measure constriction without knowing bore size. The problem arises when folks use step gauges. These don't account for under-over boring from the "nominal" gauge ID (say .729" for 12 ga.).
jack
