Thanks for sharing your gauge design. This is the gauge I use for 12 ga. The first black mark is 2 1/2", the second is 3 3/4". If it bottoms on the shoulder, the chamber is 3.0" or longer. The gauge length is 3.500". To measure the exact chamber depth, I measure from the end of the lump to the end of the barrel and subtract that reading from 3.500". The .793" diameter was chosen becuse it is the minimum specified by SAAMI.