I have chamber dimensions taken from Roy Dunlaps book "Gunsmithing". While several years old I believe these are still standard. Minimum dimensions for the 12ga are as follows; dia of outer rim 0.887", then 55° angle as Chuck stated. Note this angle is measured up from the horizontal, thus an included angle of 110°. The gauge point for depth is .0546 to a point on the angle @ 0.850" dia. An 0.020" radius is called for at the juncture of the angle with chamber body, so as Chuck pointed out there is no positive point to measure here.
While the No-Go gauges are handy for one doing a "lot" of bbl work, personally I never bothered to buy any. When I had need for gauging I would simply buy the "GO" gauge & cut varying thicknesses of shim stock to check how much clearance was there. As Chuck mentioned the breech of a dbl is not normally square to bore axis so if using this method the shim should be full gauge dia. Also note "NO" force should be applied when gauging. The gun should close easily on the Go gauge & not close on the No-Go, though most any in-tolerance gun "Can BE" closed on a no-go gauge if force is applied.
