Rick: check the image I posted. The WT of the FC is thicker than the end of the chamber. The critical number is the end-of-the-chamber
before the forcing cone, and it's usually not hard to tell by "feel" and the increase in WT when the gauge makes that transition.
.170" would be much thicker than any 12g end-of-the-chamber thickness I have measured; about what you would expect for 10g.
Is the gun a No. 2 frame? If a No. 3, that number might be possible and would explain the relatively thick 9" from breech numbers, and we can all take a big sigh of relief
Threaded Mode