Got that "20 in the 12" from "Wilhelm Gregor," the disappearing Alaskan hogrider. I tried the "slip the bullet to find the rifling" method suggested by Jim and got 2-27/32" OA case rim to case rim. No doubt the 20 ga. rim is in the cone. Howsomeever, what is important to me is that the wall of the 20 shell base is a slip fit in the 12 crimp area. The crimp is not allowed to collapse and will not collapse if it either hits the cone of protrudes into same. If there is no reduction in the OD of the fully open crimp (and that full diamater is maintained in my view by this "gauge") then there's no appreciable occlusion of the cone and likely no appreciable pressure excursion. I have at least two 12 ga. guns which will NOT close on this gauge so it sure as the devil is a GO-NO-GO some of the time some of the guns. Note also that the 20 base is a tight enuf fit in the 12 crimp area to hold its position in Jim's "test" so I think it's effective in holding the open crimp OD. Incidentally, the wall of a plastic hull is considerably thinner than the wall of a paper hull; a fortuitous advantage of new school ammo in old school guns. Wilhelm's invention does give a feel for "fit" if not "bottom of hole" if the barrels are off the gun and only hand pressure used to chamber the gauge with the extractor already retracted. OMO.
jack
