My friend Miller doesn't need my support to be proven correct. However, I was curious as to how far off this 20/12 method could be. I assembled the "gauge" and inserted it into a chamber of my favorite Lefever. This chamber is exactly 2-3/4" to the start of the FC(I lengthened these chambers, myself). The 12/20 combo was compressed to a length of 3.004", giving the false impression that the chamber was 1/4" longer than it actually is. The 20 ga. rim diameter was .763", .035" smaller than the forward end of the 12 ga. chamber should be (.798"). Not a usable gauge, IMO.
