There's GO NOGO and then there's mensuration. Wilhelm Gregor's "20 in a 12" hull lashup has always worked pretty well for me. If the rim of the 20 ga hull is just seated on the open crimp of the 12 ga hull and a single wrap of metallic tape added, the diameter is pretty close to .775" and the overall lgth. right on 2.75" [Win AA plastic 12/2 3/4] whereas the AA hull by itself is 2.715" overall lgth open. With the 20 rim pushed into the crimp of the 12, the diameter is .790". Something to fiddle with anyway and you don't need a lathe.
jack
