Making a Breech Plug Jig

Reset the cross slide and compound to zero then lock the cross slide.  Set the first cut to 0.005" on the compound, then with the threading dial on the 1 or 2 mark throw in the half nut and spin the chuck by hand until the saddle hits the stop.  Back off the compound and release the half nut, then run the saddle back to the 2 or 1 mark on the dial.  Set the compound for another 0.005" cut and repeat.

With my tool the total depth of cut on the compound was 0.034", but your tool won't have the same tip geometry so you might go deeper or shallower.  Once you get close, start taking cuts 0.001" at a time and check the fit after each one.  The goal is an easy but not loose fit where you can screw in the breech plug with your fingers but no appreciable wobble.  It's a good idea to get three or four breech plugs and test all of them - a hair too loose for a few is better than much too tight for others.