The Breech Plug Jig Mark II

Start with a 3/16" drill and open the hole a bit at a time to a diameter of 3/4" and a depth of 1-1/8".  If you don't have drills this big, this is a great opportunity to purchase some new toys; or you could open this up with a boring tool instead.  The exact diameter isn't critical, it just has to clear the body of the breech plug when mounted in the jig.

The next step is to bore out the collar to fit over the shoulder on the jug.  This should be an easy but not loose fit - no more than 1 to 2 thousandths larger in diameter than the shoulder.  Make sure you cut this deep enough that it will seat on the shoulder rather than on the bottom of the bore.