The Continuing Adventures of a Home Shop Machinist  
      After thinking things over I realized that it's hard to measure the depth of a drilled hole directly - the conical bottom makes it hard to determine just where the nominal bottom really is.  And since the breech plugs to be modified aren't all the same height, the depth to be drilled (as measured from the top of the plug) will vary from one to another.  

      Now with the jig to locate the plugs reproducibly, it's not too hard to drill several of them to the correct depth - if you've got a good way to set the depth of the hole for the first one and then hit that depth reproducibly for the rest.  But you've still got to get the first one right!

      The solution I came up with is to modify the jig so we can measure the actual position of a .25 cartridge in the drilled hole; we do this by adding a calibrated extended collar that we can measure down from with a depth micrometer - and in the pages to follow you'll see how I did this.

      The first step is to take that jig that was so nicely finished and cut a shoulder on it to form a seating surface for the new collar.