The Continuing Adventures of a Home Shop Machinist  

      As best I can measure, these are the dimensions of the breech plug - the original in black, and the modification shown in red.  You can see that the original primer well has a flat bottom, while the modified breech plug has a conical bottom,  I couldn't come up with a good way to measure this, so in my modification I just used the angle of a standard 118° drill point.

      The original breech plug is intended to hold a "209" primer - which isn't 0.209" diameter, they run from about 0.240" at the small end to 0.244" at the large end.  The hole for this was a loose fit on a "D" drill (0.246") but too small for an "E" (.250") - it's a pretty loose fit on the primer.  

      The modified breech plug well turned out to be a close fit on a "K" drill (0.281"), and it's an easy fit on the .25 ACP cartridges (which averaged 0.277" diameter).

      The critical dimension - the one that determines the height of the primer face - is given here with a tolerance of ±0.003".  I don't really know just how tight this tolerance is; in a sample of primers and cartridges I found a variation of about ±0.002" in their lengths, while comparing an unmodified plug/primer with the commercially modified one with a cartridge, I found a 0.010" difference in overall height.

      Again, not being a gunsmith I don't know exactly what tolerance is acceptable; but ±0.003" should be well within the capabilities of an HSM.  Bear in mind that different rifles will have different dimensions; if you want to try this yourself, make your own measurements!

      The Mark II version of the jig itself is on the right (as if you hadn't figured that out already!).