The Continuing Adventures of a Home Shop Machinist  
      By now you're probably asking yourself "What the heck is a breech plug, anyway?"  Well, if you happen to own a muzzle-loading rifle (I don't) then you've got something like this to 1) hold the powder, and b) hold the primer (or percussion cap, or whatever it is that makes the small bang that sets off the big bang (no, not the Big Bang, that's something else entirely!)).  And if you're just an HSM like be you've probably got a buddy who's a gun nut and has such a rifle...

      Anyway, my buddy the gun nut has this rifle, and he thinks that the standard "209" primer is too powerful and it gives the bullet a little kick before the powder catches, thus making the thing less accurate than the theoretical optimum.  And my buddy the gun nut bought this modified breech plug that takes an empty .25 ACP cartridge and uses its primer (quite a bit smaller) to fire things up.  And my buddy TGN wants to know howcumzit the thing costs so much and can I modify some of the cheap ones for free, or at least at no cost to him (my buddy TGN not having a close intimate relationship with the spondooliks at the moment).

      Now MBTGN doesn't know the dimensions of anything, but he does have one "after" and lots of "before" pieces of hardware, and a handful of primers and cartridges, and these big puppy-dog eyes that make an HSM want to do anything he asks.  So...

      DISCLAIMER: I'm not a gunsmith, nor a mechanical engineer, nor a professional machinist.  Since the object of this work is to reproduce an existing product, it's probably fairly safe; but  guns are dangerous things to play with and if you modify pieces of them - whether following my directions or anyone else's - you do so at your own risk!

      The pictures above show the original unmodified breech plug and its matching primer, and the modified plug with its matching (primerless) cartridge.  On the right you see both of these assembled.  The critical dimension here (in my humble opinion, not being a gunsmith, etc.) is the distance from the seating face of the plug (bottom) to the top of the primer - 1.323"±0.003" in this case.  If the primer sits too high you may not be able to close the breech; if it's too low the firing pin may not strike it, and you'll probably get a lot of blowback.