Yes, thanks Kutter and Damascus for digging deeper into it. Kirk, I think you can figure it out. The simple drawing that you made had me thinking that you may be able to skip the impression step and go to a length of turned down maybe drill rod.
I think what kutter mentioned about timing the action maybe the time consuming part. You could stick the smooth shanks of regular drill bits in the hole to get pretty close on diameters and depths. It may horrify some folks, but I bet you could turn down some rod with regular tools and time. Worst that might happen is you toss out the pin and call it practice, and hopefully it fits right if it's able to fire off a shell.
