Steve is spot on in his advice. You will likely find that a protrusion of about 0.39" works best, and the tool steel that I like is O-1 alloy steel drill rod. Be certain that when the tumbler is forward and down against the rear of the firing pin that with 0.39" protrusion (or whatever protrusion you decide upon) that the firing pin can be pulled forward from the breech face of the action by about 0.10" to 0.20" ----free travel to minimize breakage concerns. If the firing pin protrusion is too long the gun will be hard to open because the firing pin is contacting the shotshell expended primer pocket.