Otto,
In the gun I described, the mainspring was a leaf spring and a screw limited the travel.The hammer,could fall back since it was no longer under spring tension.The firing pin is on the hammer, so it falls back with it.The adjustment lets you stop it at the face of the breech block, to keep the primer from flowing into the hole.
In another rifle, I made three fireing pins before I learned you have to let the primer stop the forward progress.I took careful measurements and made it stop against a small spring(to retract it)and the frame, with the protrusion I was looking for(it was enough to ignite a primer). It acted like yours, fireing sometimes and sometimes not.I finally made it longer in the front, but the same overall length(so it would be flush at the breech face (it is a tip-up).Also hardenen it before you try it( harden it so the tip is spring hard and the back is chisel hard).It is my theory that the frame stoping the pin took too much inertia out of it, that should have gone to the primer.
I kind of specialize in busted rifles, I can't afford good ones.
Keep trying, you will finally make it work.
Mike
