Schimels had some kind of chemical blacking on the zinc cast material, which may be difficult to duplicate. However, a quick Google search shows several commercial zinc-blacking potions plus home-brew advices. One thread suggested looking for a local stained-glass shop that would have "zinc patina" material to try out.

I don't recall seeing any Schimel presented as a restored gun, so you may have the opportunity to pioneer a new field here!

Best of luck and we'll look forward to hearing how you fare.

Don R.