I found this here in the 1896-1961 section,
Stripping nickel chrome off the Registered Magnum and re=finishing.updated 9/8/09
I suppose you could soak the gun in ammonia as this would remove the plating in time. However, I never tried this and I'd don't know what damage would be done to the bare steel.
Smarter folks on plating will chime in I'm sure.
