for an antique look, some plating shops have used a strong oxidizing solution that will turn the nickel black, then carfuly and lightly polish until the desired brushed look is achieved, then seal that with a clear coat, as the oxidation will quickly wear off.
I am not sure what the oxidizer is, or of any shop currently using it.
I have on occassion taken a nickeled gun, and to remove light scratches and camo little dings, run them through my bead-blast cabinet. I only use glass beads in it so it's not real aggressive like an aluminum oxide media. I do turn the pressure down.
Now, with that being said, you do risk the chance of removing the nickle or causing it to pop off. Each manufacturer has their tolerances for the type of nickle and it's thickness. The plating I offer does let me do some touch up.
If the gun is actually chrome, more aggressive media will be needed as the glass beads will just bounce off. Aggressive media equals material removal...BEWARE!!.