Basically, the best way of removing an old plated finish is to have a plater DE-plate the gun.
To do this, they just reverse the plating process, and instead of depositing material on the gun, it's removed.
This insures a thorough job, since other methods often leave nickel or chrome in the pores of the metal, and when you attempt to have it blued or parkerized, it comes out looking like it's covered with bright freckles.
Check with some platers about a strip. Many can also refinish with blue or other finishes.