PLEASE understand since you put this under the handgun section, I wondered if you were considering nickle plating a gun that had never been nickle plated?
If you are trying to get a gun that was already nickle plated "touched up" or a restoration of the plating, then please disregard this post.
Back in the late 80's a very good friend of mine had helped out another person a great deal. That other person offered to Gold and Silver plate 2 handguns for him as a way to thank him. My good friend not knowing better, allowed him to do it on both a revolver and unmentionable modern pistol.
However, after the job was done, the parts would not fit back together to allow the handguns to be reassembled - let alone work properly. This because the plating winds up making parts LARGER than they were before plating. Even the old "Loose as a Goose" WWII M1911 would not go back together without major stoning and lapping of parts.
Thank Heaven I had already gone to the S&W Armorer's course because I had to completely re-fit the parts back on that revolver, including the side plate.
What many people don't know is that PLATED guns are normally made with different specs, so all the parts will fit and work together properly. So it is often a VERY BAD IDEA to have a Non Plated Handgun later plated and especially if the people plating the parts are not gunsmiths, themselves.
Gus