As above, the best way to strip chrome and nickel is to have an experienced plater DE-plate the metal.
They basically reverse the plating process and it removes the chrome or nickel.
Then they de-plate the copper under coat.
This is really the only way to go if you intend to have a chemical finish like bluing or parkerizing done after the plating is off.
There are other methods, but they often don't get ALL the plating out of the pores of the metal, and when you apply a chemical finish, the metal looks like it has white freckles.
Here's a couple of places that specialize in gun refinishing.
Any one of these should be able to do it, and do it RIGHT.
http://www.fordsguns.com/ (They offer just about all gun finishes).
http://www.apwcogan.com/Greetings.htm (TOP company, does it all).
http://www.techplate.com/firearms_pl.htm