I don't know how practical it would be for you to send a gun to the US to do it, but here is a recommendation from my side - Dave Norin a very good friend does superb work. There are some British gunmakers in the US who use his services to get their restoration projects case hardened. He is especially good at recreating the look of traditional British guns and Mausers, Enfields and Model 70s are the only bolt actions that he works on.

Do check his website out, too.

Cheers!