It's highly unlikely the receiver has been nitrided unless it was done "after-market", and most likely by Art's.
If there is a silver finish on the receiver, it's probably nickel plating or hard chrome.
Stripping the hard chrome, in my understanding, is easier to strip than nickel. Nickel plating has to be stripped with acid dipping. Stripping the nickel could run $100-$150. That's if it's done properly. And by properly, the stripping process has to be monitored throughout the whole process. If the part is just dipped and left, you'll have a nasty pitted receiver, that may never clean up.