Silver nitride, the coating, is probably a trade name. Silver Nitride, the chemical, is seriously unstable. Here's the best lead I've found on what the Citori "nitriding" process might be - like Chuck said but with heat:

http://techplate.com/saltbathnitriding.html

They say it's a sub-critical temperature casehardening in a molten salt. I've wanted to blue my well used 'silver nitride' Citori since it's also tarnished in the carry areas. I back burnered it as a cosmetic issue, but if someone figures out the whole thing I'd lazily follow along.

I think SKB used several finishes and coatings over the years. If tanky's gun is peeling it's obviously plated. But why silver plate a gun.. in a metal that tarnishes?