According to Nitride providers you are supposed to completely assemble the gun and fire approximately 40 rounds for break in. Then disassemble and have the barrel nitrided. It is an alternative to leaving a barrel stainless color. It's supposed to extremely prolong barrel life and be extremely corrosive resistant. As mentioned earlier, about 60 Rockwell, about .005" deep.
If it's done correctly you shouldn't have to re-chamber. Articles on shooter's forum talk of AR chambers with almost 12,000 rounds with little to no wear in the throat area or rifling.
http://blacknitride.com/index.htmlhttps://sites.google.com/site/freeballisticcalculator/black-nitride-melonite-and-qpq 
