Sorry for the slow reply but I just noticed your posting!
We tried coating the inside of barrels with NiB but found no real improvements in wear or accuracy over chrome. We have coated barrels with our Salt Bath Nitrocarburizing (melonite) and we have had great success. I have been trying to stay out of the melonite threads since we are a manufacturer but our melonite is in a class by itself. Go to our plating company website www.kcjplating.com
and take a look at our melonite process. And yes if you ask we are the Gun Division of KC Jones which is the largest commercial plater in the country. And yes we can melonite personnal guns and gun parts.