I have to admit...I don't know what "melonite" is and I've never seen it as an option while shopping for the several ARs I've put together over the last year or so.
Melonite is also known as nitrocarburization, ion bond, etc. Glock and Smith and Wesson both treat their slides with melonite. Smith and Wesson, LWRC and Troy are using melonite on their AR barrels. It is corrosion resistant and surface hardens the treated metal. All in all, it's a good choice for barrel treatment.
Nitrocarbonization, Melonite, Atranite.....etc are metal treatments that work very well just like mtdawg169 said. This is what stainless steel looks like after nitrocarbonization.
Stainless CBOB
After I had Atranite applied. Nice matte black.
Somewhat self lubricating, corrosion resistance, scratch resistant. Competition shooters have their barrels treated to prolong the life of the barrel. Had mine done by Freedom Gunworks in Camilla, Ga. http://www.freedomgunworks.com/services.html