It would probably take a metallurgical engineer to really tell us about melonite and its properties, but the fact remains, the mil spec is hard chrome. Another fact remains. In a quick run down of S&W's "law enforcement" M&P15 product line, I am mostly seeing chromed bores, no melonite. The melonite treatment seems to be reserved for the consumer grade rifles. I must confess though, until your post I thought melonite was an exterior treatment and the OPs bore was unlined.

I've never shot melonite barrels, but do have many rounds thru various chrome lined barrels. A good chrome barrel is accurate, cleans easily, doesn't rust and lasts quite a while.