Re: AR15 BARRELS ???
Wow, that's a lot of enthusiasm.
With Chrome Moly you are going to have to have a lot more concern with rust than with Chrome lined (where the chrome lining keeps the moisture away from the carbon steel) or Stainless (which isn't as prone to rust.)
My choices are Stainless, then chrome lined, and then chrome moly. If I'm paying my good money then it's not going to be for chrome moly.