User blog comment:JoeRamirez/Call of Duty Modern Warfare 3 Weapons Wishlist./@comment-4056988-20110612230518/@comment-2089069-20110613070421

The "A" in a gun model name is as you say "advancement". When a new version of the weapon is out they give it the A name like M4 is to M4A1 and MP7 is to MP7A1. It's just to make the guns different from their older versions.