This developer diary for Modern Warfare 3 discusses the changes that have been made to the progression system. Leveling now extends to individual weapons, and going a step further, players can further customize their firearms with proficiencies that can alter kickback, melee speed, and more.
Internet-based discussion can only be civil in this post-Battlefield 3, pre-Modern Warfare 3 period. If I say that phrase three times in front of a mirror, maybe it will come true. If not, my backup plan is to call upon the Candyman.