The latest trailer for Call of Duty: Modern Warfare 3 reveals a new feature in the game which will be called “weapon proficiency”. Long story short: you will now be leveling up your individual weapons in addition to your individual player rank.
Each weapon in the game now has its own experience bar which builds as you use weapons and become more adept to using them in battle. The better you do with a given gun, the better it performs in the game. Some of the things that were previously available as individual perks to players such as the option to increase your aiming when shooting from the hip and better penetration for your bullets now reverts to being a trait for your specific guns. If you want a better gun, then you need to work for it.
Remember Call of Duty fans, just over a week to go until launch!