A new trailer released for Modern Warfare 3 shows how in multiplayer mode gamers will also be able to level-up their weapons in addition to the traditional levelling-up of their characters. The levelling-up of weapons are called "Weapon Proficiencies".
The video provides the details but basically gamers, as they play and rack up points for their weapons (weapons have experience bars), will be able to upgrade or change the way that weapon behaves to correlate more with their play style.
For example, a player can adjust their weapon's kick after they achieve the right level-up. There are others like reduced sway and some that are unique to each class, for example the Assault class can now also get the 'hold breath' option like the sniper class.