Even your weapons will have experience bars in Modern Warfare 3. The unlock system has been revamped , and will work differently depending on which of the three classes you play as, but the perks and point-streak rewards will be separate from the weapon unlocks. You'll be able to earn new attachments by fighting and killing with a weapon. See the devs explain the system in the new trailer above, spotted on Evil Avatar. Infinity Ward say that the unlocks will be geared towards giving players more options in combat, though some, like the weapon stabilising upgrade, will probably just make you better.
If guns can level up, it feels as though they're starting to become RPG characters in their own right. We're hoping for loyalty side quests and proper story arcs come Modern Warfare 4.