you can unlock it with you're first killstreak award unlock
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You unlock it by getting to 10th Prestige.
you cannot unlock them you have to buy them on PSN or LIVE Marketplace
Yes, it is the final gun to unlock in the game.
ak47
Nothing happens other than getting the achievements and trophies in modern warfare 2. But in modern warfare 1, you get to unlock certain cheat codes that can be used during the game.