As with all multiplayer Call of Duty games, in multiplayer - Weapons (as well as perks and challenges) are unlocked simply by increasing your "rank".
when you unlock a weapon - you must then complete challenges with them to unlock attatchments like scopes and silencers (varies per weapon).
To do this, the player must participate in online mathes with other players or with other players on their own console offline. When you complete tasks such as kill other players (on the enemy team!) or caputure command posts in Domination, you are rewarded with experience points. It is useful to note that while challanges in their preliminary stages may not reward much exp - ofteh they have multiple stages, which reward more and more exp when you complete them.
Your rank is determined by the experience you have. As you gain ranks, it requires more and more Exp (experience points) to progress to the next level. The maximum rank on modernwarfare 2 is 70.
After this you have unlocked all weapons and challanges, and have the option of entering "prestigue" mode.
PLEASE NOTE!:
After entering presitgue, although you keep your 'titles' and 'emblems' in modernwarfare 2, when you prestigue you
LOOSE ALL YOUR WEAPONS, ATTATCHMENTS AND PERKS!!!!!
Only presituge if you are absolutely sure you want to do it all again. It still requires the same exp for each rank as it did the first time round, but you have to do it all again. (including challenges.)
To unlock more weapons you need to get to a higher level.
unlocked at lvl 1
The desert eagle is unlocked at Level 62.
lvl 13 i think lol
You unlock it at level 36.
You cant.
No.
you have to reach level 50
Get to Prestige 9.
Well you can't now as it is patched
253 by krishnokov dracnez in russia playing a domination
you can unlock it with you're first killstreak award unlock
You unlock it by getting to 10th Prestige.
you cannot unlock them you have to buy them on PSN or LIVE Marketplace
Sorry there is no alien invasion
Get to rank 70