That selection is paltry compared to the game's full arsenal, and players will initially need to tolerate that scant selection until they play the game a bit and eventually get to the good stuff. In order to unlock every gun (and most other items) in Modern Warfare 2, players first need to level up.
Unlock three new free functional weapons in Sectors F4, F7, and F18, respectively: the ISO 9mm SMG, the TR-76 Geist Assault Rifle, and the Dual Kamas Melee weapon.