In single player Co-op Story mode, some of the guns unlocked in Career mode will not be not accessible (for example, the Desert Eagle and SPAS 12); however, the terrorists in Co-op Story mode will use them. To unlock those weapons, play a mission in multiplayer Co-op mode. Pick up the terrorists' guns that are left behind after they die. Finish the level with them to unlock them at the equipment menu. You can unlock the SPAS 12, Desert Eagle, and MG36.