Each weapon is unlocked in multiplayer at a certain rank.

Weapon	How to unlock
Assault Rifle - G3	Reach Rank 25
Assault Rifle - G36C	Reach Rank 37
Assault Rifle - M14	Reach Rank 46
Assault Rifle - M4 Carbine	Reach Rank 10
Assault Rifle - MP44	Reach Rank 52
LMG - M60E4	Reach Rank 19
Pistol - Desert Eagle	Reach Rank 43
Pistol - Golden Desert Eagle	Reach Rank 55
Pistol - M1911	Reach Rank 16
Shotgun - M1014	Reach Rank 31
SMG - AK-74U	Reach Rank 28
SMG - Mini Uzi	Reach Rank 13
SMG - P90	Reach Rank 40
Sniper Rifle - Barret	Reach Rank 49
Sniper Rifle - Dragunov	Reach Rank 22
Sniper Rifle - M40	Reach Rank 04
Sniper Rifle - R700	Reach Rank 34