Unlockable multiplayer weapons are based on your Rank you receive. As you gain in rank, you will gain the ability for more and more weapons.

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