A list of what you unlock when you advance a level in Multiplayer.

Unlockable	How to Unlock
3x Frag (Perk Slot 1)	Reach Level 41
AK-74U	Reach Level 28
Bandolier (Perk Slot 1)	Reach Level 32
Barret .50 Cal	Reach Level 49
Bomb Squad (Perk Slot 1)	Reach Level 14
Boot Camp Challenges 1	Reach Level 09
Boot Camp Challenges 2	Reach Level 15
Boot Camp Challenges 3	Reach Level 18
Claymores	Reach Level 23
Create-a-Class	Reach Level 04
Dead Silence (Perk Slot 3)	Reach Level 44
Demolitions Class	Reach Level 02
Desert Eagle	Reach Level 43
Double Tap (Perk Slot 2)	Reach Level 29
Dragunov	Reach Level 22
Elite Challenges	Reach Level 51
Elite Challenges 2	Reach Level 53
Elite Challenges 3	Reach Level 54
G3	Reach Level 25
G36C	Reach Level 37
Golden Desert Eagle	Reach Level 55
Gun Challenges	Reach Level 05
Humiliation Challenges 1	Reach Level 42
Humiliation Challenges 2	Reach Level 45
Humiliation Challenges 3	Reach Level 47
Humiliation Challenges 4	Reach Level 48
Humiliation Challenges 5	Reach Level 50
Iron Lungs (Perk Slot 3)	Reach Level 26
Killer Challenges 1	Reach Level 30
Killer Challenges 2	Reach Level 33
Killer Challenges 3	Reach Level 36
Killer Challenges 4	Reach Level 39
Last Stand (Perk Slot 3)	Reach Level 08
M1014	Reach Level 31
M14	Reach Level 46
M1911	Reach Level 16
M21	Reach Level 07
M4 Carbine	Reach Level 10
M60E4	Reach Level 19
Martyrdom	Reach Level 17
Mini-Uzi	Reach Level 13
MP44	Reach Level 52
New Playlists	Reach Level 06
Operations Challenges 1	Reach Level 21
Operations Challenges 2	Reach Level 24
Operations Challenges 3	Reach Level 27
Overkill (Perk Slot 2)	Reach Level 38
P90	Reach Level 40
Prestige Mode	Reach Level 55
R700	Reach Level 34
Sleight of Hand (Perk Slot 2)	Reach Level 20
Sniper Class	Reach Level 03
UAV Jammer (Perk Slot 2)	Reach Level 11