Reach a certain rank on online multiplayer to achieve each unlockable.

Unlockable	How to Unlock
Browning M1919	Unlocked at level 61
Clan Tag	Unlocked at level 11
Custom Class Slot 10	Reach Prestige Level Ten
Custom Class Slot 6	Reach Prestige Level One
Custom Class Slot 7	Reach Prestige Level Two
Custom Class Slot 8	Reach Prestige Level Four
Custom Class Slot 9	Reach Prestige Level Seven
Double-Barreled Shotgun	Unlocked at level 29
DP-28	Unlocked at level 13
FG42	Unlocked at level 45
Kar98k	Unlocked at level 41.
M1 Garand	Unlocked at level 17
M1A1 Carbine	Unlocked at level 65
MG42	Unlocked at level 33
Mosin-Nagant	Unlocked at level 21.
MP40	Unlocked at level 10
PPSh-41	Unlocked at level 53
PTRS-41	Unlocked at level 57
STG-44	Unlocked at level 37
Type 100	Unlocked at level 25