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

Unlockable	How to Unlock
.357 Magnum	Reach level 49
Arisaka	Reach level 4
BAR	Reach level 4
Browning M1919	Reach level 61
Clan Tag	Reach level 11
Colt M1911	Reach level 3
Custom Class Slot 10	Reach Prestige Level 10
Custom Class Slot 6	Reach Prestige Level 1
Custom Class Slot 7	Reach Prestige Level 2
Custom Class Slot 8	Reach Prestige Level 4
Custom Class Slot 9	Reach Prestige Level 7
Double-Barreled Shotgun	Reach level 29
DP-28	Reach Rank 13
FG-42	Reach Rank 45
Gewehr 47	Reach Rank 6
Kar98K	Reach Rank 41
M1 Garand	Reach Rank 17
M1A1 Carbine	Reach Rank 65
MG-42	Reach Rank 33
Mosin-Nagant	Reach Rank 21
MP-40	Reach Rank 10
Nambu	Reach Rank 1
PPSh 41	Reach Rank 53
PTRS 41	Reach Rank 57
Springfield	Reach Rank 3
STG-44	Reach Rank 37
SVT-40	Reach Rank 1
Thompson	Reach Rank 1
Tokarev TT38	Reach Rank 21
Trench Gun	Reach Rank 2
Type 100	Reach Rank 25
Type-99	Reach Rank 1
Walther P38	Reach Rank 2