To unlock a new costume, simply reach the required level on single-player mode. The first two costumes are already unlocked.
Reach the indicated level in Single Player mode to unlock the corresponding weapon:
XM-2119: 1
M4E: 2
P44M: 5
CAC82: 7
TM9V: 11
SAW24: 15
M45R: 19
SPR11: 23
PM6: 26
M94R: 30
AR4A: 32
Guardian II: 34
TR14: 38
P4512: 40
ABG1: 43
TK1B: 46
CPG7: 50
G50: 54
Guardian I: 60
M209: 61
SPR3: 69
MP10: 77
SR308: 85
SG112: 93
XMW21: 99
