Finish the single-player campaign in SOCOM 3 on Ensign or higher and link your games via Crosstalk.

How to Unlock - Unlockable
Finish the South Asia area of operation in SOCOM 3 - ZX25 Burst
Finish the Poland area of operation in SOCOM 3 - STG 77
Finish the North Africa area of operation in SOCOM 3 - 9mm SMG