Press "R" for radio then any of the following codes:

 PIGSOINK - Max Overkill (Overload Weapons)
 HARMONY  - GPS Hellfire
 6969     - Mercenary
 X666     - Kill Teammate
 IMARAT   - Invisible
 IMACOW   - Time freeze
 LOADME   - Reload
 FIXME    - Fix Damage
 IPIG     - unlimited Ammo
 BAT9     - More Hellfires