During gampeplay press '~' to display the console window and then type the following codes to activate the corresponding weapon.
Unlock M468 Assault Rifle (Long Range): This 6.8mm assault rifle which is a favourite of US Navy SEALs is a high powered weapon and fires 750 rounds per minute. To enable it enter the code 1y3a4x2b442x
TAR-21 Assault Rifle (Close Range): This 5.56mm Israeli built assault rifle which stands for 'Tavor Assault Rifle - 21st Century' and is a compact and accurate weapon which fires 850 rounds per minute. To enable it enter the code 3311xbxby11y.