The K3G4 is a 20 round assault rifle that possessed the capability of penetrating flak jackets. This gun was used in Syphon Filter, 2, and 3, and was classified as a assault rifle, even though it was fired one handed. The design is nearly identical of the PK-102 assault rifle from Syphon Filter and 2, but excluding the stock.
Be aware of your ammo count, more so than any other weapon. The K3G4 will shred through enemies in a split-second but leave you vulnerable to counter-attacks during reloads.
Reserve this weapon specially for flak jacketed enemies when you're in an emergency. It should only be used if your ammunition is zero for every other weapon or if you really have to take down enemies as fast as possible (case in point, when stopping the missile in Syphon Filter).
Aside from the Desert pistols and the Barrett M82 50-cal sniper rifle, it is the only weapon in the game that can kill flak jacket-wearing enemies without headshots, leaving the armour for the player to pick up.