The Heckler & Koch HK51 is a compact variant of the Heckler & Koch G3 rifle. It has a short barrel and collapsible stock, making it more maneuverable in close quarters. It fires the powerful 7.62x51mm NATO round, giving it high firepower, but the rifle's short length gives it high recoil and limited range. The HK53 later succeeded the HK51.
— In-game description
The HK51 is a purchasable Battle Rifle for the Criminal Enforcer kit. It is a downsized G3A3, and has high maneuverability in close quarters, but its heavy recoil is difficult to control due its small size. A Stock can be attached to reduce the weapon's recoil. A Muzzle Brake or Compensator are also well-suited to this weapon.