The "HK51" is blanket term for a compact variant of the Heckler & Koch G3 rifle. They are not officially made by Heckler & Koch, and are generally G3 rifles converted with short barrels and collapsible stocks, making them 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.
"A cut-down G3A3 which has been modified to take MP5 furniture and accessories. Smaller size makes this weapon maneuverable in tight quarters, but recoil is harder to control."
— In-game description
The HK51 is a purchasable Battle Rifle for the CriminalEnforcer 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.