The HK51 is an extremely short carbine based on the G3 rifle, developed by Heckler and Koch in Germany. It fires the powerful 7.62x51mm NATO cartridge but features a barrel shortened to approximately 21 centimeters, making its overall length significantly shorter than a standard G3.
The severe barrel shortening gives the HK51 extremely heavy recoil, large muzzle flash, very loud report, and a high rate of fire despite its compact dimensions. These characteristics make controlling the weapon, especially in full-auto mode, highly challenging.
The HK51 never entered mass production and was instead manufactured in limited numbers for certain special forces units and security forces, as well as the custom market. It uses the same 20-round magazines as the G3.
Today, the HK51 is regarded as one of the rarest and most distinctive members of the G3 family, delivering the high firepower of the 7.62 NATO round in an extremely small package, albeit at the cost of difficult handling for the shooter.