HK79

The HK79 an underslung grenade launcher that was cut from Call of Duty 4: Modern Warfare.

Call of Duty 4: Modern Warfare
The HK79 was supposed to be exclusive to the G3, much like how the AG36 was supposed to be on the G36C. However, it was cut from the game like the AG36. The HK79 was available in early versions of the game, confirmed by localization strings in game files.