While obviously a helmet Cannon isn't very practical, the U.S. Military in recent years did see the utility in mounting other devices to the helmet, such as lights, radios, night vision goggles, and most notably HMDs, or Helmet Mounted Displays, which could be considered an evolved form of this.

Essentially, HMDs were used by helicopter pilots to aim the gun directly below the cockpit by allowing them to turn their head and aim via motion-tracking, as seen Here. They use a camera on the gun and relay it into a display on the helmet so the pilot can get an accurate view from the gun. Essentially, it's like the Helmet Cannon, but without the issue of recoil.