M301 grenade launcher

"The only thing I can think of that would make the MA5C any sweeter would be an under-slung shotty or a forty mike mike."

- Anonymous UNSC Serviceman

The M301 40mm Grenade Launcher, also known as the "forty mike-mike" among troops was a United Nations Space Command weapon designed to fire explosive grenades. It can be attached to the MA5B and MA5C Assault Rifles and NA4 Flamethrower.

In assault rifles, the launcher is fitted under the barrel, replacing the flashlight attachment standard to the MA5 series. Hellbringers were equipped with this to launch flashbang grenades used to stun and surprise enemy troops.

Trivia

 * The M301 wasn't confirmed as a weapon attachment until the release of the Hellbringer action figure, clearly sporting the grenade attachment that wasn't seen in previous artwork.
 * The M301 may be related to the modern-day M203 grenade launcher, an underslung weapon that could be attached to assault rifles and other weapons. Since Hellbringers don't appear to carry an entirely different weapon apart from their NA4 Flamethrower, the M301 was indeed more likely a grenade launcher attachment rather than a stand-alone weapon.
 * It was originally meant to be part of the MA5B during the development phase of Halo: Combat Evolved, while it was still intended for the PC. As such, its canonicity was debatable until mentioned by Bungie in a description of the MA5C.

List of appearances

 * Halo Wars