M301 grenade launcher
From Halopedia, the Halo wiki
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 the M6634 flamethrower. In service with the UNSC Marine Corps, the M301 grenade launcher is commonly given suppression, anti-armor, and anti-vehicle roles.
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. The M301 grenade launcher is capable of firing 40mm grenades at targets within a 99-foot (30 meters) zone, and up to 164 feet (50 meters) vertically—depending on wind, trajectory, and skill. The grenade launcher's rare tendency to detonate in the hands of unskilled operators has led to the M301 seeing limited use. It has seen increased use within the last hundred years.
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