M319 grenade launcher
From Halopedia, the Halo wiki
The M319 Individual Grenade Launcher, more formally known as the Individual Grenade Launcher, Caliber 40mm, M319, is a single-shot, break-action explosives launcher produced by Misriah Armory and used by the United Nations Space Command, specifically the UNSC Army. Due to its simplicity, ruggedness, and firepower, it has been a mainstay weapon in the Army since its adoption in 2263 and has seen action in several armed conflicts during nearly three centuries of use, with the Human-Covenant War as the most prominent. It proved very effective during the Insurrection; although it is less effective against energy shielded Covenant foes, it can be used to deadly effect against unshielded targets. Because of the weapon's reliability and power, it will likely continue to be fielded by the Army. Sometime after the Human-Covenant War, SPARTAN-IVs used this weapon in a simulation against the Flood aboard the UNSC Infinity.
The M319 IGL fires high-explosive dual-purpose grenades. The grenade launcher has two firing modes. The first launches a single grenade: after hitting a surface, the grenade detonates on impact with an opponent or after about two seconds, in which case the grenade will explode after bouncing off the ground once. The second firing mode shoots a grenade that explodes when the user wishes. This is performed by holding down the trigger, which fires the grenade but merely primes it, leaving the detonation contents inert; the grenade will only explode after the trigger is released. This enables the user to lay a trap if the grenade is bounced properly into position, and essentially creates an explosive with a dead-man's trigger, (a trigger that will go off once it is released, e.g. when the user dies). This function is also helpful if the user wishes to fire at targets that are farther away than the detonation timer would normally allow.
Also, the grenade emits an electromagnetic pulse upon detonation, similar to that of the plasma pistol, that can temporarily disable vehicles and energy shields, but only when used in the manual detonation mode.
As can be seen in some early concept art, the M319 was originally planned to be a revolver type grenade launcher like the XM510 grenade launcher seen in Halo Wars. The artist who created this concept art, Isaac Hannaford, revealed that the design was changed to the current single-shot, breech-loaded design because the original was thought to have "too many grenades," which presumably might make it overpowered.
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