M319 grenade launcher

The M319 Individual Grenade Launcher (more formally Individual Grenade Launcher, Caliber 40mm, M319), comically known as the "Pro Pipe" by Bungie employees, is a single-shot explosives launcher in use by the United Nations Space Command, specifically the UNSC Army. It has seen action in several armed conflicts, having been in service for nearly three centuries, with the Human-Covenant War as the most prominent. It was used during the Battle of Reach.

Description
The grenade launcher has two firing modes. The first launches grenades at around normal grenade speed, which detonates on impact with an opponent or after normal grenade time after hitting a non-opponent surface. The second firing mode shoots a grenade that explodes when the player wishes. This is performed by holding down the trigger, which fires the grenade but it is merely primed, it will only explode after the trigger is released. This enables the player to lay a trap 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 player dies). Like the Halo: Reach frag grenade a player will take a little damage if hit by the grenade when it is merely primed.

Also, the grenade acts as a form of EMP that could bring down or temporarily disable vehicles, but only when used in the firing mode that allows the player to fire the grenade in a dormant, primed state.

Trivia

 * It appears to be roughly based off of the M79 grenade launcher, a weapon used throughout the Vietnam War. This is due to its single-shot break-action configuration.
 * The functions of the grenade launcher are similar to that of the Demoman's grenade launcher in the Valve multiplayer game Team Fortress 2.