M363 Remote Projectile Detonator

The unidentified UNSC pistol, or Rocket Pistol, or DMG, is a single-shot, muzzle-loaded rocket/grenade launcher and has been seen in the Halo 4 teaser and on an action figure at the 2012 New York Toy fair. John-117 used it to create an opening in the wall while using thruster pack to get out of from.

Weapon dynamics
The "Unidentified UNSC Pistol" seems to fire a propelled adhesive rocket or grenade. The projectile leaves a tell-tale trail and sticks to objects, making it akin to the Covenant plasma grenade. When fired, a trigger/button rises at the back of the pistol and is possibly the detonator. It can be used to remotely detonate the explosive by pushing it.

The weapon holds only one projectile at a time and is reloaded by sliding a new rocket/grenade onto the front of the pistol, making it a muzzle-loaded weapon.

Trivia

 * It is likely that John-117 has had experience using this weapon, since he can be seen fluidly operating it during the teaser trailer. It is possible the unidentified weapon is an old weapon, entering service prior to, or during, the Human-Covenant war.
 * Due to the fact that it is a single-shot weapon, and is muzzle-loaded, it is likely that this Micro Grenade Launcher was originally intended to be something akin to a flare gun.
 * In OXM june magazine It was said that they have got to to test the DMG which might be the rocket pistol, DMG = Debris Mover Gun
 * A leak of the DMG can be seen Below

Appearances

 * Halo 4 Teaser at E3 2011 )
 * 2012 New York Toy Fair
 * Halo 4 ViDoc: Making Halo 4