M363 Remote Projectile Detonator

The unidentified UNSC pistol, or rocket pistol, is a single-shot, muzzle-loaded rocket/grenade launcher and has been seen only in the Halo 4 teaser. John-117 used it to create an opening in the wall, a way out of the disintegrating.

Weapon dynamics
The "Unidentified UNSC Pistol" seems to fire a propelled adhesive rocket or grenade. The projectile leaves a telltale trail and sticks to objects, rather than bouncing off or exploding on contact. 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 experience using the 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.
 * The handle on this weapon closely resembles a SMG handle.
 * On the teaser, Master Chief reloads the pistol. On the old teaser,He throws the pistol away.

Appearances

 * Halo 4 Teaser at E3 2011 )