M363 Remote Projectile Detonator

The Rocket Pistol (name unconfirmed) is a single-shot, muzzle reloaded rocket or grenade launcher. It has only been recently seen in the Halo 4 Teaser Trailer, with Master Chief using it to blow up a door on the UNS ''Forward Unto Dawn. ''It is presumably a UNSC weapon due to being found on a UNSC ship.

Weapon Dynamics
The Rocket Pistol seems to fire a rocket propelled adhesive rocket or grenade, as can be seen by the tell tale rocket trail behind the projectile and how it stuck to the door of the ship, rather than bouncing off or exploding. When fired, a back portion of the pistol raises up. This is the detonator that is used to remotely detonate the rocket/grenade by pushing it back down onto the pistol, pressing the button or switch that causes the rocket/grenade to explode.

The weapon holds only a single projectile at a time, and is reloaded by sliding a new grenade/rocket onto a rod on the front of the pistol, technically making it a muzzle loading weapon.

Trivia

 * There is not yet an official name for the Rocket Pistol.
 * It seems that Master Chief has experience using the Rocket Pistol, as can be seen how fluidly he operates and fires it in the teaser trailer. This may mean that it may be an older weapon that saw service during the Human-Covenant War, or possibly even earlier.