In the actual movie you never see them using the grenade launcher In the Command Centre sequence when the Aliens break the perimeter two of the marines use the grenade launcher and Hicks demonstrates it to Ripley prior to the facehugger attack in the medlab. The launcher is again used by Ripley in the Queen's Chamber when she destroys the egg sack. The grenade launcher is a 5 shot launcher for impact or timed airburst grenades (similar to ones available to modern military) it works in the same manner as a pump shotgun. The black fore-grip slides back to cock the weapon (for some reason there's an ejector port on the side - probably because it's a hacked up pump shotgun under there) the loader for the launcher is under the grip in the main slide of the launcher. There is a firing trigger for the grenade launcher just in front of the rifle magazine. For the rifle itself, the cocking handle is above the grip and behind the magazine and operated seperately to the grenade launcher.