The Remington Model 870 is an American-made pump-action shotgun chambered in 12 gauge.
The Remington 870 was designed to directly compete with the very successful Winchester Model 12, and as such was a modern, streamlined, rugged, reliable, and relatively inexpensive shotgun. The 870 features a bottom-loading, side ejecting receiver, tubular magazine under the barrel and internal hammer. The weapon is used to this day by several US military and police forces, as well as UK special forces.
In Aliens, cut-down Remington 870 shotguns were used for the pump-action grenade launchers on the M41A Pulse Rifle, although the weapons were concealed within the shroud and foregrip of a Franchi SPAS-12.
In Alien3, the M41A returned, and although the grenade launcher was not used in the movie, the live-firing weapon used during filming was the so-called "Delta" prop from Aliens, and as such still featured a working Model 870 inside it.