M870

From the Metal Gear Wiki

The Remington 870 is a pump action shotgun which has gained widespread use across civilian, law enforcement and military sectors. The fourth major design of the pump action shotgun by the weapon manufacturer, sales of the weapon have been consistent since it was initially introduced in 1950. There have been hundreds of variations on the M870. In MGS4, although it can be customized with only one attachment it can carry alternate types of ammunition.