The Remington Model 870 is a pump-action shotgun. The 870 features a bottom-loading, side ejecting receiver, tubular magazine under the barrel, dual action bars, internal hammer, and a bolt which locks into an extension in the barrel.
If your Model 870 serial number ends with "M" you have a 12-gauge Magnum receiver that is capable of shooting 2 3/4-inch and 3-inch shells as long as your barrel is chambered for 3" shells.
If your serial number ends with "A", you have a 12-gauge Super Magnum receiver that is capable of shooting 2 3/4-inch, 3-inch and 3 1/2-inch shells with a barrel that is chambered for 3 1/2" shells.
If your serial number ends in "N" or "U", you have a 20-gauge Magnum receiver that is capable of shooting 3-inch shells with the correct barrel.