Mossberg 500 is a series of shotgun

Shotgun

A shotgun is a firearm that is usually designed to be fired from the shoulder, which uses the energy of a fixed shell to fire a number of small spherical pellets called shot, or a solid projectile called a slug...

s manufactured by O.F. Mossberg & Sons

O.F. Mossberg & Sons

O.F. Mossberg & Sons is an American firearms manufacturer, specializing in shotguns, rifles, scopes and firearm accessories. From the 1940s through the 1960s, it also produced a line of .22 caliber target and sporting rifles....

. The 500 series comprises widely varying models of hammerless, pump action

Pump action shotgun

Pump-action shotguns, also called 'slide action repeating shotguns' or 'slide action shotguns' are a class of shotguns that are distinguished in the way in which spent shells are extracted and fresh ones are chambered. The weapon has a single barrel above a tube magazine into which shells are...

 repeaters, all of which share the same basic receiver and action, but differ in bore

Caliber

In guns including firearms, caliber or calibre is the approximate internal diameter of the barrel in relation to the diameter of the projectile used in it....

 size, barrel length, choke options, magazine capacity, and "furniture" (stock

Stock (firearm)

A stock, also known as a buttstock or shoulder stock, is a part of a rifle or other firearm, to which the barrel and firing mechanism are attached, that is held against one's shoulder when firing the gun. Stocks are also found on crossbows though a crossbow stock is more properly referred to as a...

 and forearm

Forearm (firearm component)

In firearms, the forearm is a section of the weapon between the receiver and the muzzle. It is used to hold the firearm steady and is usually made out of wood or composite material...

) materials. Other model numbers included in the 500 series are the 590, 505, and 535.
Introduced in 1961, all model 500s are based on the same basic design.