How a Mossberg 500 Shotgun Works

Wikipedia: The Mossberg 500 (M500) is a series of pump action shotguns manufactured by O.F. Mossberg & Sons.[1] The 500 series comprises widely varying models of hammerless repeaters, all of which share the same basic receiver and action, but differ in bore size, barrel length, choke options, magazine capacity, stock and forearm materials. Model numbers included in the 500 series are the 500, 505, 510, 535, and 590. The Revelation 310 and the New Haven 600 were also variations of the 500 series produced by Mossberg under different names. By 2021, 11,000,000 M500s had been produced, making it the most-produced shotgun of all time.[2]

Designed in 1961 by Carl Benson, the Mossberg 500 was created mainly for use by hunters, but quickly found itself in use by law enforcement because of its reliability and low cost.[2] In the 1970s, the M500 was submitted for military use, but failed to meet the MIL-SPEC 3443E protocol,[2] which involves firing 3,000 rounds of 20-gauge shells with no more than two malfunctions.[3] During the testing, some M500s were created to better match the MIL-SPEC, which came at an increase in cost.[2] M500s began to be purchased by the US military in limited numbers, with the first order consisting of approximately 5,900 shotguns for the Navy and Coast Guard.[4]

To maintain its low cost, Mossberg continued to produce the M500 for civilian and police markets, and in 1987, designed the M590 specifically for military use.[2] In response to a Navy request, the M590 was modified, with a thickened barrel, extended magazine, barrel shroud, and bayonet lug. The new model, designated the M590A1, became the standard shotgun in the United States military.[2]

Replacement for the M500 came as early as 1999,[5] when the Marine Corps adopted the semi-automatic M1014 Combat Shotgun, which became standard issue by 2001.[6] In 1997, the Army began to develop the M26 Modular Accessory Shotgun System, an underbarrel shotgun designed for usage on the M4 carbine, and which could also be used as a standalone weapon. The MASS first entered service in 2010.[7] Despite the competing shotgun designs, including with other pump action models such as the Remington Model 870 and Winchester Model 1200, Mossberg shotguns continued to see usage in various American military branches until the end of the 2010s.[8][9]

M500 and M590 shotguns were first used during the 1991 Gulf War,[4] wherein they were used to defend US supply depots.[10] In the wars in Iraq and Afghanistan, the M500 shotguns were primarily used for breaking locks and hinges during door breaches.[11] In 2009, U.S. Army Special Forces Groups procured Military Enhancement Kits to provide a standardized shotgun configuration based on the Mossberg 500.[12] The kits included a collapsible stock, "shotgun retention system", 1913 receiver rail, fore end rail system and breaching barrels. A total of 1301 shotguns were converted with the first unit being equipped in July 2009. The majority of the kits convert the standard issue shotgun to a 14" compact model with a 16" accessory breaching barrel, H92239-09-P-0113.[13]