in Iraq and Afghanistan, M500 shotguns were primarily used for breaking locks and hinges during door breaches. In 2009, U.S. Army Special Forces Groups procured Military Enhancement Kits to provide a standardized shotgun configuration based on the Mossberg 500. 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. Model 500 options The name "Model 500" covers an entire family of pump shotguns designed to chamber "magnum" shells. The standard model holds five or five shells in the magazine and one in the chamber. The Model 500 is available in 12 gauge, 20 gauge, and .410 bore, with the 12 gauge being the most popular and having the most optional features available. A 16 gauge model was introduced in 1963 and later discontinued. Finishes The standard finish for the Model 500 is an anodized aluminum alloy receiver and a polished and blued barrel. Some models come with a matte black matte-anodized receiver, and a matte blued barrel. Some 500 models are anodized to look parkerized, with parkerized barrels. This is also true of the 590 series since an aluminum receiver cannot be parkerized. Mossberg also offers camouflage painted models, in a number of different patterns. Stocks are either wood or composite, with the composite stocks being matte black or camouflage to match the rest of the gun. A special model called the Mariner is available with the Marinecote finish, an electroless-nickel finish that is highly corrosion resistant. Mariner models use the black composite stocks. Model 500 vs. Model 590 vs. Model 590A1 The primary difference between the Model 500 and Model 590 is in magazine tube design. The Model 500 magazines are closed at the muzzle end, and the barrel is held in place by bolting into a threaded hole at the end of the magazine tube. Model 590 magazines are designed to be opened at the muzzle end, and the barrels fit around the magazine tube and are held on by a capnut at the end. The Model 500 magazine facilitates easy barrel changes, as the barrel bolt serves no function other than holding the barrel in place. The Model 590 magazine