O-rings, also called a duck plug, located in the tube magazine. This is to comply with U.S. migratory bird laws. This dowel reduces and regulates the number of shells that can be loaded in the magazine to 2. This can be removed by taking off the barrel, pointing the shotgun downward, and shaking it back and forth lightly until the dowel shows out of the end of the magazine tube. It must then be pulled out, as the O-rings prevent it from just falling out. Except for the Maverick 88, all current production Mossberg models have a pre-drilled receiver for installation of an upper Picatinny rail for mounting various optics such as red dot sights. Some models can be bought with the rail and accessories already installed. Some Mossberg models, like the 590, have a bayonet lug. Maverick Arms subsidiary Mossberg also markets a less expensive shotgun under the Maverick Arms name, the Mossberg Maverick 88, in blued finish, with synthetic stocks, and in appearance, it is virtually identical to the 500 model. Maverick and Mossberg shotguns share many interchangeable parts, but Maverick shotguns differ in some ways, such as lacking sling swivel studs and having cross-bolt safeties instead of tang safeties, which makes the trigger group non-interchangeable with the Model 500 shotgun. The one-piece forearm can be replaced with OEM or aftermarket parts with the addition of an action slide tube, as the factory forearm has the action bars pinned into place. The Maverick series also does not come drilled and tapped for rail mounts like the 500 models. There are two basic models of the 88, the 88 field and the 88 security, with a cartridge capacity of six or eight shots, and they cannot have their magazines easily extended without machining. Model numbers 500A = 12 gauge 500AB = 12 gauge, Dual Action Bar 500B = 16 gauge (has been discontinued) 500C = 20 gauge and 12 gauge 500D = 28 gauge (never went into production) 500E = .410 bore Users Mossberg produced two U.S. military versions of the model 500, the 500 MILS and the 500M MILS, the difference being the location of the upper sling swivel. The model numbers contain a U.S. prefix to the serial number. These particular models (500 MILS & 500M MILS) contain all metal parts, are parkerized with a 20-inch barrel. The magazine capacity is 6+1. : Used by naval infantry