rounds in the magazine tube, and has a single brass bead front sight with no rear sight. Production began in 2017 with a 12 gauge model, adding 20 gauge and .410 bore models later. Legality In the United States, the Shockwave does not fit the definition of a shotgun as defined by the ATF, which says that shotguns have barrels over in length and are made to be fired from the shoulder. It avoids restriction as a short barrel shotgun under Title II of federal firearm laws by remaining over in length and not being made to have a shoulder stock. The ATF refers to the Shockwave as a non-NFA firearm. While legal without a tax stamp at a federal level, regulations also vary by state, thereby prohibiting purchase in some states. Accessories and combinations The Mossberg 500 has always been marketed as a multi-purpose firearm. Mossberg sells a wide variety of accessory stocks and barrels, allowing many configurations to be made (including, in the past, a bullpup configured model 500). Mossberg is also the only company to ever offer a double-action-only model. The model 590DA offers a longer, heavier trigger pull, to reduce the chance of an accidental discharge by the operator, and was targeted at the police market. With the appropriate parts, the same Model 500 can be a field gun, a slug gun, defensive weapon for civilian, police, or military use, trap and skeet gun, or .50 caliber (12.7mm) rifled muzzleloader. Mossberg has also sold "combination" sets, with a single receiver and more than one barrel. Common examples included a field barrel packaged with an cylinder bore barrel for defensive use, or a field barrel and a slug barrel, or a slug barrel and a .50 caliber muzzleloading rifle barrel. A unique item offered by Mossberg for the Model 500 is a line launcher kit. It uses special blank cartridge to propel a shaft with an optional floating head and a light rope attached to it; a canister hung below the barrel to hold the line spool. A test of the Mossberg 500 with line launcher by the BoatUS Foundation showed an average range of over with the floating head. Distances of are claimed for the non-floating long distance head. All Mossberg models including the 835, 535, 500, 505 and 590 (except for Special-Purpose and Law Enforcement models) are shipped with a wooden dowel with two rubber