facilitates easy cleaning and parts replacement, as removing the nut allows removal of the magazine spring and follower. Both the Model 500 and Model 590 have polymer trigger guards and safeties, and standard barrels. The Model 590A1 has an aluminum trigger guard and safety, a heavier barrel, and a bayonet lug (although some versions of the 590 also have bayonet lugs), intended for military use under extreme conditions and rough handling; the metal trigger guard was added in response to the 3443G materials requirements, and the heavy barrel was added at the request of the Navy to prevent barrel deformation if the shotgun caught in the closing of heavy steel ship doors. Some 590A1s have a sling swivel on the back of the bayonet lug, some do not. The 590A1 usually has a swivel mount on the stock even when there is no corresponding mount on the forward end of the weapon. The 590A1 is generally sold through military and law enforcement channels, though in most jurisdictions the and barreled models are available and may be legally purchased by any non-prohibited persons. 590A1s with 14" barrels are Title II firearms, and may be purchased only by non-prohibited persons in NFA-legal states, after completing BATF transfer forms. The riot gun versions of the pistol grip Model 500 (Persuader, Cruiser, Road Blocker, and Mariner) are available with an and barrel depending on magazine capacity. The 590 is only available with a barrel and flush-fit magazine tube. The 590A1 is available with a , 18.5-inch, or barrel. One model sold as Model 590-SP (catalog item 51663) is technically a 590A1, as it uses the heavy barrel and military trigger group, but unlike models designated 590A1 it is sold on the civilian market. Unlike Model 500 and 590 shotguns (with the exception of ghost-ring sight 590 models), Model 590A1 shotguns cannot be easily fitted with the common factory 500 heat shield, due to the heavier barrel. A heat shield and bayonet lug are required for military 3443G Type I riot shotguns, and some 590A1s are so equipped. The 590A1 heat shield, p/n 16335P, is available for purchase from Mossberg for about the same price as the 500 heat shield. Bantam and Super Bantam models The standard Model 500 uses a length of pull (LOP) for the stock, which is suitable for adult shooters of average or greater size. The Bantam models use a LOP