Mossberg 500 is a series of pump action shotguns manufactured by O.F. Mossberg & Sons. 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. Introduced in 1960, all model 500s are based on the same basic concept designed by Carl Benson. Originally using a single action bar that was known to bind and even break, this was changed to dual action bars in 1970, following the expiration of Remington's patent on the double action bar design. A single large locking lug is used to secure the breech. The magazine tube is located below the barrel, and is screwed into the receiver. The slide release is located to the left rear of the trigger guard, and the safety is located on the upper rear of the receiver (often called a "tang safety").