The SR-556 is designed around the original AR-15, however it does have its own unique features. The most significant of these is the fact that the SR-556 utilises a "two-stage" piston mechanism, rather than the direct mechanism used on the AR-15. The use of the "two-stage" piston is intended to result in a smoother action and firing experiance, with four regulators used to control gas flow. The SR-556 also features an integral lug, replacing the gas key of the AR-15 on the bolt carrier.
The SR-556 is fitted with a collapsable stock, designed to improve how the shooter can hold the SR-556 for the best control. This collapsable stock is also fitted with a Mil-spec buffer and spring system in the extension to absorb the effect of recoil on the user. The SR-556 is also fitted with four picatinny rails to allow a wide range of after-market parts to be mounted onto the frame. Factory mounted picatinny rails are also fitted with covers to protect the users hands.
The barrel of the SR-556 is bored with six 1:9 ratio rifling twist grooves. The barrel is chrome lined to protect the inner surfaces from corrosion, with several other components also finished in chrome. The barrel, measuring 410mm (16.1in), is also fitted with a removable flash-supressor to better improve the efficency of the design. The iron sights fitted to the SR-556 consist of a set of folding, adjustable ghost sights, intended to be used if a different set of sights or scopes are not installed.
The SR-556 is chambered, like the original AR-15, for the 5.56mm NATO round. This popular round is fired from an updated version of the STANAG Magazine with a capacity of 30 rounds. Another version of the SR-556 (the SR-556C) is fitted with a 10 round magazine.
The standard version of the SR-556 featuring all of the above features and sold three 30 round magazines. Various components are also chrome plated.
The carbine version of the SR-556, shortened to an overall length of 785mm (31in), without the stock extended. The bolt is chrome plated while the four picatinny rails are modified to include handguards. The SR-556C is (0.5lb) lighter than the original SR-556.
The economic version of the SR-556, with a much lower price than the original SR-556. The SR-556E is also designed around ergonomic considerations, such as extended handguards, grooved pistol grip and rapid deploy sights. A simpler collapsable stock is also included, reducing cost.
The SR-556VT is a modified version of the SR-556, featuring a solid stock and target style (light pull) trigger. The SR-556VT is also manufactured with a tighter 1:8 ratio rifling twist to improve accuracy as well as an extended charging handle to improve the ability to mount scopes.
The SR-556 is not used, in any form, by any military organisation. Instead the SR-556 has found use in the hunting and (lately) target shooting markets, where the recently released SR-556E version has become an economically viable option.
The SR-556 is not the first move Ruger has made into using an existing design and attempting to improve it. In the 1990's, with Uziel Gal at the helm of the design section of Ruger, the Ruger MP9 attempted to reinvent the Uzi, while the 1950's Ruger Blackhawk was heavily based on the Colt Peacemaker.