The Ruger SR-556 semi-automatic rifle is based on a gas-operated AR-15 design.
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The Ruger SR-556 is a relatively new commercial market addition to the Armalite AR-15 family of automatic, self-loading rifles which originated in the 1950s under the direction of Eugene Stoner and produced the venerable M16 assault rifle for the US military (among other variants). The SR-556 was the first AR-15 style firearm manufactured by Sturm, Ruger & Company, Incorporated and mimics the original in style and arrangement to an obvious extent. One key differentiating feature between the SR-556 and the original AR-15 approach is use of a two-stage, chrome-plated, piston-driven gas-operated system with rotating bolt - a departure from the AR-15's original direct impingement-rotating-bolt method of operation. The SR-556 is marketed to law enforcement and military parties and has proven popular in the civilian market as well - though it carries a relatively steep entry price.
Ruger's approach to the SR-556 is such that fouling is minimized through use of a venting system for the built-up gas. The resultant gas is vented through the bottom of the gas block which clears the bolt carrier and is designed to keep said carrier free of contamination while additionally being easy to clean. As such, the Ruger SR-556 is marketed as a highly reliable, gas-operated firing solution in direct competition with all other AR-15-style firearms and users have noted its exceptional accuracy (even for AR-15 beginners) and strong "out of the box" performance.
The SR-225 is primarily chambered for the .223 Remington cartridge as well as the 5.56x45mm NATO cartridge while support for the 6.8 Remington was later announced. The weapon showcases a weight of 7.9lbs (3.6kg), slightly heavier than most AR-15s on the market, and features a collapsing stock which allows the 36 inch length to be compacted down to a handier 32.75 inches. The collapsing stock is an M4 Carbine-style design with six preset positions. The Mil-spec 41V45 barrel assembly measures in at 16.2 inches and sports six grooves while being cold hammer-forged and chrome-lined in its construction. The firearm is designed to accept STANAG curved detachable box magazines (MAGPUL PMAG) in 10- or 30-round counts depending on the variant in play (see final paragraph). Troy Industries "Folding Battlesights" are flip-up, low-profile iron sights fitted along the top of the receiver and atop the handguard for ranged accuracy. Like other rifles, the SR-225 is designed to be fired from the shoulder in a conventional fashion - the primary operating hand on the pistol grip and the supporting hand under the handguard ahead of the magazine well to complete the three-point hold. The gas cylinder is mounted above the barrel and is capped by a 4-position chrome-plated gas regulator to control pressure.
The SR-556 is currently marketed in three major production forms divided into their respective subvariants. The "SR-556 Standard" includes the "SR-556FB" with collapsible stock and 30-round magazine and the "SR-556SC" with 10-round magazine sans slotted muzzle brake - both available in black synthetic finish. The "SR-556 Carbine" - a shortened form of the base SR-556 Standard - is available as the "SR-556C" and "SR-556CLA", the former weighing 0.23lbs more than the latter and differentiated by its redesigned handguard and full-length accessory rails. The "SR-556 E-Model" is available as the "SR-556E" with collapsible stock in 30-round magazine form and the "SR-556ESC" in 10-round magazine form sans the slotted muzzle brake. Both field a single accessories rail across the receiver top while the handguard is cleanly-designed with slotted perforations for cooling.