Sturm, Ruger & Co. has a long tradition of offering modern interpretations of classic firearms. However, it was only in recent years that Ruger delved into the exceedingly popular AR market here in the United States.

The result was a series of rifles that were ideal for the LE professional looking for a high-quality AR patrol carbine or duty rifle that could be counted on to perform when needed. And, in classic Ruger form, the new offering was not simply a direct copy, but rather an innovative new take on the popular design. The result was the SR-556 series of 5.56mm rifles, featuring the basic overall design of the AR-pattern rifle but with a unique twist: a short-stroke piston system of operation.

In the original AR-15/M16 design, the rifle was operated by a direct gas impingement system. In this system, gas is tapped from a port in the barrel, located in the gas block/front-sight base area, and is driven rearward through a hollow tube to impact directly against a gas key affixed atop the bolt carrier assembly. This force would drive the bolt carrier rearward to cycle the action. The benefit of this system was light weight and often good accuracy. A potential downside of this system is that any extra gas and fouling is dumped directly into the receiver.

Ruger instead incorporated a piston-operated system of cycling, so all of this gas and fouling is kept forward of the action and above the barrel. Gases now simply impact against the face of the piston and do not travel back into the action. Additional fouling or gas is vented out of the bottom of the gas block, and the piston system and the bolt carrier are both chrome-plated for corrosion resistance.

Driving Forces

The SR-556’s piston system is designed to interface with a strike place machined integrally to the bolt carrier. The force of this impact causes the bolt carrier assembly to drive fully rearward and cycle the action.
Although there are several piston-operated ARs on the market today, Ruger’s is unique in that it employs a two-stage piston. The two-stage element of the piston is designed to offer a smoother recoil impulse, accomplished through a smaller, tapered projection on the piston’s face.

Gas impacts this smaller piston first, which then moves rearward to allow more gas through the larger opening to impact against the larger diameter of the gas piston. This results in a theoretically smoother recoil impulse, by opening the action with a small amount of gas at the start and before the full force of the gas pours through to fully cycle the action.

The gas system features a regulator knob with four positions—0, 1, 2 and 3. The 0 position completely blocks the gas port and effectively makes the rifle a single-shot. Settings 1 through 3 range from smaller to larger ports that allow incrementally more gas through to cycle the rifle under a variety of conditions. The owner’s manual indicates that most commercial ammunition will cycle reliably with the regulator set at the 2 position. Adjusting the knurled regulator is accomplished by either turning it by hand or inserting a tool into the crosshole.

Beyond the gas system, the rest of the SR-556 is quite similar to any other standard AR-pattern rifle—albeit a very well-appointed one. The carbine features a split upper/lower receiver system, as does the AR, with the lower being configured in the standard AR pattern. Both the upper and lower are made from aircraft-grade aluminum with a matte black hardcoat anodized finish. All controls are exactly where you would expect them to be, from the manual safety to the magazine release to the bolt catch lever. The upper receiver features the familiar forward-assist/shell-deflector configuration as well as a hinged dust-cover door over the ejection port. The lower accepts all standard AR-pattern magazines.

The standard SR-556 comes with an extremely high-quality, free-floated, quad-rail forend system with a top rail that mates up with the flattop upper receiver. The bottom and side rails run the length of the handguard. The rifle also comes with three quality rail covers. The pistol grip is a Ruger-branded Hogue Monogrip with a soft rubberized surface and generous finger grooves. The buttstock is a standard-style, six-position, collapsible unit. The flattop rifle also comes with a rugged set of folding back-up iron sights. The rear unit is windage adjustable, while the front unit is elevation adjustable. The rifle also comes with three 30-round Magpul magazines.

Extreme Duty

Long a fan of Ruger firearms, I jumped at the chance to attend a Gunsite carbine course featuring the SR-556 Carbine variant of the SR-556 series. The SR-556C is basically the same 5.56mm rifle as the standard SR-556, but instead features a 14.5-inch barrel with an integrally machined muzzle brake/flash suppressor to bring the barrel’s overall length to 16 inches. The barrel itself is cold hammer forged and chrome lined, featuring a 1-in-9-inch rate of twist. The result is a carbine that is 1.75 inches shorter than the standard SR-556 and, as the quad-railed-forend version, weighs in at 7.4 pounds. (An even lighter SR-556CLA model with a modular rail forend weighs in at 7.17 pounds.)

It was with this very carbine that I soon set about running the Gunsite course. Over several days in very hot and dusty conditions, I ran the SR-556C through numerous range drills as well as in both indoor and outdoor shooting simulators. I came to appreciate the short length and lighter weight of the SR-556C and was extremely impressed that it ran in those conditions without a single hiccup (or any cleaning or maintenance). Once the class was over, I made the decision to purchase the SR-556C as I was so impressed with its performance.

Once I had the carbine back at home, I took it to the range and put it through a formal testing and evaluation protocol. I had definitely tested its ruggedness and reliability at Gunsite, but wanted to wring it out for accuracy results as well. I equipped the carbine with a Leupold Mark II 3-9x40mm optic and set up on the bench with a selection of Black Hills, Federal and Winchester ammunition. I favored lighter bullet weights due to the 1-in-9-inch twist of the bore. At a distance of 100 yards, I fired three 5-shot groups with each of the three types of ammo. The accuracy of the SR-556C was quite good considering its compact dimensions. It showed a preference for the Black Hills load, with its tightest group coming in at 0.99 inches.

Piston Performer

All in all, I think it is safe to say that Ruger has made a fine addition to its line with the SR-556 series. By bringing the ever popular AR-pattern rifle into its line and giving it a unique Ruger twist with its two-stage piston system, the company has, in my opinion, developed an outstanding patrol carbine for LE officers. I was very impressed with the SR-556C and could see it performing admirably as an LEO’s close-at-hand tactical carbine. With its capable performance and compact handling characteristics, I suspect many officers would agree with me. For more information, visit

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