Ruger SR-556

Ruger's introduction of its first AR-15 variant about a year ago--the piston-driven SR-556--was cause for an awful lot of excitement on the firearms-related Internet forums I frequent. 

By Greg Rodriguez

Ruger's introduction of its first AR-15 variant about a year ago--the piston-driven SR-556--was cause for an awful lot of excitement on the firearms-related Internet forums I frequent.  Some argued the merits of the piston-driven system over the traditional AR's direct gas impingement operating system, while others were just shocked that a company as conservative as Ruger would introduce an AR at all.  Well, the truth is, the SR-556 isn't the first Ruger offering to be inspired by a military firearm.  The Ruger Model 77 was inspired by the Mauser '98.  The Ruger Mini-14 was inspired by the M1 Garand and M14.  And one can even make a case for suggesting that the Ruger Standard 22 Auto pistol and the Security-Six double-action revolver were both inspired by military guns. 

Ruger's new SR-556 carbine was strongly influenced by the military M-4 carbine.  But, just as it did with the Mini-14 and Model 77, Ruger wanted to improve on the original design.  So rather than producing yet another AR from a parts kit, Ruger's engineers set out to make a totally new AR-15 that would run with relentless reliability. 

The main knock on the AR is its direct gas impingement operating system, which vents unburned powder and gases back into the rifle's action.  This causes a great deal of fouling, sends noxious gases back into the shooter's face, and makes the action very hot.  That heat helps bake on fouling, and the combination of excess heat and fouling adversely affects reliability.  Keeping the gun clean will help prevent this problem, but the middle of a firefight, when heat and fouling are most likely to cause a malfunction, is not the time and place to be cleaning your carbine. 

The obvious fix for the direct gas system's problems is a piston operating system.  Several makers have introduced piston-operated guns or aftermarket piston systems that vent gas and unburned powder out the front of the gun.  Some of those systems are pretty good, but the folks at Ruger decided to design a gas piston operating system of their own. ( Go to shootingtimes.com for a detailed description of Ruger's SR-556 piston system.) 

The SR-556's piston system disassembles easily for cleaning.  Note that instead of a carrier key, the bolt carrier has a bump machined into it.  The transfer rod strikes this protrusion and drives the bolt backwards in the firing process. 

The SR-556 is built on a forged, aircraft-grade aluminum lower.  The controls are nothing new to anyone familiar with the AR platform.  A standard safety and bolt release are in the usual location on the left side of the receiver, and the fenced magazine release is on the right.  The grip is a soft, rubber model from Hogue.  The trigger is a standard, single-stage unit.  A six-position, M-4-style collapsible stock is also standard. 

The SR-556's upper receiver is also machined from an aircraft-grade aluminum forging.  Its case deflector and A2-style forward assist are in the usual locations.  The upper is a Picatinny-spec, flat-top design to facilitate the mounting of sights or optics. 

SPECIFICATIONS

Manufacturer:

Sturm, Ruger & Co. Inc., www.ruger.com

Model:

SR-556

Caliber:

5.56mm NATO/ .223 Remington

Barrel:

16.12 in.;  rifling: six grooves, 1:9 RH twist

Overall Length:

32.75-36.00 in. 

Weight, empty

7.94 lbs. 

Stock:

Six-position collapsible M-4 type

Length of Pull:

10.25-13.5 in. 

Finish:

Matte black

Sights:

Troy Industries flip-up front and rear; integral Picatinny rail

Trigger:

7 lbs.,  15 oz.  pull (as tested), single-stage

Safety:

Two position

Price:

$1,995

The SR-556 has a 16.25-inch, hammer-forged barrel of mil-spec 41V45 steel chambered for the 5.56mm NATO round.  The barrel is chrome-lined for durability and has a 1:9-inch twist rate, which is ideal for stabilizing bullets of 50- to about 69-grains.  The barrel measures 0.70 inch at the muzzle.  It is tipped with an unusual muzzle brake reminiscent of the brake on Ruger's full-auto AC556. 

The SR-556 comes standard with a quad rail handguard system from Troy Industries made specifically for the SR-556.  The handguards have circular vents for cooling, and the accessory rails are Picatinny-spec and numbered to ensure consistent placement of lights, sights, lasers, vertical foregrips, etc.  The top of the forward-most portion of the handguard is cut out for the gas block.  The sides and lower portion of the handguard extend forward and enclose the gas block for a very clean, nicely finished look.  Four soft rubber rail covers protect your hand from sharp edges and heat. 

Relentless reliability and fine accuracy make the SR-556 one serious fighting gun.  The author's well-worn 3X30 Trijicon ACOG was the perfect match for the little carbine. 

That Troy Industries handguard is not the SR-556's only value-added feature.  It comes standard with three of Magpul's outstanding 30-round polymer PMAGs; a set of Ruger-branded flip-up iron sights from Troy Industries; and a black, nylon carrying case. 

Rapid-Fire ReliableSince piston guns are touted as the answer to AR reliability issues, I devoted the majority of my testing to trying to make the SR-556 fail.  To stack the odds in my favor, I brought the rifle and 600 rounds of assorted 5.56 and .223 ammunition to the local police range for a rapid-fire reliability test.  I thought about running the gun dry, but I added a few drops of lubricant in the interest of fairness. 

In hopes of heating up the rifle, two buddies and I took turns loading magazines and shooting the SR-556 as fast as we could find the target and break the shot.  We fired everything from 55- and 62-grain 5.56mm military rounds to 69-grain .223 match ammunition.  The rugged carbine fed, fired, and ejected every load flawlessly and hit where we pointed it. 

At the end of my firing session, I took the rifle apart and examined it.  The bolt was cool to the touch, and it wasn't near as dirty as a direct gas gun would have been after digesting the same number of rounds.  And the little bit of soot and crud that had accumulated on the bolt wiped right off. 

They have always been known for reliability, but mediocre-at-best accuracy has always been the knock on piston guns.  The Ruger hit where we pointed it and seemed to group well during the reliability portion of the testing, but I cleaned it thoroughly and returned to the range to see how the SR-556 performed from the bench. 

The SR-556's adjustable gas regulator has four positions.  The knob is knurled and has a through-hole that allows a cleaning rod or punch to be inserted to assist in turning a sticky regulator. 

I tried loads from Black Hills, Federal, Hornady, and Winchester in my accuracy evaluation.  All shot very well, and three loads averaged less than 2 inches for five, five-shot groups.  Winchester's 55-grain Ballistic Silvertip load was the accuracy champ, with a five-group average of 1.38 inches and a best group of just 0.89 inch.  That's pretty impressive accuracy for an AR-15 with a 3X optic intended more for CQB than bench work. 

The Ruger SR-556 I tested earned high marks across the board.  Its overall quality is on par with any of the top-name makers, and its accuracy is as good as I could reasonably expect from any AR carbine.  Value-added features like the Troy Industries sights and handguard also make the new Ruger really stand out.  Fit, finish, and overall quality were outstanding.  All the controls worked smoothly and engaged positively, and the trigger pull was fairly crisp and clean despite a pull weight of 7 pounds, 15 ounces. 

The SR-556 isn't a cheap gun, but its combination of accuracy, reliability, and added-value features make it more than worth its $1,995 retail price. 

RUGER SR-556 ACCURACY

Factory Load

Velocity (fps)

Standard Deviation (fps)

Best Group - 100 yard Accuracy (in.) 

Average - 100 yard Accuracy (in.) 

.223 Rem. 

Hornady 55-gr.  TAP

3042

16

1.22

1.65

Winchester 55-gr.  Ballistic Silvertip

2996

19

0.89

1.38

Black Hills 60-gr.  V-Max

2874

21

1.73

2.47

Federal 69-gr.  Match

2805

15

1.14

1.80

WARNING: The loads shown here are safe only in the guns for which they were developed.  Neither the author nor InterMedia Outdoors, Inc. assumes any liability for accidents or injury resulting from the use or misuse of this data.  NOTES:Average accuracy is for five, five-shot groups fired from a sandbag benchrest and a rear bag.  Velocity is the average of 15 rounds measured 15 feet from the gun's muzzle with a Shooting Chrony chronograph. 

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