There’s no doubt about it; the AR-15 has gone mainstream. Over the last several years, the growth in demand for black rifles has lured many manufacturers (including the big guys) to introduce their own versions of the venerable Eugene Stoner direct-gas-impingement design. Now that the market has matured, Gun Tests magazine is seeing engineering departments introduce versions that are claimed to improve upon the original AR-15 through the use of gas-piston systems.
They recently tested the Ruger SR-556FB, $1995.
Here’s what they said:
In the original AR-15, gases are vented back through a gas tube to work the bolt and bolt-carrier group. The entire assembly works as its own piston. Since all of the gas is vented back to the bolt, fouling and heat build up in the mechanism. A piston system uses exhaust gas to work an exterior piston and operating rod assembly, greatly reducing the amount of spent gas that reaches the gun’s action. In our tests all three of our guns ran with less fouling than we see with our regular AR-15s.
The Ruger SR556 uses adjustable gas pistons, allowing the user to change the piston setting to optimize its operation for different ammo and operating conditions.
How We Tested
All of our testing was done in the climate-controlled atmosphere of Bass Pro Shops in Grapevine, Texas, sparing us from the ravages of a record cold snap. We started by mounting a Sightmark 3-9 X 42 (SM13016) Scope on a LaRue Tactical LT-104 QD Scope Mount. This lever-throw mount allowed us to move the scope quickly but securely among our flat-top equipped test rifle. The Sightmark had a smaller frame that balanced well with our gun, and featured an illuminated reticle to aid in low-light settings. Our target range was 50 yards.
The second portion of our testing involved utilizing iron sights at a shorter distance of 25 yards. We proceeded to fire 30-round magazines at a range-limited speed of one round per second to evaluate the gun’s handling and responsiveness in a dynamic situation.
After that, we also checked to see how cool the gun ran. The primary claims associated with gas-piston guns are that they run cooler and cleaner than comparable AR-15 direct-gas-impingement designs. It was fairly easy to disassemble the gun to look for carbon build-up, but verifying operating temperatures required a different approach. We devised a test protocol for this measurement through the use of an infrared thermometer. We decided on three points of measurement for our gun: the bolt, the chamber, and the end of the barrel. We verified the room temperature and the surface readings on the gun prior to testing. Then 30 rounds were fired through the gun and surface temperatures were taken immediately thereafter. The Ruger showed temperature changes of 8 degrees and 4 degrees, respectively.
We could see that there could be long-term benefits in the cooler operating temperature of the gas-piston system, although we did not see the AR rise to an exceptionally high level. A more extensive thermal test will be conducted in the near future.
Here’s how the rest of our evaluation turned out:
Ruger SR-556FB Model 5902 5.56mm NATO/223 Rem., $1995
Ruger’s foray into the AR market was announced with a flourish in May 2009. Instead of entering the crowded realm of AR-15 clones, Ruger chose instead to design its own proprietary semiauto piston system and incorporate it into full-featured offering. The SR-556FB comes only one way from the factory and that is fully equipped. It appears that Ruger attempted to cater to the new AR owner by making a number of equipment choices for the buyer.
The SR-556FB sought to differentiate itself from other black rifles right out of the box, cradling the gun in a custom cloth case with the red Ruger logo on full display on its front. The case wasn’t bad, but we’d like a bit more padding and a much heavier zipper than the one on our sample. Our gun also included a pair of Magpul polymer 30-round magazines, a design we found to be quite dependable.
Beginning our inspection, we found that the 16.1-in. barrel was chrome-lined and made of chrome-moly-vanadium steel and (thankfully) topped with an AC-556 flash suppressor. These parts were all manganese-phosphate coated along with the gas block and regulator. The lower receiver was made of aluminum and hard anodized. Ruger chose to chrome-plate the bolt assembly, which made for a nice appearance. The Ruger emblem was also engraved on the bolt as well. In fact we noted the Ruger logo appeared in no less than eight different places on our test gun—a definite branding effort.
The SR-556 had the standard AR-15 configuration of charging handle, mag release, dust cover, forward assist, and left-side-only lever safety. The stock was Ruger-branded, but was the standard M4 variety as well.
The other parts of SR-556 were upgrades over a standard AR. A comfortable Hogue Monogrip adorned our gun, and a Troy Industries 11-in. Quad Rail wrapped around the barrel of the Ruger. This left plenty of space for all manner of accessories to be mounted. Troy Folding Battle Sights graced the top of our gun. The rear sight had dual apertures and was adjustable for windage, and came paired with a partially hooded front that offered elevation adjustment. We like this sort of sight configuration because it allows them to be co-witnessed with red dots and other CQB optics. This additional hardware, along with a heavier piston system, brought our gun in at 7.9 lbs.
The trigger on the SR-556 was a single stage version that broke very cleanly, but at a rather robust 8 lbs. We’d like at least a couple of pounds shaved off this pull weight.
The primary design difference between the SR-556 and a standard AR-15 is the Ruger proprietary gas piston system. The transfer or op rod was noticeably larger than a standard gas tube and was electroless tin plated. The gas block has a rotating dial on its end with four positions labeled 0 thru 3. This allows the gas output to be tuned to optimize the gun’s action with a variety of ammunition. At the fully open or 0 position, the gun becomes a single shot. For our purposes, we found the middle or 2 position worked well with all of the ammo we tested.
Accuracy testing revealed the Ruger handled all of our ammo relatively well, turning in its best group with the Remington 55-gr. MC at 1.2 inches, and fared only slightly worse with the heavier 63-gr. Silver State loading. Recoil was dampened well, and the gun functioned with no malfunctions. The Ruger SR-556 also fared well in our action shooting. The Troy sights proved to be much easier to line up and adjust than other models. We found the gun to be steady and relatively easy to point, although our testers did find the gun to be balanced pretty far forward because of the long quad rails.
Gun Tests Team Said: It was obvious that Sturm Ruger spent quite a bit of time on the design and execution of the SR-556. New shooters will find its package well-equipped and easy to use. Experienced AR shooters will appreciate the inclusion of quality stuff that works.
