In a land of black rifle owners who have universally awarded Eugene Stoner holy status, loyalty to the AR weapon system is absolute. All Cold War rivalry aside, the same can be said for Mikhail Kalashnikov, who took the American M1 Garand and German StG-44 and made it the most popular weapon system in history. In 1978 SIG Sauer – then Schweizerische Industrie Gesellschaft (SIG) entered competition to build the Swiss Army’s Stgw57 using the 7.5x55mm Schmitt-Rubin cartridge. In 1981 SIG Sauer’s design was favored and the designator was changed to SG 541 and chambering in 5.56x45 NATO was adopted. In 1983 the SG 541 was officially adopted and the designator was changed to the infamous SG 550 in 1984 – thus giving birth to the SIG 550 series rifles. Unless you have no appreciation for Swiss weaponry, these rifles need no introduction as they are legendary, proven, and intuitive to highest degree.
Fast forward to 2004, the U.S. ‘Assault Weapons Ban’ had lapsed and the firearms industry was open to retail the black rifle again. In 2007, the U.S. subsidiary of SIG Sauer (then SIG ARMS) opted to develop a black rifle unlike that of the traditional Colt AR weapon system, the SIG 556. The U.S. manufactured semi-automatic rifle, based on the SIG 550 and aspects of the AR weapon system that merges reliability and modularity and is very much an identity in itself.
Like the SIG 550 series, the SIG 556 action is based exclusively on the Kalashnikov’s AK-47 rotating bolt and full-length gas piston (note: the SIG 556 Pistol, Patrol, and Classic SBR have shortened operating rod). The only difference is incorporating Swiss tolerances in machining and moving the recoil spring from the back of the action (as on the AK) and integrating it to the operating rod at the front of the system – in a 5.56x45 NATO (.223 REM) weapon system vs. 7.62x39 Russian. The SIG 556 features a two position regulator at the gas block, one setting for normal operation, and a second for use when the rifle has heavy fouling or in adverse conditions (rain, cold, etc.). The overflow gases escape through a port located atop the gas tube, just behind the front sight. This overflow port keeps the carbon filled gases at the far-end of the weapon system, keeping the majority of the operating rod and more importantly the bolt carrier group cool during operation. Overall this gas piston/rotating bolt system is incredibly reliable and given it’s influenced by the AK weapon system, and has proven to be - perhaps the most reliable on the planet.
Upper Receiver - the upper receiver is a carry-over from the SIG 550 series, it is stamped (pressed), made of high carbon steel, and Nitron coated to protect against corrosion.
Trigger - the trigger is steel and two stage (spring assisted) with a pull of 6lbs (DMR) and 8lbs (all others). After about 3/8 inch of travel it capitalizes on a spring assisted trigger stop (ideally at the second stage) at the rear of the trigger well that engages the back of the medium-width trigger shoe. This trigger is unlike the AK or AR weapon systems and is purely unique to the SIG 550 series rifles.
Barrel – The SIG 556 barrels are made from cold hammer forged (CHF) steel and nitrocarburized (Nitride treated - not chrome lined). While unconfirmed by SIG Sauer, it’s rumored that they contracted FNH to make barrels for the SIG 556 at their plant in South Carolina using a specific Ammonium Nitride treatment. Barrel life can be difficult to clarify due to lacking specifics from SIG Sauer; however the SG 550 barrels had to be rated for military use (includes automatic fire) at 15,000 rounds –minimum. Seeing as SIG Sauer used a similar process for the SIG 556 barrels, one should expect to see 10,000-15,000 rounds before loss of accuracy (throat erosion). As a semiautomatic weapon system the barrel should maintain utility beyond 15,000 rounds - some professional estimates giving it a 20,000-50,000 round lifespan. SIG 556 barrels vary in length depending on the specific model being referenced and are all .57 inch in diameter (DMR model being the exception). The barrels have a 6 groove, 1 in 7” right-hand twist and have a thread pitch of 1/2”x28 - meaning the SIG 556 can utilize the vast array of muzzle devices designed for AR weapon systems.
Speaking of the AR weapon system influence, this is where the SIG 556 gets interesting. The SIG 556 escapes influence of the SIG 550 series in its lower receiver. The lower is made of 7075-T6 forged aluminum alloy, CNC machined, and has a hard coat anodized finish. The lower has an AR influenced magazine release and features a STANAG magazine well – thus accepting a menu of magazine options. The safety is ambidextrous; however the magazine release only accommodates right handed shooters. Like the AR weapon system, the SIG 556 lower receiver detaches from the upper receiver by removing two pins (pivot and take-down). The SIG 556 incorporates the same threaded rear as found on the AR weapon system, meaning it can accept all stocks made for the AR, as well as a proprietary folding stock as it lacks a functioning buffer. Utilizing a similar bolt catch design as that on the AR weapon system, the SIG 556 bolt catch is simple and well placed. The SIG 556 SWAT and Patrol models feature a hand guard that is an OEM 1913 rail system (not applicable to the Classic or Sports Configuration models). This adds modularity for those looking to mount weapon system accessories such as fore grips, 1913 rail mounted bipod, lights, lasers, and other task-centric accessories.
Griffin Tactical makes an outstanding aftermarket charging handle that replaces the SIG 556 OEM part. The charging handle and offers a steeper degree which accommodates for optics and is made of 4340 allow steel.
Any 1913 rail mounted flip sights are ideal, thus removing the dovetail mounted front hooded sight from the gas block. This allows for upgrading to tritium assisted reference dots and compactness if using a primary optic. Recommend looking at Troy Front or Rear BUIS or Magpul BUIS.
Button-actuated take down and pivot pins. These allow for field stripping the weapon without the use of tools to remove the screws on the OEM pins. These pins are either Swiss-made by SIG Sauer or aftermarket made in the U.S.
For those looking to upgraded to an AR style stock for utility and comfort, there are many options. Particularly a stock that still offers adjustable settings yet has an improved fit at the cheek weld. The Magul CTR and UBR stocks offer an outstanding upgrade to the OEM stocks.
The OEM pistol grip is not a required swap, however if the shooter requires a more rubberized texture or finger grooves, Hogue offers an aftermarket option for the SIG 556 that is ideal.
For those who require a 1913 rail system, you have the option of the Samson rail or the Troy rail. Both are admirable options as an upgrade over the SCM or Classic OEM hand guards.
Technical Specifications (merged among all models)
Caliber: 5.56x45 NATO
Action Type: Semi-automatic
Operating System: Gas piston operated, rotating bolt
Overall Length: 20.5-39.2”
Overall Weight: 6.3-8.2lbs
Barrel Length: 10”, 16”, 18”
Rifling: 6 groove, 1 in 7” or 1 in 8”
Muzzle Device: AR-style 1/2” in 28 RH TP
Sight Radius: 14.4”-18.1”
Magazine Type: STANAG
Special Features: Swiss-style folding stock, rotary diopter sight, SIG 550-style furniture
The SIG 556’s barrel at a 1 in 7 twist is more accepting of heavier grain bullets as it caters to the specifications of the NATO 62 grain bullet. Being as it is, the SIG 556 rather enjoys the 62 grain + bullets over the 55 grain option as grouping is much better overall. The SIG 556 will also perform very well with 60 grain bullets from performance ammunition makers like Hornady and Black Hills, grouping on par if not better than its 62 grain NATO companions. Overall accuracy can be considered sub-MOA using the correct ammunition, meaning it is not ideal to expect tight and consistent groupings with the 55 grain bullet.
Recoil and Operation
The SIG 556 is admiringly front-heavy when you add a 1913 rail system, grips, and take into account the robust full size gas piston system. Coupled with weight distribution, the gas ejection port on the topside of the gas tube acts to push the weapon system downward while muzzle rise would in effect, lift it. This makes for outstanding follow-up shots, and allows the shooter to stay on target with no interruptions to their sight picture. The felt recoil is absolutely minimal, especially with a quality aftermarket stock. One thing that the SIG 556 lacks is the annoying buffer spring “twang” typical of the AR weapon system.
The SIG 556 has many features that play into its outstanding reliability. The gas piston system itself is reliable and less prone to fouling versus the AR weapon system’s direct impingement gas operation. The gas piston system allows for cooler operation, less carbon build up, and consistency. The AK-style rotating bolt has two main lugs that lock it into breech; this is only enhanced more when the bolt and chamber become polished to the operation. The firing pin is spring assisted which not only acts as an internal safety measure, but also makes slamfires a non-issue. The upper receiver features full length steel rails that allow for efficient movement of the bolt carrier group. The pronounced yet simple bolt catch makes using any STANAG magazines and non-issue in terms of the bolt riding forward after the last round. This is an issue for some AR weapon system owners as they have to match the bolt catch with the magazines being used due to differenced in tolerance.
The SIG 556 rather enjoys the distance starting at 25 meters and performs to task at 300 meters. The rated maximum effective range from the manufacturer is 100-400 meters. Given the operation of the weapon system, it can be employed rather usefully in a close quarter battle (CQB) situation as it handles very well in rapid fire and at close distance.
The SIG 556 catches strong criticism from genuine SIG Sauer traditionalists, as they failed to deliver a U.S. made SIG 550 series rifle from the start. In 2012 SIG Sauer released the SIG 551 A1 which features the SIG 550 style lower, accepts the rare yet requested SIG 550 series magazines, and finished with the traditional SIG 550 gray finish. Those dedicated to the AR weapon system are critical of the SIG 556 in terms of the heavy gas piston operating system and the lack of a chrome lined barrel. In terms of accuracy, the SIG 556 may not deliver the same results past 300-400 meters – however proves to be incredibly reliable in the meantime. The SIG 556 in the Classic and SCM variants are naked to what most consumers would want standard on a black rifle at the given price point. However, the SIG 556 SWAT and Patrol variants go a little further.
The initial versions of the SIG 556 were assembled in the U.S. using many Swiss-made components. In some weapon systems, primarily those manufactured in 2009, the bolt carrier forms a factory-induced habit of pinning where it meets the trunnion. While this has been documented by numerous SIG 556 owners, it has shown no effect on the weapon system and will eventually cease once the bolt carrier is seated. In short, due to tolerances (yes, quality control) some SIG 556 rifles may require a period of breaking in while internal mechanisms become polished.
Personally, my only complaint (and a minor one at that), is when the brass ejects from the chamber it hits the side of the upper receiver - along the lower-left corner of the ejection port. As a result, it causes heavy wear marks where the brass cases are impacting the side of the steel upper, thus wearing away at the finish – cosmetic concern.
I think it is important to remember that the SIG 556 (by lineage) came before the Magpul/Bushmaster ACR, FNH SCAR, or the H&K G36 – something that should be considered when looking at alternatives to the AR weapon system. What SIG Sauer did was take the simple AK gas piston and rotating bolt operating system, make a few adjustments and package it into a modern black rifle. In doing so, the SIG 556 creates a fine balance between the reliability and efficiency of the AK and the modularity and accuracy of the AR weapons systems. The SIG 556 is without a doubt built like a tank and it has decades of development behind it.
I’ve owned my first SIG 556 for 6 years now, and it undoubtedly had a direct affect in my interest in weapons. I like the weapon system so much that I invested in a second one in 2011. Both of my SIG 556 rifles get regular range time, and I estimate that I’ve put 1,500 to 2,000 rounds through each of them. Interestingly enough, I should add that I have NEVER had a failure to feed (FTF) or failure to eject (FTE) from either weapon system. I’ve run brass and steel ammunition with no prejudice. I’ve even successfully run ammunition that had light primer strikes from an AR platform - firing each time.
The Reluctant “xi”
In 2012-2013 SIG Sauer released the predecessor to the SIG 556, designated the SIG 556xi. By 2014 SIG Sauer had discontinued production of the original SIG 556. The SIG 556xi brings add ambidextrous controls and increased modularity to the OEM furniture; however it lacks the robustness of the SIG 556 which has shown to play directly into the reliability. The SIG 556xi is currently offered in 5.56x45 NATO and 7.62x39 Russian.