By Todd Burgreen, GUNS Magazine
When it comes to choosing an assault-style rifle, there is little doubt that the US consumer is fixated on the AR platform. As of late, the AK series seems to be gaining a grudging acceptance as well. Each rifle type has burgeoning ammunition and after-market accessory sales to support this statement. Both also have stereotypes that are not necessarily true. AKs are more accurate and have better ergonomics when viewed through a fighting rifle lens. ARs are much more reliable than most give them credit for. It’s easy to forget there are other viable options outside the iconic AK and AR platforms. In fact, the rifle design that has become one of my favorites blends aspects from both the AR and AK, combined with unique characteristics of its own. I am talking about the SIG SAUER 556, specifically the SIG556 SWAT Patrol model.
A little background on where the SIG556 design originates. Certain weapons can morph into legendary status in consumer minds. This is often initiated by restricted supply of the weapon, which only serves to exacerbate the desire to possess. This can be caused by limited production, or more likely onerous import restrictions that have been imposed on the market by the federal government. The SIG 550/551 series of rifles is definitely such a weapon. A quick search of forums, books, anecdotal reports and articles shows the SIG 550/551 labeled as the “World’s Best Assault Rifle,” which only heightens an individual’s desire to possess such a widely-acclaimed weapon. Many in the US market waited on SIG SAUER’s entry into the “black” rifle market. The hope was to get a civilian-legal version of the legendary Swiss Army SIG 550 series of rifles. SIG SAUER designers chose a compromise in the form of the SIG556. The SIG556 trigger housing was altered from the 550 series to accept AR-15 magazines. Overall, this is a sage decision considering the growing number of AR-15 magazines existing in the US. Many would find the need to invest in a different magazine type a negative, considering existing low price and availability of AR magazines in the US. The SIG556 avoids this by being compatible with AR magazines. However, the proven 2-position, adjustable gas-piston operating rod system found on the 550 series was maintained in the
The SIG SG 550/551 design dates back to the late 1970s, as the Swiss sought to replace their Stgw 57 battle rifle. From the beginning, specifications highlighted the desire for a modular design with various model variants expected, such as compact and marksman-type weapons. The SIG SG 550 (20.8″ barrel) and the SG 551 (14.3″) carbine version were adopted in 1983, put into production in 1986, with final widespread introduction into the Swiss service in 1990. The SG 550 series was chambered in the Swiss equivalent of the 5.56mm in lieu of the earlier 7.5mm round and the experimented-with 6.45×48. The SG 550/551 functions via long-stroke adjustable gas-piston with a rotating bolt/carrier group are very similar to the AK’s in configuration, with Swiss refinement and tweaks. For example, the SG 550 barrel is screwed into the receiver, compared to the AK’s barrel being “pressed” into a front trunnion. The SIG recoil spring is located in front of the action, versus AK’s spring being behind the bolt carrier. A gas cylinder with a gas channel directs gasses tapped from firing a cartridge to a piston head that pushes the piston and bolt carrier rearward, working the action. The piston moving backward removes its alignment with the gas channel, cutting off the supply of gas acting on the piston. Surplus gases are directed out of an exhaust port. This system eliminates the “over”-gassed characteristic inherent in the AK, making the 550/551 run smoother and thus more accurately and less prone to wear over its service life. Accuracy requirements were stringent with the SG 550/551, reflective of the Swiss emphasis on marksmanship by its citizen soldiers. Literature discovered during research of this article indicated random SG 550 rifles tested before leaving the factory had to deliver no greater dispersion than 4.3″ windage and 2.8″ elevation groups at 300 meters from the bench using Swiss GP90 service ammunition.
The SG 550/551 gas system is adjustable via a 2-position valve. One setting is for normal operation and the second is for more adverse conditions, stemming from fouling or weather conditions. Another important nuance offered in the SG 550 series over the base AK is the adoption of a hinged lower/upper receiver style. This allows for a permanently-attached diopter drum rear sight via soldering at the rear of the receiver, compared to the AK’s sight location in front of the action due to the removal of the dust cover. A longer sight radius translates into more accurate fire placement. Another benefit that may not have been fully appreciated in the 1980s is the easy mounting of optics on rails incorporated into the upper. There’s hardly a battle rifle in use today that doesn’t sport a red dot or low-powered optic of some type. Lastly, the folding stock on the SG 550 provided proper cheek alignment no matter if a soldier was prone, kneeling or standing; this is not an easy accomplishment and a credit to the designers. In summary, the Swiss got the SG 550/551 design right, as is evident by the acclaim and positive reports it has received over decades of use by various units and armed services. These features are transferred over to the US-made SIG556.
The SIG operating rod system is often referred to as the “Porsche” of AK designs due to
the tighter tolerances and better workmanship found in SIG SAUER weapons compared to other
manufacturers using the AK as a basis of design.
Best Of Two Guns
The SIG SAUER 556 rifle has always struck me as a valid option to both AR-15/M16 and AK platforms since its introduction in the US. While the SIG556 product line accepts AR-15/M16 magazines, it is closer to the Kalashnikov in actual operating method. The SIG operating rod system is often referred to as the “Porsche” of AK designs due to the tighter tolerances and better workmanship found in SIG SAUER weapons, compared to other manufacturers using the AK as a basis of design. The gas-piston operating system keeps the action cleaner, cooler and overall more reliable. It also allows the use of side folding stocks to reduce overall dimensions of the rifle when needed. Many will find the 556’s ability to accommodate a side folding stock (unlike the AR-15) an added incentive. These are all positive attributes, especially considering the recent rush to create gas-piston ARs because of the increased reliability factor.
Next, the design