Black Rifles & Tactical Guns

Gun Test: SIG 556 SWAT Patrol

Guns and Gear Contributor
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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
556’s design.

Some History

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

Any department, individual officer or security-conscience civilian looking for an LE patrol carbine will instantly appreciate the SIG SAUER 556 SWAT Patrol rifle. The functionality of a rifle utilizing AR-15 magazines, incorporating a proven gas-piston operating rod system with a heritage tied to the legendary SIG 550, is hard to deny. The SIG SAUER 556 SWAT Patrol integrates excellent accuracy from the 16″ barrel with efficient handling and maneuverability. Thus, no matter the situation the rifle is deployed in, whether it is a rural setting with longer distances encountered or more urban requiring CQB-style tactics, the SIG SAUER 556 SWAT Patrol can satisfy
the mission.

The SIG556 SWAT Patrol features a “flat-top” upper receiver with a Picatinny rail for mounting optics or other aiming devices, combined with an aluminum quad rail fore-end that is ideally suited for mounting accessories. One unique feature of the SIG556 product line is the SIG SAUER Rotary Diopter Sight System (RDSS). The SIG RDSS with a rotary drum rear sight and front post is more substantial than most iron sight configurations. A shooter is well served by the RDSS if an optic device is not utilized or goes down. The RDSS stems from the 556’s SIG 550 heritage and is befitting a service rifle, which relies solely on its open sights to engage targets. However, recent trends in fighting rifles would indicate the design intent for the SIG556 SWAT Patrol is to mount some sort of low-powered magnified optic or red dot as the primary sighting tool. Thus the RDSS mounts on the Picatinny rail in lieu of the permanent nature of the original SG 550/551 diopter sight.

Thought-Out Design

The SIG556 SWAT Patrol chambered in 5.56 NATO weighs 7.5 pounds, measures 36″ with the stock unfolded and 27.1″ folded. The folding rear stock is an immediate advantage compared to a traditional AR rifle, with its buffer tube preventing a folding-stock option. The A2 flash suppressor uses a standard .5×28 thread pattern for mounting. SIG SAUER installed a 2-stage trigger and ambidextrous safety on the 556 SWAT Patrol; they also chose to use aircraft-grade aluminum alloy forgings for the redesigned trigger housing. This saves weight in the 556 series compared to the 550 series. The SIG556 SWAT Patrol has 16.25″ military-grade, cold-hammer-forged barrel with 1:7″ twist. The SIG556 SWAT Patrol benefits from further weight saving with SIG SAUER’s decision to use a shorter gas piston system compared to its other SIG556 models. The shorter gas piston is similar in length to the piston featured on the SIG556 pistol variants. Think of the standard AK-length piston compared to what is found in an AK Krinkov variant.

Over a pound is shaved from the 556 SWAT Patrol due to this shorter gas piston and resultant shorter forearm furniture. For many, this translates into a handier rifle that is not so front heavy. SIG SAUER accepts that rails are here to stay on the modern fighting rifle, because they allow for a user to configure the rifle as they see fit. The fact SIG decided to equip the 556 SWAT Patrol with a compact fore-end quad rail gives a nod to the realization that high-intensity flashlights and vertical forward grips serve a role and have a place on a fighting rifle. However, the want for full-length quad rails (which adds weight and unbalances the rifle by making it front heavy) is resisted. The ability to mount infrared laser designators, night vision devices or other sorts of tactical hardware is not something most of us require in a fighting rifle. If this capability is needed, there are other rifles in the SIG SAUER lineup to suit your needs. The SIG556 SWAT Patrol’s 7.5 pounds of weight and minimal barrel length gives it great balance and handling.

The 556 SWAT Patrol upper hinges open with bolt group and gas piston removed (per provided directions), and the trigger group is exposed in the lower. Barrel/chamber cleaning is easily accessible, as is the bolt group and piston-operating rod. Overall, cleaning of the SIG556 takes little time, with the bolt group needing little more than a wipe off and re-lubrication. The Nitron finish on the upper and hard coat anodized aluminum lower keeps exterior concerns to a minimum. SIG SAUER is now using a folding stock reminiscent of the design found on the classic SIG 550, compared to other types used on early 556 variants. The factory-folding stock once deployed is very sturdy and doesn’t exhibit any side-to-side or up-and-down wiggle. One interesting side note is the upper of the SIG556 bears the serial number of the weapon compared to the lower of the AR-15. I took advantage of this nuance by switching out the factory lower with one that featured a non-folding LMT SOPMOD stock ala the AR. This was derived from a previous SIG556 SBR project converting a 556 Pistol into a SBR. As alluded to at the beginning of the article, the SIG556 is a favorite platform. This allowed for comparison of the two stock designs. The use of the other SIG lower with LMT SOPMOD stock offers the benefit of adjustable length-of-pull so a user can find what best suits individual needs, especially if wearing body armor.

Anyone looking for a hard-hitting, high-capacity carbine would do well by picking
the SIG556 SWAT Patrol, especially if ammunition is selected wisely. Short work
was made of an EVTC target of opportunity with this SIG rifle.

Combat Optics

It was decided to take full advantage of the SIG556’s flattop upper by installing different optics for testing. A Trijicon ACOG 3.5×35, model TA11H with green horseshoe reticle was mounted. I also decided to mount a Leupold Mk4 CQ/T for testing. Both Trijicon and Leupold’s reticles offer the capability to engage multiple targets in rapid sequence (unlike open sights), and at same time provide adequate accuracy out to a couple hundred yards. This is made possible by superimposing an aim point on the target; this does not totally obscure the target because the reticle dot is not so large. As many “maturing” shooters can attest, the single-focus plane with the red dot is easier to shoot accurately than coordinating front and rear sights. The 5.56/.223’s flat trajectory aids in making center mass hits out to 300 yards, without having to compensate excessively for bullet drop.

Next, the evaluation 

The 556 SWAT Patrol was evaluated at Echo Valley Training Center (EVTC). The private facility has multiple 100-yard enclosed bays and a 360-degree drive-in range all capable of handling numerous students conducting square range drills, or more dynamic/fluid types of training. In conjunction with the individual training bays, EVTC features multi-stepped target berms that are strewn with reactive steel targets from TacStrike, fluid-drained automobile, and moving targets at ranges varying from 100 yards out to 350 yards. Range T&E with the SIG556 SWAT Patrol followed an established protocol for combat rifles. After a quick sight-in of the SIG RDSS open sights, evaluation commenced with a function test involving firing several magazines in rapid succession at various TacStrike steel man targets and vehicles. While not unique, this is a good way to establish a baseline for reliability. This was done with the RDSS open sight to offer a chance of familiarization with the irons. While definitely a superior iron sight in terms of precision compared to most other types, the RDSS is large and had the tendency to obscure my field of view when engaging targets at

CQB distances. I found the RDSS could be left on the rifle and serve as backup iron sights if desired; co-witnessing with the RDSS isn’t likely, but it depends on optic chosen by the end user.

Realistic Evaluation

Further range testing of the SIG556 SWAT Patrol consisted of repeating numerous drills and exercises experienced via training with Norone Corp., Jason Falla’s Redback One, Storm Mountain and Tactical Response. Firing while moving, as well as behind cover, reloading drills, transitions between shoulders depending on cover orientation and engaging multiple targets arranged around “no-shoot” targets all helped put the 556 SWAT Patrol through its paces. The 556 SWAT Patrol’s minimal recoil and handling characteristics allowed for fast double and triple taps on selected targets, especially at CQB distances. The Trijicon assisted in engaging targets at close distances while the green horseshoe reticle was easy to pick up rapidly. At same time, it allowed for more than enough accuracy out to a couple hundred yards, due to the green reticle superimposing an aim point on the target while not totally obscuring the target.

Magazine change drills turned out to be a combination of AR and AK manipulation techniques. The SIG556 SWAT Patrol does have a bolt hold-open feature after the last round is fired. So it was possible to slam a fresh magazine home and hit the bolt hold-open lever with your left hand to chamber a round. For the occasion when a magazine was changed without the bolt being held open or training doctrine dictated running the bolt every time, the user had to resort to a couple different methods ala the AK. Either reach over or under the rifle with your left hand, work the bolt, tilt the rifle over with the right hand on the pistol grip and work action with left hand, or switch rifle over to left hand and work bolt with right hand. None of these techniques were particularly hard to master, but different from what dyed-in-the-wool AR shooters will be accustomed to.

Fittingly, the SIG556 SWAT Patrol’s trigger feel is a cross between an AR and AK. It is 2-stage as in that travel is encountered when first pulling it to the rear. However, the break comes as a surprise due to the final stage of sear release being light in nature before the round is sent. The trigger can be staged for precise shots, yet retains a measure of surety for CQB engagements. The reset is quite minimal, allowing the shooter to engage rapidly once familiar with design. A couple drills and a handful of magazines is enough to get comfortable with the operator trigger interface.

Shannon Campbell, co-founder of Norone Corporate, was on hand for some of the 556’s evaluation. Norone Corporate is a weapons and security applications training entity that began as a collaboration of former FED contractors, USMIL and active competitors who gathered a vast amount of tactical experience to bring to bare and share to specific clientele. Norone provides tactical weapons training for protection teams of FED contractors and LEO as well as for some of the largest global security providers; today, Norone has narrowed its focus to stay at mid-level and provide specific training to enhance individual skill development, making any team member an asset for any environment. With several sojourns overseas as a private security contractor, Shannon is a good opinion source. Shannon was appreciative of SIG556 SWAT Patrol’s handling and reliability.

The SIG SAUER 556 rifle is appealing as a valid option to both AR-15/M16 and AK
platforms in the US. While the SIG556 product line accepts AR-15/M16 magazines,
it is closer to the Kalashnikov in actual operating method.

Flawless Reliability

Ammunition tested with the SIG556 SWAT Patrol was a combination of Black Hills Ammunition loads, Hornady 60-grain TAP loads, Wolf Ammunition Military Classic 55-grain FMJ, Winchester 55-grain FMJ, Federal 55-grain FMJ, Federal Premium Law Enforcement 55-grain Triple Shok and 62-grain Bonded SP loads. The 556 SWAT Patrol kept all loads tested under 2.5″ at 100 yards with multiple Black Hills loads — 50-grain TSX and 77-grain Match — and produced near-MOA groups out to 200 yards. This accuracy level justifies mounting a magnified optic such as the Trijicon ACOG or Leupold’s Mk4 CQ/T, depending on intended mission. Overall, the accuracy level was more than acceptable considering the barrel’s thin .5″ profile. Accuracy test protocol consisted of five 3-shot groups with each ammunition type; group sizes were averaged. Velocity figures ranged from 2,700 to 3,000 fps over an RCBS chronograph.

Next, the final thoughts

Several of the more dynamic drills experienced during the T&E showed the SIG556 SWAT Patrol at its best; it showed unrelenting reliability even when scorching hot after multiple magazines fired consecutively. The SIG SAUER 556 SWAT Patrol’s gas piston operating rod system proved its worth in keeping the bolt carrier group cooler and cleaner compared to direct impingement AR rifles. I purposely neglected to clean the 556 SWAT Patrol over the course of the review, and only kept the rifle lubricated. At no point did I feel the need to adjust its gas system into its second position to compensate for a dirty rifle. The 556 Patrol’s gas piston steadily ejected brass briskly forward (briskly being an understatement) and to the right several feet away from the firing point.

While the SIG556 SWAT Patrol isn’t going to displace the AR or AK in terms of popularity based on pure numbers sold, its owners will be well served by its capabilities. A certain satisfaction will be gained by knowing the SIG556’s heritage and how it incorporates and enhances features from both AR and AK; even better, the 556’s own nuances benefit those who choose to use it.

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