The gas-operated Sig 556 uses an improved version of the AK’s operating rod system, with a bit of FN-FAL thrown in. The steel upper receiver houses the barrel, bolt and operating rod. The aluminum-alloy lower receiver holds the lockwork, pistol grip and stock. The two connect in a fashion similar to the M16, in this case using a push-pin cross bolt at the back and a two-piece threaded screw connection at front.

The direct ancestor of the 556 is the highly regarded Sig 550. Importation of this excellent rifle was stopped in the ’90s by both the Bush Sr. administration and the subsequent (and now expired) 1994 Assault Weapon Ban.

The Sig 556 is manufactured in New Hampshire by Sig Sauer (formerly called Sigarms). Wisely, I think, they chose to modify the rifle to accept M16/AR-15 magazines. Current steel M16/AR-15 magazines from quality manufacturers (Brownells being a notable example) are excellent in every way, reliable and durable.
The 556 is a gas-operated autoloader, powder gas is tapped off the barrel to a gas block, similar to that of an FN-FAL, then drives an operating rod housed in a tube above the barrel, which in turn drives the bolt carrier. The recoil spring wraps around the operating rod. Lockup is via a rotating bolt in the front of the bolt carrier, a cam on the bolt running in a cam track in the carrier to control locking and unlocking.

Like an FAL, the volume of gas reaching the operating rod can be adjusted. With the FAL, an adjustment ring on the gas block is adjusted to provide sufficient gas volume to reliably drive the action. The 556 is similar in concept but simpler in function. At the front of the gas block is a flat knob. Oriented vertically it provides the correct amount of gas for proper functioning under normal conditions. If the system gets sluggish after extensive firing (more likely with the military full-auto versions), you can turn the knob to a second position.

The owner’s manual advises if it becomes necessary to use this second position to ensure reliability, it is an indication the rifle should be disassembled and cleaned at the earliest possible opportunity. Using the second position all the time would increase wear on moving parts.

The manual says recoil is increased as well. Maybe it does, but the difference is pretty subtle. I fired a couple of magazines at the second position and couldn’t feel any difference, but a .223 doesn’t kick much anyway.

The operating rod system is the main functional difference between the Sig 550 series and the M16. The M16 and AR-15 use a direct gas impingement system. Powder gas tapped from the barrel travels down a gas tube and impinges directly on the bolt carrier, driving it back to extract and eject the fired case and cock the action.
The operating rod system as used by the AK and FAL designs grabs a good whiff of gas the instant the bullet passes the gas port and immediately jolts the operating rod into motion. The bolt and receiver heat up much more slowly during extended fire and fouling powder gases aren’t carried into the receiver. All theory aside, the reliability of this system under the most extreme conditions has been so well demonstrated it hardly merits further discussion.

On the negative side, we no longer have a true free-floating barrel, possible with the AR-15. As the operating rod separates from the gas block, the barrel gets a little tweak which affects its natural vibration. Military AKs have never been noted for accuracy. Even custom AKs and FALs only shoot groups of 1-1/2" to 2" at 100 yards.

In practice, high-quality materials, close tolerances and fine workmanship play a big part. Operating rod designs such as the M1 Garand and M14, properly tuned up, give wonderful accuracy. And so it is with the Sig 556. Sig workmanship is second to none. I’ve been through the manufacturing facility and the high-tech equipment used to measure tolerances is amazing.

In practical terms the test 556 produced excellent accuracy. The fast 1:7" barrel twist is best suited to heavier bullets. The 556’s best accuracy was with Black Hills ammunition loaded with 69-grain Sierra MatchKing bullets. Five-shot groups at 100 yards averaged 1.05" ranging from a worst of 1.23" to a best of 0.84".

Light bullet loads usually function reliably, and sometimes give very good accuracy even in a fast-twist barrel. Sometimes, though, one of the fast-spinning bullets won’t have settled down at the 100-yard mark and you’ll get flyers. I experienced this using 50-grain Ballistic Tip bullets. Several times I’d get four shots in 1/2" to 3/4"and the fifth shot an inch or more out. It may have been shooter error, though I got flyers even with shots called as good.

Speaking of calling the shots, for accuracy testing I fitted a Sightron 4-16X scope. Even at 16X it was easy to call the shot. This has to be the softest recoiling .223 I’ve ever fired. Admittedly no .223 kicks hard, but I believe the rifle’s weight, plus the “equal and opposite reaction” to the gas pressure shoving the operating rod back helps reduce felt recoil.

The 556 telescoping stock can be adjusted to four positions, with just over a 3" difference in length from the closed to fully extended position. It locks solidly in whatever position is selected.

I really like the feel of the pistol grip on the 556. My hands are not particularly large, but I find standard M16 pistol grips much too small. The 556 grip is longer from back to front and fits my hand just right to get my trigger finger squarely across the face of the trigger.

The ambidextrous thumb safety is ideally placed. Magazine release button is likewise well placed for easy access by the trigger finger, and has a “fence” around it to prevent accidental magazine release. The bottom of the triggerguard is hinged and can be opened for use with a gloved hand.

The trigger pull is heavier than I like, but weight aside, trigger quality is excellent, and that is coming from the original trigger curmudgeon. What I demand more than anything from a trigger is consistency, and here it excels.

It has been described as a 2-stage pull, but in fact it is 3-stage. An initial take-up of 3-1/2 pounds moves the trigger smoothly to a point of increased resistance. Another pound of pressure moves it back until the rear of the trigger contacts a trigger “stop” in the back of the triggerguard.

This “stop” is actually spring loaded, not fixed. As you continue to apply pressure the trigger is moving back against the resistance of both the trigger return spring and the stop spring. On the test rifle the sear released at 7-1/2 pounds.

If this sounds complicated, it really isn’t. Trigger movement is consistent, predictable and smooth. There are no uneven, unpredictable “steps” during trigger movement, no feel of grittiness. Clearly the lock work components are well designed and well made.

For precision shooting the shooter quickly presses through the first two stages until the trigger contacts the stop, then smoothly presses through the last three pounds. For fast shooting just press straight through, as one would with a double-action revolver.

Disassembly and reassembly is straightforward and holds no special difficulties. The steps are well described in the operating manual, so I won’t copy them here. The bolt handle isn’t very big and the rifle won’t work without it, so don’t drop it off your desk and have it bounce behind the gun safe. Of course that would never happen.

Notes On Shooting

A combination of time pressure and cold weather limited shooting to just over 350 rounds. Other than cleaning the bore a couple of times during accuracy testing this was done without cleaning. There were no malfunctions of any kind.

As mentioned, I tried switching the gas knob to position “2” just to see how it felt, but there was no need from a reliability perspective. After firing a magazine, I switched back to position one. Except under arduous military conditions with no opportunity for routine cleaning, the second position won’t likely see much use.

The bolt face extractor is a massive affair and grabs a big hunk of the case rim. Ejection is via a fixed-blade ejector. The bolt must be moving fast at the instant the ejector hits the case, as fired cases are thrown out with considerable vigor. Ejected cases marked up the rifle’s finish a bit just behind the ejection port.

At first I thought the marks were just brass that would wipe away, but alas no, the actual finish has been removed. It’s not pretty, but I guess like the first scratch on a fine walnut stock it is something that must be endured. Fired cases were not damaged, only an occasional slightly flattened case mouth which did not affect reloadability.

Most shooters will want some sort of optical sight. However there is a neat rear sight which folds down into a recess in the sight rail. The front sight blade also folds down out of harms way. In the event an optical sight breaks down in the field you can always remove it, flip up the iron sights and stay in the game.

Other than accuracy testing, I shot the 556 with a Trijicon ACOG sight attached and the combination of the Sig rifle and Trijicon sight is truly impressive. You can engage a group of targets at 10 or 15 yards with amazing speed, in bright light or dim, almost as fast as the trigger can be operated. Then you can grab a rest and with the next trigger press engage and hit the A-zone of an IPSC target at 400 yards.

The quad rail handguard is useful for mounting accessories. It does make the 556 a bit muzzle-heavy. The accessory forward pistol grip with flashlight on the test rifle was very useful in fast handling. There’s a “trigger” on the forward grip which is pressed to illuminate the Sig flashlight (a cross-bolt safety prevents the trigger from being depressed unintentionally).

Incidentally the forward pistol grip has a storage compartment, as does the rear pistol grip and the stock called “battery storage compartments” in the manual.
The Sig 556 should prove very popular with its outstanding reliability and excellent accuracy. I can see it doing well in 3-gun competition. It should excel as a police duty weapon or for personal defense. It should also be popular with firearm enthusiasts who admire superb design and exceptional workmanship.

The latest version of the Trijicon Advanced Combat Optical Gunsight (ACOG) consists of scope, base and red-dot sight. This ACOG scope is a 4x32 with illuminated reticle. In daylight, light is gathered by a fiber optic cable on top of the scope body and carried to the reticle. The reticle also has a tritium insert and is visible even in complete darkness. Of course, if it is too dark to positively identify target and backstop, you probably shouldn’t be shooting.

The scope is attached to a quick-detachable base which fits a Picatinny rail, standard on the Sig 556. An excellent mount, quick to use, strong and secure, removing and reattaching it between shots showed it holds zero reliably.

Trijicon sights are in use by the military in Iraq and other combat zones, where soldiers may have to do building searches one day and engage the enemy at long range the next. By adding a Trijicon red-dot sight above the main scope body Trijicon has made this an amazingly versatile sight.

As has been amply demonstrated in open-division practical shooting competition, red-dot sights have no equals in fast, close to medium range shooting. Red-dot sights have a non-critical eye relief and work just as well whether mounted on a rifle, handgun or way out on the barrel rib of a shotgun.

Their use is natural and intuitive, just place the dot where you want to hit and press the trigger.

Eye relief on the ACOG 4x32 scope is specified at 1.5", which seems on the short side, though plenty adequate for a light recoiling round such as the .223. In practice, eye relief is relatively non-critical. I could get the full field of view even with my eye about 2" from the ocular. Field of view is 36.8' at 100 yards.

This is a remarkable sight, capable of rapidly engaging targets at close range, placing accurate shots at long range, and doing so in very dim ilght to bright daylight. Trijicon quality, reliability and durability have been well proven in service with American soldiers under severe conditions. At $1,500 the price is a bit breathtaking, but it does include the scope, red-dot sight, detachable base and Pelican carrying case.

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