SIG’s line of 556 carbines created quite a buzz at the 2006 SHOT Show, as evidenced by the long line at the SIG booth and a lengthy backorder list. Unfortunately, the mountain of orders stacked up so fast it created production delays (since remedied). To make a long story short, my T&E sample didn’t show up until December 2007, but the carbine I finally unboxed proved to be well worth the wait.

The 556 is based on the popular 550-series design of SIG’s sister company, Swiss Arms. The 550 series uses Swiss Arms’ two-position adjustable gas-piston operating-rod system, which is known for its unflappable reliability. The key? Unlike the AR-15’s direct-gas-impingement system, the 550 series’ piston-driven system vents gases and unburned powder out the front of the weapon rather than back into the action. This results in a cleaner, cooler-running system that is more reliable and easier to clean.

Piston-driven systems are also less fickle with short barrels and suppressors, which means the 556 can easily be made into a short-barreled rifle (with the proper paperwork) or adapted to suppressor use. I didn’t have the test gun long enough to mount a can on it, but its standard half-inch x 28-thread pitch would work perfectly with my Jet titanium suppressor.

SPECIFICATIONS
MAKER:	SIG
TYPE:	Gas-Operated semiauto
CALIBER:	5.56 NATO
CAPACITY:	30
BARREL LENGTH:	16.25 inches
OVERALL LENGTH:	37.5 inches (with stock extended)
WEIGHT:	8.7 pounds
SIGHTS:	Flip-up front/rear plus Picatinny rail
STOCK:	Magpul collapsible
FINISH:	Matte Black
PRICE:	$1,800

A Design Refined
One notable deviation from the 550 series is the 556’s trigger housing, which accepts standard AR-15 magazines. The new design is machined from an aircraft-grade aluminum-alloy forging, which shaves a pound of weight from the original design. The triggerguard swings down for gloved use.

But SIG did more than update the 550 series’ trigger housing when it designed the new 556. First, as much as I like the 550’s diopter sights, SIG ditched them for a lighter, more compact flip-up design. The front sits atop the gas block, and the rear is recessed into the integral Picatinny rail.

The top of the receiver is not the only part to sport a rail; the handguard is a quad-rail design, with full-length rails on the top, bottom and both sides. It appears to be true 1913-spec, as all the accessories I tried—even some notoriously fickle ones—fit perfectly on the receiver and handguard. The included vertical foregrip with integral SIG Sauer tactical light also slid on and off the gun easily.

Other features include an ambidextrous safety selector, a two-stage trigger, an AR-style protected magazine release on the right side of the receiver and a bolt-release lever on the left side that is big enough to provide a substantial amount of leverage. Closing the bolt—which locks back on an empty magazine—was a breeze with the oversize lever.

A robust, rubber-coated charging handle provides a good purchase in high-stress situations. An ergonomic polymer pistol grip with integral storage and a Magpul collapsible buttstock are also standard.

Disassembled: The operating rod (second from top, just below top rail cover) and gas tube (just below it) are the keys to the 556’s reliable, adjustable gas piston and Kalashnikov-inspired operating system.

The 556 sports a 161?4-inch cold-hammer-forged barrel that SIG calls “military grade.” It is chambered for the NATO-spec 5.56mm rather than the commercial .223 Remington and features a 1:7 twist, which allows operators to employ the heavy-for-caliber bullets that are so en vogue. The barrel has a thin, .573-inch profile that flares out to 3?4-inch at the muzzle, where a flash suppressor is threaded onto the barrel.

Adjustment of the SIG Sauer’s gas system is accomplished via a two-position gas valve located at the front of the handguard. The vertical position is for normal use, while the slanted position, which allows more gas to be released against the operating rod, is for use when the gun is heavily fouled. I had no need to adjust the valve in my testing, but the substantial lever was easy to turn, yet unobtrusive.