The Sig 556 is an interesting and impressive rifle. The two most successful military rifles since WWII, in numbers at least, have been the M16 and the AK-47. stretching a point a bit, it can be claimed the Sig 556 shares some of the virtues of both.
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 fullauto 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...