The SIG556xi is based on the SG550, the standard-issue rifle for the Swiss military, which was originally adopted in 1990 to replacing the rather ungainly Stgw 57. The SIG556xi keeps the modular abilities of its predecessor but adds touches that will be more appreciated by American shooters.

This is a gas-piston-operated, semi-automatic rifle (although select-fire models are available for sale to government agencies) using a rotating bolt that resembles the bolt found on an AK. The gas system also features three settings to control the amount of gas that is fed into the gas tube and piston.

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If the rifle becomes extremely fouled, to the point where reliability suffers, the second setting directs more gas into the system to increase reliable cycling. This is only a temporary measure, and the system should be returned to the normal first setting as soon as it is convenient. The extra gas from the second position places more wear and tear on the rifle and increases recoil. The third position provides the proper amount of gas for properly cycling with a suppressor mounted.

The 16-inch steel barrel is nitride treated and can be easily removed by the user in the field without tools. Barrels are available in 10-, 14.50- and 16-inch lengths, and shooters can switch between 5.56mm NATO, 300 Blackout and 7.62x39mm barrels. The latter requires the use of a different bolt and lower receiver to accommodate standard AK magazines. Since only the upper receiver is serialized, you can have as many unrestricted lower receivers as you like.

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The handguards are also removable and replaceable to fit mission needs. Polymer, carbon-fiber and aluminum handguards are available to accommodate rails for mounting accessories. The polymer, side-folding stock makes transport much easier and optional receiver endplates allow for the use of standard AR extension tubes and your choice of the vast array of AR stocks. The upper receiver is steel and made from a stamping, like the AKM. Steel and aluminum flip-up diopter sights are standard, with the rear sight adjustable for windage and the front sight adjustable for elevation.

The rifle is almost completely ambidextrous, with a reversible charging handle, an ambidextrous safety selector and an ambidextrous magazine release. The lower receiver is made from aluminum and features an integral, extended triggerguard for use with gloves. The polymer pistol grip has internal storage space for spare batteries or parts and a very aggressive texture on the sides for a firm grip.

For more on the SIG556xi, keep your eyes peeled for a full test and evaluation in an upcoming issue of SPECIAL WEAPONS FOR MILITARY & POLICE. To subscribe, visit

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