SIG SG-550, SG-551, SG-552 And SG-556 SWAT Assault Rifle

The SIG SG-551 Assault Rifle is a carbine version of the SIG SG-550 AssaultRifle, with a shorter 14.6-inch barrel, no bipod, and a rudimentary cheekpiece on the stock, but otherwise identical to the SG-550. A variant of the SIG SG-551 Assault Rifle, the SIG SG-551 SWAT, is identical except for a redesigned stock and the addition of a permanently-mounted MIL-STD-1913 rail to allow a greater range of sights to be mounted. The SIG SG-552 is a short-barreled carbine for use by special operations personnel; it also has a permanently-mounted MIL-STD-1913 rail, and can still mount an underbarrel grenade launcher or fire rifle grenades, despite the 10.7-inch barrel. A three-round burst mechanism has been added to the normal selector modes. The barrel is equipped with a muzzle brake instead of a simple flash suppressor. It also has a folding stock, and is known as the Commando. This version was not introduced until 1998.

SIG SG-550 Assault Rifle

SIG SG-550Sniper Rifle

 In addition to these variants, semiautomatic versions of the SIG SG-550 Assault Rifle and SIG SG-551 Assault Rifle also exist for civilian sales. They are called the SG-550SP and SG-551SP; the SG-551SP version is rarely found outside of Switzerland, however, due to the short length of its barrel and the plethora of government regulations in various countries. The SG-550SP is often found outside Switzerland minus its flash suppressor and sold with 10-round magazines (particularly in the US prior to the sunset of the Assault Weapon Bans).

SIG SG-552 Swat Assault Rifle

SIG SG-551 Swat Assault Rifle

SIG SG-552 SEALS Assault Rifle

 Finally, the SG-556 is a version of the SG-550 designed for sales in the US, and built in SiGArms’ facilities in the US. Internally, the SG-556 is virtually identical to the civilian versions of the SG-550; however, they have some changes both to suit US firearms regulations and to suit the tastes of American shooters. The cold-forged barrel is 16 inches long, and it is tipped with a flash suppressor which is similar (but not exactly the same) as that of an M-16A2. The fore-end is coated with ribbed, non-slip rubber.

The receiver is topped with a MIL-STD-1913 rail, and three more are on the front of the weapon. Both the front and rear iron sights are military-type and flip down if other optics are mounted. (They can also be removed entirely.) The SG-556 is designed to use standard AR-15/M-16 magazines (the standard SG-550 series uses proprietary magazines). Law enforcement versions have a removable rubber handgrip/fore-end, which can be replaced with a fourth MIL-STD-1913 rail. The SG-556 was introduced at the 2006 SHOT show and began sales the following fall. Six versions are built: the Classic, the DMR (Designated Marksman Rifle), the Classic SWAT, the SCM (Sport Configuration Model), the Patrol Rifle, and the SWAT Patrol Rifle.

SIG SG-552 Commando

SIG SAUER SG Assault Rifle Variants

All SG-556s use an almost identical action to the SG-550 series, but are modified to make a conversion to automatic fire virtually impossible. The Classic is a basic-configuration that is, ironically, one of the newest versions of the SG-556. The stock of the Classic may be a fixed, skeletonized stock adjustable for length of pull, a similar skeletonized stock that folds to the right, or a Vltor 5-position stock, similar in design to that used on the M-4 but more adjustable and with a compartment in the rear large enough to house a standard cleaning kit. The Classic is equipped with a small red-dot-type sight with a 1.5x magnification and a
folding rear diopter sight. The front sight is fixed.

The cold-forged barrel is 16 inches long, and it is tipped with a flash suppressor which is similar (but not exactly the same) as that of an M-16A2. The fore-end and pistol grip are coated with ribbed, non-slip rubber. The receiver is topped with a MIL-STD-1913 rail. The Classic SWAT is very similar, but has a handguard with four MILSTD-1913 rails as well as one above the receiver.

SIG SG-556 SWAT Assault Rifle

This Variant SIG Assault Rifle was developed in response to a Swiss need for a new service rifle to replace the STGW-57; in Swiss service, this assault rifle is called the StG-90. It was adopted in 1984. The weapon was designed for light weight, balance, and accuracy, using plastics for the buttstock, handguard, and pistol grip, as well as the magazines. The stock is folding and skeletonized to reduce weight.

SIG SG556 Assault Rifle

Variants SIG SG556 Assault Rifle

The SIG SG-556 SWAT magazines are clear so ammunition levels can be checked, and are equipped with studs and lugs so that up to three of them can be clipped together for rapid changing. The sights consist of day and illuminated night rear sights (one flips the sights to change between the two), and a hooded post-type front sight (also with a tritium inlay). The sights are adjusted for elevation and windage from the rear sight only, and when either the day or night sight is adjusted, the other sight is simultaneously adjusted. There is a STANAG-compatible sight mount for optics of the NATO sort, and this mount can also accept a MIL-STD-1913 rail. The SG-550 is notable for its long 20.8-inch barrel; in addition, a bipod is standard equipment. Various Western-type 40mm underbarrel grenade launchers may be mounted.

The DMR uses a match-grade, cold hammer-forged 21-inch heavy barrel, tipped by a target crown instead of a flash suppressor. The handguards and fore-end are designed to be as non-slipped as possible; they have a flared area at the bottom that is ribbed to ensure a positive grip. The stock does not fold, but is adjustable for length of pull and has an adjustable cheekpiece. At the front end of the handguards are three short lengths of MIL-STD-1913 rail; the bottom rail is normally occupied with the equipped light bipod that is adjustable for height and cant. There is also a MIL-STD-1913 rail atop the receiver; the DMR does not normally come with iron sights, though it comes with a telescopic sight. The SCM is constructed in much the same way,
but with no bipod, a telescopic sight is not included, and the barrel is 16 inches; the stock is also fixed. The Patrol Rifle is essentially a Classic with an option for quad-MIL-STD-1913-rail fore-end and a stock that both slides and side-fold; the SWAT Patrol Rifle is a patrol rifle with a skeletonized side-folding and sliding stock, more finely-adjustable sights, and a quad MIL-STD- 1913 rail for the fore-end as standard.

In mid-2009, SiG-Sauer introduced the P-556 – one of those “pistol” versions of a rifle. The P-556 is for the most part an SG- 556 Classic with a short, 10-inch barrel and no stock, nor any attachments for a stock. The P-556 has a MIL-STD-1913 rail above the receiver, and is tipped with a flash suppressor similar to that of the M-16A2/M-4 (a model often called the “A2” flash suppressor). Though the P-556 does not have any stock attachments, shooters have quickly discovered that it is possible to add stock attachments and convert into a short-barreled rifle (it’s legal in many places, including most of the US, if you pays the money
and does the paperwork), and several kits are available to do this. Some generic figures for stocked P-556s are presented below. The P-556 could, of course, be tricked out even further.

Early in 2009, SiG-Sauer also introduced a version of the SG-550 series in .22 Long Rifle, called the SIG-522. The SIG-522 comes in two rifle versions, the SIG-522 Classic and the SIG-522 SWAT, as well as a pistol version, the P-522. The SIG-522 Classic largely follows the lines of the SG-550 series, and many parts of the SIG-522 are interchangeable with those of the SG-550 series, particularly the furniture and receiver halves. The folding stock is the same as that used on the SG-556 Classic, as are the pistol grip, handguards, and trigger group.  The furniture is almost totally of polymer, though the stock is light alloy encased in polymer with a non-slip buttplate. The upper receiver is topped with an integral MIL-STD-1913 rail, and short lengths of MIL-STD- 1913 rail are attached to either side of the handguards near the front and above the gas block; backup iron sights which attach to the receiver’s rail as well as the gas block’s rail are available.

The 16.6-inch barrel is tipped by a flash suppressor which is similar in design to a standard SG-550-series rifle. Magazines are designed to look like standard 20 or 30-round 5.56mm magazines, but have inserts to fit the smaller rounds. The SIG-522 SWAT is for the most part the same, but has a four-point MIL-STD-1913 rails on the handguards; the top rail forms a continuous rail with the MIL-STD-1913 rail atop the receiver. The pistol counterparts, the P-522 Classic and P-522 SWAT, essentially follow the lines of their rifle counterparts as well as the P-556 pistol. Barrel length for the P-522 versions is 10.6 inches, and as with the P-556, kits exist to attach a stock to the P-522 to turn it into a short-barreled rifle.