SIG551-A1, chambered for the widely used 5.56 x 45mm NATO caliber using Swiss Polymer magazines. The SIG551-A1 features a distinctive swiss folding stock, and an M1913 rail for accessory attachment. The 16” barrel features four-groove rifling and a twist rate of 1:7”. The SIG551-A1 measures 36.1” but can fold to 26”,rotary diopter sighting system