From sigs website:
SIG556® ER features a 16 military grade cold hammer forged barrel, chambered in 5.56mm NATO, with a twist rate of 1 in 7. The flash suppressor uses a standard .5 x 28 TPI thread pattern, ensuring compatibility with after-market accessories. The forearm housing the gas operating system is a vented non-slip polymer featuring the SIG TriRail design with three integrated Picatinny rails for mounting accessories. The optional flip up front combat sight is adjustable for windage and elevation. The Picatinny rail equipped receiver is made of high strength steel with a durable wear-resistant Nitron® finish. The Picatinny rail features an optional flip up rear sight that works in concert with the optional front combat sight, providing iron sight performance and a back up sight option for optics. The trigger housing is machined from an aircraft grade aluminum alloy forging with a hard-coat anodized finish designed to survive extreme conditions. The rifle comes equipped with a smooth two-stage trigger, ambidextrous safety and is designed to accept standard AR magazines. A 30-round magazine is supplied with each rifle. The collapsible buttstock features a rubber butt pad, multiple sling attachment points, and a waterproof battery compartment. The ergonomically designed pistol grip features an integrated storage compartment.