From the Sig Sauer website:
The Fulfilled Potential Of The AR-15/M16
One of the most tested and proven weapons systems in the world achieves ultimate refinement with the introduction of the SIG516 rifle. With its chrome-lined, nitride-finished, military grade barrel and adjustable, three-position gas regulator, the SIG516 incorporates all of the most desirable mechanical improvements. Add to these a free-floating quad rail forend and barrel, and you have the most advanced, versatile AR-based rifle available.
Note: Actual production rifle may differ slightly from photos.
• Gas piston operating system
• Three-position gas regulator
• 5.56 x 45mm NATO
• Free-floating military grade chrome lined barrel
• M1913 Picatinny flat top upper
• 7075-T6 Aircraft grade aluminum upper & lower receiver with hard coat anodize finish
Update: I have done a post that covers the entire Sig Sauer SIG516 model line.