Having a Sig556, but not a 516, and knowing Sig, it probably has sights. The good news is it's removable if needed.
http://www.defensereview.com/sig-sauer-sig516-short-stroke-gas-pistonop-rod-ar-weapons-series/Firearms blog lists a spec sheet with flip up BUIS. and pics, 7" barrel option?

Nice...
http://www.thefirearmblog.com/blog/2009/09/16/sig516-sig-sauer-enters-the-ar-15-market/Specifications
Caliber 5.56mm NATO
Fire modes single shot, 3 round burst and fully automatic (except marksmen models which are semi only)
Controls Ambidextrous
Gas system piston system with 3 or 4 position gas regulator
Upper Receiver Flat top with picatinny rail
Lower Receiver forged 7075-T6 aluminum
Trigger Milspec for all models except the Marksmen which have 2-stage match trigger
Capacity Takes standard AR-15 magazines
Finish black hard coat finish
Barrel chrome lined, cold hammer forged, nitrate finish
Barrel Twist 1:7" right hand
Stock collapsible SOPMOD buttstock
Weight 6 lbs. (PDW) - 7.28 lbs (Patrol) - 8 lbs (Precision Marksman)
Rails Free floating aluminum alloy M1913 picatinny quad rails
Sights flip-up BUIS (backup iron sights)Magazine Ships with 30 round AR-15 magazine. Marksmen models ship with 10 or 20 round magazine.
Other Features 0.5x28TPI threaded muzzle
MSRP (Price) not known