Quoted:
I have updated the blog post with the specs.

There are six different models. I think that they look a lot like the Ruger SR-556 with a piston and 3/4 position gas regulator.

Uppers will be sold, but I am unsure if they mean to LEO/military or civilians.


http://www.thefirearmblog.com/blog/2009/09/16/sig516-sig-sauer-enters-the-ar-15-market/

Edit: corrected mistake.

The SIG516 Tactical Rifle line has three different models:

   * SIG516 PDW : 7" barrel. A personal defense weapon (pictured above).
   * SIG516 CQB : 10" barrel. A Colt M4 Commando equvelent.
   * SIG516 BASELINE CARBINE : 14.5" barrel. A M4 Carbine equvelent.
   * SIG516 Patrol. 16" barrel.
   * SIG516 Tactical Marksman: 16" barrel A squad level sniper / designated marksmen rifle.
   * SIG516 Precision Marksman: 20" barrel. Possibly a true sniper rifle, or designated marksmen rifle.

They are all piston operated and feature an adjustable gas regulator like the Ruger SR-556.

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

Upper receiver assemblies will be sold. Just before anyone gets their hopes up, while it is likely that Sig will sell civilians models, this has not been confirmed.