I had the opportunity to shoot a SIG 556 today. I had my AR at work and did a side by side comparison.
Ergonomics: SIG 556 has a terribly small bolt release, fine motor movement skills needed to work. Ambi safety was positive. Slight weight forward bias, felt lighter than standard M4. SIG has an adjustable gas/piston system. If your rifle starts to foul or rounds aren't ejecting, you can manually adjust the piston system to open for increased reliability, staying in the fight, until you can do a cleaning. Mag release, just like an M4. Advantage: tie.
Trigger: SIG 556 has a very loooong trigger pull. Felt like a 3 stage trigger, not gritty, just loooong. Once at the break point it was very light, but would take some training (untraining) to get used to. Definitely advantage to the 2 stage AR trigger.
Shooting: SIG 556 was very flat, with very little felt recoil. Still nothing like an AR platform with a JM comp. Comparing stock muzzle break / flash hider, the SIG has the advantage. Advantage: SIG
Price: The retail at the store on the SIG 556 is $1299.99. For less money you can get an AR15 rifle that will do the same thing and still have money left over for ammo and mags. Advantage: AR
Hope this helps.
Any other info or questions you might have, post away.
