Do they accept regular AR mags ?
I had a chance to T&E a Sig 556. IMHO -
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Ergonomics - Bad. Placement of the selector is less than optimal, doesn’t allow quick change to safe if you have large hands. Very front heavy. The handguards and stock were strange and no one could get comfortable with continued use. The magwell seemed less than optimal as compared to an AR.
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Accuracy - Not on par with an AR with the loads we use (TAP). Groups were irregular; I attribute part of this to the trigger.
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Dependability - We had constant FTE and misfeeds. Charging handle broke off .
I’d pass as the QC seems less than great. I remember a figure being kicked around at 1.1K. For that I’d pick up a Colt 6920; or if your set on a piston system, the Colt 1020 when they bring it to market. FWIW
ETA - To answer your question, yes, it used standard AR mags
Wow, great info, thanks for the reply. My buddy was talking about he had a few suppose to be coming in, if they ever ship. I’ve really become quite in love with my M&P now after firing it more. Plan on taking her out tomorrow for a few hundred rounds of lovin’ 
The ads make the 556 look mighty appealing though. I think I’ll just keep saving for an ACOG for my M&P.
Played with one at my local indoor range. Did not shoot, only handled.
Did not have any iron sights. Counter guy said it didn’t come with any. This, if true, bothers me as there is a front dove tail and it should come with basic iron sights.
Not fond of the AR type stock that comes with it. For that matter, why didn’t Sig go with the original folding stock that the 550 series was designed around in the first place?
For the money I can build and scope a really nice AR.
I’ll pass.