If there is anything I can say about the construction of this rifle is that it is Robust and Very Reliable. This "C" model is in fact half a pound lighter in the front which makes a very noticeable difference in comparison to the original or "FB" model. Ruger accomplished this by Fluting the barrel and shortening the overall length of the barrel. The "FB" model comes with a 16" unfluted barrel PLUS the length of the flash-hider. This "C" model come with a fluted 14.5" barrel with a Flash hider permanently installed onto the end of it to make the OVERALL length 16".
The SR556 weapons system is just as reliable as any other mid-high end AR weapons system out there. I have over 2000 rounds through my SR556C with no F-T-Feed or F-T-Extract. I had 4 Primer strikes that failed to fire the round, but it turns out it was the cheap and unreliable Russian 5.56 ammo I was shooting. The only other competition the SR556 has is the SIG516 and I've held and shot both before I purchased my SR556C.
The Ruger just felt more solid and their customer service is second to none. Sig has had some quality control issues over the last few years with many of their products across the board being hit or miss. If you're on the fence about it, just do it. You will not regret your purchase.