I used a 551 with an iron sights for duty. I finally put an EOTECH on it and, while great for entry rifle purposes, I noticed my shooting fundamentals getting sloppy. The 551 was heavy, but solid. My team switched to 556 SWAT and we were among the first to get them. I also hurried out and bought my own SIG556 16" barrel. It came with the nail file sight, and the front flip up that doesn't lock in one place. Surprisingly, it shot pretty well with the emergency combat sights. The HALOTECH sight reticle that came with the rifle was flat out scary to shoot with rounds going from a quarter sized shot group to off the target (paper plate). It moved all over and I took it off and never had the chance to fall in love with my rifle. To complicate matters, the new duty 556 SWATs were falling apart literally, with no help or concern from the manufacturer. That eventually chaged after about a year of butt stocks falling off etc...We tried MAGPUL's flip up sights, but they wouldn't co-witness through the EOTECH. They also wouldn't mount on the rounded font forend of my own 556. With no options other than Aurora for my own 556, I called SIG and explained this history. I thought I was in the process of getting duly blown off, but they put me in touch with a regional rep. He was very helpful and got me a set of the SIG diopter and front hooded sight. I broke the front sight putting it on, and pleaded for another one, glad to pay for it. (You can't buy just the front hooded sight from SIG). He laughed and let me exchange for another one that was not broke. I paid for the sight to be mounted (and the gunsmith also had some trouble, but more brains and didn't break it). I finally got to break in my rifle with 200 rounds on good ol' iron sights and was shooting a nice tight group and am now in love with my rifle. Hope someone finds something useful in this saga.