A couple of months ago I traded for a SIG 556 Classic with no front sight. Like so many others before, the previous owner had felt the stand up, hooded front sight was in the way and had removed it. Then lost it. :-(
I began the search for a replacement sight and soon found MFI. They offer a folding front sight that seemed to be exactly what I was looking for. They make more than one height and style of front sight because of the different rear sights the 556 has come with. The web site very clearly pictures the different styles of rear sights and which front sight works with each one.
I ordered the one to go with the lollipop rear sight on my rifle and waited. The sight arrived in 4 days. Excellent service.
The sight is extremely well made with no machining marks. The folding detent is positive and clicks into the up and down position nicely.
It fit the dovetail on the gas block perfectly. Just tight enough to require a drift to get into place but not so tight as make you worry that you will mar the finish.
The adjustment screw and locking pin were equally well fitted.

I am very pleased with this sight and would highly recommend it to anyone needing a front sight for their SIG 556.

http://mfiap.com/i-14389262-mfi-sig-classic-556-556-r-550-1-2-front-flip-up-folding-sight-windage-adjustment-wheel-see-instructions-for-installation.html

I have heard good things about MFI products. I did not like the way the Sig USA sights looked.   I was going to get one of their front sights but came across a good deal on a 552 front sight.  I know it does not match up perfectly with the popsicle rear but I am putting a red dot on it anyway.   I love my 556.  I should only have another week or two for my Form 1 then it will be chopped!!!