Re: Sig 556
I have had one for a few years now. It has proven to be very reliable, though not particularly accurate. I would say 2 MOA+ with most ammo. Fine for combat, but not very good for target work.
You can shave some weight off the front if you add the 551 handguards. Makes a noticeable difference in handling vs the "fish gill" unit that the gun ships with.
Some of the early ones came with Swiss parts, and are considered to be a bit more "valuable" as a result. You might hunt for a used one. For more info, you can check out the sigforum or sigarms556 web sites.
I like mine. It is a bit heavy, even with mods, and probably wouldn't be my "go to gun", but it is fun to shoot and as I said, seems to be reliable.
I am a big fan of the Stgw 90, and this is probably as close as I will ever get to owning one unless I hit the lottery or something.
Greg