by David on Thu Apr 12, 2012 9:56 am
I don't have any experience with the 516, but it's a piston-driven AR, and that's a platform you're either interested in or not. I can't compare SIG's version to anyone else's because I've never handled one.
The 556 and 551 are very different from the 516, and any AR, however. I bought the 551 because I wanted the original Swiss configuration. It compares favorably to the 556 in that the finish is much more durable, and the stock is solid as hell, unlike the crappy, rattly stock that comes on most 556s. The 556 has a few advantages over the 551, however, that I think most shooters in the U.S. will appreciate. It uses regular, good 'ol AR15 magazines, which are so common and cheap you practically trip over them around here. The 551 uses a proprietary Swiss magazine, which is harder to find, more expensive, and only work in one rifle. It is a fantastic magazine, but there are those drawbacks.
The 551 also is not set up to be a ninja rifle. There is one top rail, on the receiver only, suitable for an optic but no room for anything else, and no side rails. You can get a rail system for it, but why bother? If you need that, then the 556, which comes with one, is by far the better choice. Just be prepared to put a different stock on it, and deal with all the paint being chipped away around the ejection port, and you're golden. SIG does sell a "classic" version of the 556, however, so if you like the classic forend and don't need rails, that's a nice "in-between" compromise that gets you that old-school look with AR15 magazine compatibility. I really like that rifle, but I'm thinking of selling mine since the 551 sort of takes its place now.