Yes, I have an AC556 - blued 13" folder. I really like it. I've fired thousands of rounds through it and have not had anything break. It's reliable and durable. In fact, I mainly shoot Wolf steel-cased poly-coated .223 which works just fine.
One dissapointing thing that you should know - Ruger has officially stopped doing refurbs on these guns. They may or may not accept them for repair, but you can no longer just send them the gun and tell them to do a complete refurb on it. Fortunately they don't break very often, but if they do, parts are difficult to find. Parts sets are nearly impossible to find right now, and if you find one you will pay dearly for it.
Still, in my opinion, the AC556 is a pretty good deal for a machinegun that fires a rifle cartridge - prices have come down some, which is unusual. The other decent deal out there are FNC's - but you really have to look to find a good price on one. And with an FNC, you really need to know who did the conversion. The only other thing worth considering is an UZI - but that's a subgun and you need to educate yourself on all of the different kinds of UZI conversions. For me it was a toss-up between the AC556 and an UZI. I decided I wanted something in .223 and not .45 or 9mm, so that's what decided it for me.
I never had a big desire to go the M16 route. But, if I did, I'd want one that is a true conversion that uses a mil-spec autosear, not a DIAS or a lightning link. Besides, it's always been out of my price range.
In any event, I've been extremely pleased with my AC556....here's a picture of it in a Sage stock with Ace folder.
