There is an interesting thread
http://sigforum.com/eve/forums/a/tpc/f/9666031561/m/956106649
on SigForum from a guy who teaches a tact response class for contractors going overseas (some neat pix). In that thread he says that very, very few of the piston driven ARs make it through. I thought that was a very interesting comment and not at all what I expected as I have heard nothing but good things from my friends with the Sig 556 - of course, I doubt they are tough on them at the range as these guys are at work. I don't know that would stop me from buying one.
(I dont know if I can link to another forum so I will apoligize in advance)
ETA - the quote is about 2/3 of the way down the first page.