I have no experience with the SIG516, though it looks good. But as far as the SR-556's compared to other AR's I'm assuming your referring DI AR15's. Well first off the SR operates with a piston system and does keep out about 95% of the fouling that DI AR's dump into BCG and upper and lower receivers. The SR's also don't transfer the extreme heat to BCG caused by the gases. Now I'm not knocking the traditional DI system, it works, just that the SR's piston system is evolution. Ruger built the SR's solid as a rock. But with the piston system also comes some weight, Ruger also uses a medium/heavy barrel which also adds weight compared to a DI carbine. Ruger introduced the SR-556c to address the weight issue, it is 1/2lb lighter and a little shorter than the standard SR-556. Both SR's are a great choice, DI's are good also but I really hate the cleaning that they require. I think you are on the right track in choosing a piston AR.