Sig Sauer 556/516
not a good gun at all, they are plagued with reliability issues and are considered by many to be the absolute worst combat style semi auto ever released.
I have one of the first Sig 556's with the long gas piston. Had it since 2008 no issues at all. I like it more than my ar that I just got. Only complaint I have is the weight, one reason I got a ar and the lack of customization you can do. I have never shot one of the shorter gas system sigs so I can not comment on those.
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