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The 516 appears to be pure AR lower, with Sig's own gas-piston upper, though even that is different from the 556.
Having owned both the Sig 556 and the Ruger SR-556, I can honestly say, the piston guns run a bit cleaner, but the added weight out front gets to be annoying, at least for me.
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Make mine lean, mean, and 9x19!
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