You sir, what have YOU DONE???!!! Lol, but seriously. The Sig 516 is just an AR with a short stroke piston gas system (and one of the cheaper ones mind you) while the 556 uses a long stroke piston system; the difference between the two aforementioned is that in a short stroke system the piston strikes the BCG such as in a LWRC, REC 7, etc, while in a long stroke the piston is attached to the BCG like in a AKM or PWS AR's. Also as earlier mentioned, the 556 is apart of the Sig 550 series of weapons; the family of weapons uses a proprietary magazine HOWEVER the 556 uses a different lower reciever to enable it to accept STANAG mags (in the common tongue, AR15/M16 type mags but there are tons of weapons in NATO that use them, just for you nitpickey rat @#$%s).