As you probably know, Vince, the Sig 556 is a not a true AR-15 platform (gas piston vs direct impingement) but does share some features and parts interchangeability.
I was just looking at an AR-10 built by ADDAX the other day on Calguns. Stunning.
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2004 NASA AIX Mustang LS2 #14
1964 Lincoln Continental
2014 4 tap Keezer