Nice weapon, looked to hold about 1-2 MOA when I shot it, not exceptional by today’s standards. But, big points for coolness factor.
SIG's 550 series are select fire assault rifles (machinegun). I'm told there are very few in private hands, I believe the 550's went into production in 1986, the same year as the "Firearm Owners Protection Act of 1986", effectively banning their sale to any regular folks and making them very rare. "Pre-86" are the only weapons you can hope to purchase.
Sig provided a civilian version. The SIG Sauer 556 SWAT Rifle, semi-automatic of course.
AR-15 magazines, SIG 550 look, and retained the two-position gas adjustable gas piston.
There used to be a guy on the hkpro forum, he was on the wet side of OR and had the 550 series for demo to LEO's. He used to show up and you could shoot it.
Best lead I have: I think his name was Greg/Ken/Hank, Class II manufacture/Class III dealer, and a HK21 guru if that helps. I bet he still has them, I would.
Regards,
Steve