Which riffle on which SIG SG 550 is based is a 5.56×45mm NATO assault rifle developed in the early 1970s by Schweizerische Industrie Gesellschaft?
SIG SG 550 The SG 550 is an assault rifle manufactured by Swiss Arms AG (formerly a division of Schweizerische Industrie Gesellschaft now known as Sig Holding AG) of Neuhausen, Switzerland. "SG" is an abbreviation for "Sturmgewehr", or "assault rifle". The rifle is based on the earlier 5.56mm SG 540 and is also known as the Fass 90 ("Fusil d'assaut" 90/"Fucile d'assalto" 90) in French/Italian or Stgw 90 in German ("Sturmgewehr" 90). 
General-purpose machine gun A general-purpose machine gun (GPMG) is an air-cooled, fully automatic, belt-fed weapon with a quick change barrel that can be used in a variety of roles, from bipod- or tripod-mounted infantry support, to deployment as a helicopter door gun, or a vehicle-mounted support weapon. Modern GPMGs fire full-powered rifle cartridges such as the 7.62×51mm NATO, 7.62×54mmR, 7.5×54mm French, 7.5×55mm Swiss, 7.92×57mm Mauser, etc.