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). 
10 cm schwere Kanone 18 The 10.5 cm schwere Kanone 18 (10.5 cm sK 18) was a field gun used by Germany in World War II. The German army wanted a new 10.5 cm gun as well as 15 cm howitzer which were to share the same carriage. Guns are heavier than howitzers due to the longer barrel. This also led to the 15 cm sFH 18. As such both weapons had a similar weight and could be carried by a similar carriage. By 1926 Krupp and Rheinmetall had specimen designs, and prototypes were ready by 1930, but was not fielded until 1933–34. Both Krupp and Rheinmetall competed for the development contract, but the Wehrmacht compromised and selected Krupp's carriage to be mated with Rheinmetall's gun.