The SG C42 was designed in the late 1970s in response to a commission from the Swiss Army for a new assault rifle to replace the aging StGW.57. The SG C42 was offered in a variety of different configurations, including a short-barreled carbine and a bullpup modification, and was chambered for experimental 6.45×48mm cartridge. When the development of the 6.45×48mm bullet was cancelled, the SG C42 was redesigned for the 5.6mm Swiss cartridge and tested against the SIG SG 550 in 5.56×45mm NATO. Ultimately, it was beaten out by the SG 550, which was adopted as Switzerland's service rifle in 1990. Consequently, development of the SG C42 ceased.
The SG C42 was a gas-operated assault rifle, constructed largely from stamped sheet steel with polymer furniture. The weapon's unusual ergonomics were designed to facilitate for winter gloves.