The 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 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 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 C42 ceased.
The 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.