Dan Coonan first designed his pistol in graduate school as a drafting project: a 1911 pistol chambered for the .357 Magnum cartridge. No easy feat, designing a self-loading pistol magazine to cycle a big rimmed revolver cartridge! This was in 1977, and he went about creating a business to manufacture them a few years later with the first guns available in 1983. The company lasted until 1998, and then a new firm (Coonan Inc) was reformed in 2009 and lasted again until 2019.

The Coonan .357 Magnum Automatic is essentially just a 1911 pistol stretched for the new cartridge, but there are a number of more subtle design changes made to improve its reliability, accuracy, and manufacturability. An external extractor was used, and shortly after production began the barrel link was replaced by a fixed cam (like the High Power). Today the guns are out of production (total production was something like 10,000 examples), but have a reputation for excellent accuracy and reliability, belying the difficulty of their core design.