The Bofors 40-mm gun is an anti-aircraft, multi-purpose autocannon designed in the 1930s by the Swedish arms manufacturer AB Bofors. It was one of the most popular anti-aircraft systems during World War II, used by most of the western Allies as well as the Axis powers. The cannon remains in service as of 2013, making it one of the longest serving artillery pieces of all time as well as the most wide spread. During World War II, In order to supply both the United States Army and Navy with a much greater number of guns, Chrysler began mass production. Over the lifetime of the production, their engineers introduced numerous additional changes to make the manufacturing process more efficient, eventually reducing the overall time needed to build a gun by half. In U.S. Army service, the single mount Bofors was known as the 40-mm Automatic Gun M1. The U.S. version of the gun fired three variants of the British Mark II high explosive shell as well as the M81A1 armor-piercing round, which was capable of penetrating some fifty millimeters of homogeneous armor plate at a range of 500 yards. Created in 1944, this technical manual reveals a great deal about the 40-mm Automatic Gun M1's design and capabilities. Intended as a manual for those charged with operation and maintenance, it details many aspects of its sighting equipment, ammunition, accessories much more. Originally labeled restricted, this manual was declassified long ago and is here reprinted in book form. Care has been taken to preserve the integrity of the text.