The Bofors 40 mm Automatic Gun L/60 (often referred to simply as the "Bofors 40 mm gun", the "Bofors gun" is an anti-aircraft autocannon, designed in the 1930s by the Swedish arms manufacturer AB Bofors. The gun was designed as an intermediate anti-aircraft gun, filling the gap between fast firing close-range small calibre anti-aircraft guns and slower firing long-range high calibre anti-aircraft guns. For its time, the Bofors 40 mm L/60 was perfectly suited for this role and outperformed competing designs in the years leading up to World War II in both effectiveness and reliability.
40 mm L/56 Mark 1 / 40 mm L/56 Mark 2 — US Navy water-cooled variants with interchangeable barrel assemblies.[b] Both were used for all twin and quad mountings. -Wikipedia