In 1922, the Swedish Navy ordered 2-lb. “Pom Pom” guns as anti-aircraft guns. By 1928, the 2-pounders were inadequate and he Swedes began seeking out a replacement. They approched Bofors, a Swedish steel producer, about a replacement cannon. Bofors got to work on their 40 mm design and fired it off in 1930.
The development wasn’t without problem. The shells are huge, and to get the rate of fire the Swedish Navy demanded, they had to lighten them up. They tried a zinc casing that burned up when fired, but this just crudded up the inside of the barrel. Finally they tried a rear ejection system and a gravity-assisted “throwing” mechanism that would get the next shell into the breach. Eureka!
Now you had a gun capable of firing a 40 mm shell at about 120 rounds per minute. Mounted to a carriage that can turn 180 degrees and raise the barrel between -5-degrees and +90-degrees, the Swedes had a formidiable weapon to put aboard ships and submarines.
By the end of the 1930s, the world was outfitted with Bofors. As the nations scrambled to choose sides, the Bofors would be used on both sides as an anti-aircraft, anti-ship and anti-tank weapon. In fact, the Bofors is still in use today. You can find them on the mighty AC-130 gunships pointing down at the ground. The L70 variant, that Bofors designed at the end of WWII with a higher rate of fire and faster muzzle velocity to be able to stop the faster jet aircraft, is still used today on the Swedish CV90 Army Combat Vehicle, and by the Republic of Korea.
At the same time Bofors introduced the uprated L70 40mm gun, they introduced a 57mm gun. This gun, arguably part of the family tree stemming from that first gun in 1930, is still used by the Swedes, Americans, Brits, Canucks, Finns, and many others as a naval anti-aircraft gun.
[Image Credit: Public Domain]