The Lahti L39 was the Finnish answer to the need for an anti-tank rifle, developed just before the Winter War. The rifle was created by noted Finnish designed Aimo Lahti, who had pressed for it to use a 13.2mm cartridge. However, arguments for using a 20x138B cartridge won out, based on hopes to use that cartridge in both antitank and antiaircraft roles, as well as testing that showed the 20mm projectile to have greater terminal effect.
The L39 was not available for use in the Winter War (having been adopted barely 2 months before the Russian attack), but was used extensively in the Continuation War. While improved tank armor quickly became thick enough to protect against the round, it was used for a variety of anti material roles, attacking machine gun positions, bunkers, light vehicles, and more. In 1944, an anti-aircraft version was also produced, firing in full auto and using 15-round magazines.
Today, ammunition is available from a few companies, typically using lathe-turned new cases and surplus 20mm Vulcan projectiles.
Boys AT Rifle: https://www.full30.com/video/9b1276adfb1ad70270819f43a393b61c
PzB-39 AT Rifle: https://www.full30.com/video/323640202084431e6d4dd2c208b4c4a5
GrB-39 AT Rifle: https://www.full30.com/video/adf2d599d1ba4b436810567436229f7a
Mauser 1918 Tankgewehr: https://www.full30.com/video/820785934d86e231ed83bd9ba0b8a6a7
Bofors 37mm AT Gun: https://www.full30.com/video/b2f6d40d056b8da10f1b19b595cde412
US M6 37mm AT Gun Motor Carriage: https://www.full30.com/video/dc5f7a3f17c7afa9313a03bd18fc6910