The 20×138mmB or Long Solothurn cartridge is a type of ammunition used mainly for anti-aircraft and anti-tank weapons during World War II.

The designation means the caliber is 20 mm, the length of the cartridge case is 138 mm and B indicates it is a belted case.

The loaded cartridge weighs 300 grams (10.7 oz).

[1] The 20×138mmB cartridge was developed by the Waffenfabrik Solothurn, a Swiss front company of Rheinmetall in the early 1930s from a less powerful cartridge 20×105mmB cartridge.

The 20×138mmB cartridge is used in the following weapons: