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The 20×138mmB "Solothurn Long" is a belted centerfire cartridge that was mainly used in anti-aircraft and anti-tank weapons during World War II. It was developed by the Swiss Solothurn Firearms Company during the early 1930's. The designation means the bullet diameter measures at 20 mm, the length of the cartridge case is 138 mm and "B" indicates that it is a belted case. A loaded cartridge weighs in at 300 grams (10.7 oz).


  • Case type: Bottleneck, belted
  • Bullet diameter: 20 mm (.787 in?)
  • Neck diameter: 20.9 mm (0.82 in)
  • Shoulder diameter: 25.5 mm (1.00 in)
  • Base diameter: 26.86 mm (1.057 in)
  • Rim diameter: 28.48 mm (1.121 in)
  • Rim thickness: 26.86 mm (1.057 in)
  • Case length: 138 mm (5.4 in)
  • Overall length: 202.7 mm (7.98 in)

Ballistic Performance

Bullet Weight / Type Velocity Energy
1851.88 gr. (120 grams) 2,950 ft/s (900 m/s) 34,655 ft lbs (47,000 J)
  • Note: We tested the numbers above from the source and they don't seem to add up correctly, since they're showing 35,794 ft lbs in the energy calculator. More accurate information is needed.


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


  • Solothurn anti-tank rifles: S-18/1000 and S-18/1100


  • FlaK 30 and FlaK 38 single-barrel, and Flakvierling quadruple-barrel anti-aircraft guns
  • KwK 30 and KwK 38 vehicle-mounted weapons
  • MG C/30L aircraft gun


  • Cannone-Mitragliera da 20/65 modello 35 (Breda)
  • Cannone-Mitragliera da 20/77 (Scotti)


  • Lahti L-39 anti-tank rifle
  • Lahti L-40 anti-aircraft gun


  • Nkm wz.38 FK anti-tank vehicle-mounted gun