The 20 mm calibre is a specific size of cannon or autocannon ammunition.
There are few weapons (aside from shotguns, large game hunting rifles, and heavy caliber muzzleloading "rampart" or "wall guns" popular in the early mid 19th century European militaries) which have been built that fire projectiles between 12.7mm (such as the 12.7mm NATO) and 20 mm caliber, though there were several 13mm heavy machine guns used during World War II, such as the MG 131; the 14.5 mm caliber is still used by some Soviet machine guns such as the KPV and anti-tank rifles such as PTRS, PTRD, and NTW-20.
Since 20mm is the cutoff point where most nations switch from bullets to shells, it has come to also generally be the cutoff point between weapons classified as a machine gun or a cannon. This can vary, however: for example, in World War II, Japan classified any weapon over 11mm as a cannon; thus, in Japanese records, their 12.7mm Ho-103 aircraft gun is classified as an autocannon, as it used explosive shells to overcome its lower muzzle velocity compared to American 12.7mm/.50BMG weapons). A machine gun does not fire shells, so historically a weapon that fires shells below 20mm can be a cannon. A true definition will always mention whether the weapon fires bullets or shells in addition to its caliber.
A very small number of anti-tank rifles have been produced in 20 mm and larger calibers.
20 mm caliber cartridges have an outside bullet diameter and inside barrel diameter of 0.787 inches (20.0 mm). Projectiles or bullets are typically 75 to 127 mm (3–5 in) long. Cartridge cases are typically 75 to 152 mm (3–6 in) long. Many but not all 20 mm rounds have an explosive filling and detonating fuze.
As an example, the RT-20 (rifle) fires a 20×110mm 130-gram projectile at a muzzle velocity of 850 m/s. For a simple slug this gives a muzzle energy of 47 kilojoules.
Like most cannon ammunition, 20mm caliber weapons are typically used against large targets such as vehicles, buildings, or aircraft. Though lethal against individual soldiers, 20mm ammunition is so large and heavy that its effects are inefficient against relatively small targets.High Explosive (HE)
High Explosive Incendiary (HEI)
Armour Piercing Incendiary (API)
Armour-Piercing Discarding Sabot (APDS)
High-Explosive Fragmentary tracer (HEF-t)
Penetrator with Enhanced Lateral Effect (PELE)
Target Practice - Inert projectile (i.e., PGU-27A/B). Used for training. (TP)
Target Practice Tracer - Inert projectile with tracer material in base for visual trajectory tracking (i.e., PGU-30A/B). (TP-T)
Each weapon is listed with its cartridge type appended.Vidhwansak: 20×82mm
Anzio 20mm rifle: 20×102mm
Mk4 20mm cannon: 20×110mmRB Oerlikon 'S'
HS804 20mm cannon: 20×110mm
Mk16 20mm cannon: 20×110mm
RT-20 bolt-action rifle: 20×110mm
Denel NTW-20: 20×82mm Mauser or 20×110mm Hispano caliber
M61 Vulcan: 20×102mm (PGU-28/B)
M197 Gatling gun: 20×102mm
M39 cannon: 20×102mm
GIAT M621: 20×102mm
Oerlikon KAA & KAB: 20×128mm (formerly the Oerlikon 204GK and 5TG respectively)
Oerlikon KAD: 20×139mm (formerly the Hispano-Suiza HS.820)
GIAT M693/20 mm modèle F2 gun: 20×139mm
Rheinmetall MK 20 Rh 202: 20×139mm
Denel Land Systems GI-2: 20×139mm
Becker Type M2 20 mm cannon: 20×70mm RB
Bofors m/40: 20×145mmR
Bofors m/45: 20×110mm
Bofors m/49: 20×110mm
Berezin B-20 20×99mm
Breda Model 35 20×138mm B
Colt Mk 12 cannon: 20×110mm USN (Mk 12 is an advanced derivative of the HS.404)
Hispano-Suiza HS.404 and derivatives: 20×110mm
Lahti L-39: 20×138mm B (Solothurn Long)
Madsen 20 mm anti-aircraft cannon: 20×120mm
Mauser MG 213: 20×135mm
MG FF/M cannon: 20×80mm RB
Mauser MG 151/20: 20×82mm
Nkm wz.38 FK designated: "Heaviest Machinegun" "Hmg" or "Najciezszy karabin maszynowy, Nkm," AA/AT 20×138mmB autocannon
Oerlikon FF: 20×72mm RB
Oerlikon F, FFL: 20×110mm RB
Rheinmetall FlaK 38: 20×138mm B forerunner of Hispano-Suiza HS.820 post-war 20×139mm round
Scotti 20 mm: 20x138mm B
Solothurn S-18/100: 20×105mm B
Solothurn S-18/1000: 20×138mm B
Type 99 cannon : 20×72mmRB (model 1), 20×101mmRB (model 2)
Ho-5 cannon : 20×94mm
Cartridge type indicates the diameter of projectile and the length of the cartridge that holds it; for example 20×102mm is a 20 mm projectile in a 102 mm long case. Only rarely do two designers use the same case length, so this designation is usually definitive. Some cartridge types have additional letters or information about them listed.