20 mm caliber is a specific size of popular autocannon ammunition. The dividing line between smaller-caliber weapons, commonly called "guns", from larger-caliber "cannons" (e.g. machine gun vs. autocannon), is conventionally taken to be the 20 mm round, the smallest caliber of autocannon. All 20 mm cartridges have an outside projectile (bullet) diameter and barrel bore diameter of approximately 0.787inches. These projectiles are typically 75to(-) long, cartridge cases are typically 75to(-) long, and most are shells, with an explosive payload and detonating fuze.
Weapons using this caliber range from anti-materiel rifles and anti-tank rifles to aircraft autocannons and anti-aircraft guns.
Twenty millimeter caliber weapons are generally not used to target individual soldiers, but rather objects such as vehicles, buildings, or aircraft.
Each weapon is listed with its cartridge type appended.
| Neopup PAW-20 |  South Africa | 20×42 mm | |
| Denel NTW-20 | South Africa | 20×82 mm Mauser | |
| Denel Vektor GA-1 | South Africa | ||
| Vidhwansak |  India | ||
| Anzio 20mm rifle |  United States | 20×102 mm | |
| Arash anti-materiel rifle | |||
| M61 Vulcan | United States | ||
| M197 electric cannon | United States | ||
| M39 cannon | United States | ||
| GIAT M621 |  France | ||
| ZVI PL-20 Plamen |  Czech Republic | ||
| Nexter Narwhal 20A | France | Based on the M621 | |
| Şahi 20-102 |  Turkey | ||
| XM301 | United States | [2] | |
| Denel NTW-20 | South Africa | 20×110 mm Hispano | |
| Metallic RT-20 |  Croatia | ||
| Truvelo CMS 20x110mm | South Africa | ||
| Yugoimport-SDPR M71/08 |  Serbia | Single barrel development of Zastava M55 | |
| Oerlikon KAE (KAA/KAB) |  Switzerland | 20×128 mm | Formerly known as Oerlikon 204GK/5TG |
| Meroka CIWS |  Spain | ||
| Oerlikon KAD | Switzerland | 20×139 mm | Formerly known as Hispano-Suiza HS.820 |
| GIAT M693/20 mm modèle F2 | France | ||
| Rheinmetall Rh 202 |  Germany | ||
| Denel Land Systems GI-2 | South Africa | ||
| Nexter Narwhal 20B | France | Based on the 20 mm F2 |
| Weapon | Country of origin | Cartridge | Notes | |
|---|---|---|---|---|
| Becker Type M2 20 mm cannon | 20×70 mm RB | |||
| Oerlikon FF | 20×72 mm RB | |||
| Type 99 cannon, model 1 | ||||
| Ikaria-Werke Berlin MG FF/M cannon | 20×80 mm RB | |||
| Mauser MG 151/20 | 20×82 mm | |||
| Ho-5 cannon | 20×94 mm | |||
| Berezin B-20 | 20×99 mm R[3] | |||
| ShVAK | ||||
| Helenius RK-20 APH |  Finland | Derivative of the Helenius RK-97 12.7 mm anti-materiel rifle | ||
| Ghan-Krnka fortress rifle | 20.3×95 mm R | Bullet diameter 21 mm | ||
| Type 99 cannon, model 2 | class=nowrap | 20×101 mm RB | ||
| Solothurn S-18/100 | 20×105 mm B | |||
| Bofors m/45 | 20×110 mm | |||
| Bofors m/49 | ||||
| class=nowrap | Hispano-Suiza HS.404 and derivatives | |||
| Oerlikon F, FFL | 20×110 mm RB | |||
| Polsten | 20×110 mm RB | |||
| Colt Mk 12 cannon | 20×110 mm USN | Advanced derivative of the HS.404 | ||
| Madsen 20 mm anti-aircraft cannon | 20×120 mm | |||
| Type 97 automatic cannon | 20×124 mm | |||
| Mauser MG 213 | 20×135 mm | |||
| Solothurn S-18/1000 anti-tank rifle | Switzerland | 20×138 mm B | ||
| Solothurn S-18/1100 anti-tank rifle | Switzerland | |||
| ST-5 20 mm AA gun | Switzerland | |||
| FlaK 30 and FlaK 38 single-barrel AA | Forerunner of Hispano-Suiza HS.820's post-war 20×139 mm round | |||
| Flakvierling quadruple-barrel AA | ||||
| MG C/30L aircraft gun | ||||
| Rheinmetall KwK 30 and KwK 38 | ||||
| Cannone-Mitragliera da 20/77 (Scotti) | ||||
| Lahti L-39 anti-tank rifle | Finland | Solothurn Long | ||
| Lahti L-40 anti-aircraft gun | Finland | |||
| Nkm wz.38 FK anti-tank vehicle-mounted gun |  Poland | Designated "heaviest machine gun", or "Najcięższy karabin maszynowy, Nkm", AA/AT (actually an autocannon) | ||
| Type 98 20 mm AA machine cannon and Type 4 20 mm twin AA machine cannon | 20×142 mm | |||
| Bofors m/40 | 20×145 mm R | |||
| Pansarvärnsgevär m/42 | 20×180 mm R | Recoilless rifle |
The usual nomenclature of ammunition indicates the diameter of projectile and the length of the cartridge that holds it; for example, 20×102 mm is a 20 mm projectile in a 102 mm long case.
Though this designation is often assumed to be unique, this is not always the case, e.g. there are three different 20×110 mm types which are not compatible. These may be distinguished in that some cartridge designations may include additional letters or names as a suffix, e.g. the various different types of 20×110 mm might be distinguished as 20×110 mm Hispano, 20×110 mm RB and 20×110 mm USN.
