Twenty millimeter caliber is a common firearm bore diameter, typically used to distinguish smaller-caliber weapons, commonly called "guns", from larger-caliber "cannons" (e.g. machine gun vs. autocannon). All 20 mm cartridges have an outside projectile (bullet) diameter and barrel bore diameter of 0.787 inches (20.0 mm). These projectiles are typically 75 to 127 mm (3–5 in) long, cartridge cases are typically 75 to 152 mm (3–6 in) 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.

Usage

A 20×102 mm round (second from left) with .50 BMG rounds, golf ball, and a stick of 168-pin SDRAM computer memory

Twenty-millimeter-caliber weapons are generally not used to target individual soldiers, but have targets such as vehicles, buildings, or aircraft.

Types of ammunition

• High explosive (HE)
• High explosive incendiary (HEI)
• Armor-piercing (AP)
• Semi-armor-piercing high explosive incendiary (SAPHEI)
• Armor-piercing discarding sabot (APDS)
• High explosive fragmentation tracer (HEF-T)
• High explosive high capacity (HE-M)
• Penetrator with enhanced lateral effect (PELE)
• Target practice - inert projectile (i.e., PGU-27A/B)^[1] 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)

20 mm weapons

Each weapon is listed with its cartridge type appended.

Current weapons

Weapon	Country of origin	Cartridge	Notes
Neopup PAW-20	South Africa	20×42mm
Denel NTW-20	South Africa	20×82mm Mauser
Denel Vektor GA-1	South Africa
Vidhwansak	India
Anzio 20 mm rifle	United States	20×102mm
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
Metallic RT-20	Croatia	20×110mm Hispano
Yugoimport-SDPR M71/08	Serbia		Single barrel development of Zastava M55
Oerlikon KAA/KAB	Switzerland	20×128mm	Formerly known as Oerlikon 204GK/5TG
Meroka CIWS	Spain
Oerlikon KAD	Switzerland	20×139mm	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 20mm F2

Historical weapons

Weapon	Country of origin	Cartridge	Notes
Becker Type M2 20 mm cannon	German Empire	20×70mm RB
Oerlikon FF	Switzerland	20×72mm RB
Type 99 cannon, model 1	Japan
Ikaria-Werke Berlin MG FF/M cannon	Nazi Germany	20×80mm RB
Mauser MG 151/20	Nazi Germany	20×82mm
Ho-5 cannon	Japanese Empire	20×94mm
Berezin B-20	Soviet Union	20×99 mm R[2]
ShVAK	Soviet Union
Helenius RK-20 APH	Finland		Derivative of the Helenius RK-97 12.7 mm anti-materiel rifle
Ghan-Krnka fortress rifle	Russian Empire	20.3×95mm R	Bullet diameter 21 mm
Type 99 cannon, model 2	Japanese Empire	20×101mm RB
Solothurn S-18/100	Switzerland	20×105mm B
Bofors m/45	Sweden	20×110mm
Bofors m/49	Sweden
Hispano-Suiza HS.404 and derivatives	Switzerland
Oerlikon F, FFL	Switzerland	20×110mm RB
Polsten	Poland	20×110mm RB
Colt Mk 12 cannon	United States	20×110mm USN	Advanced derivative of the HS.404
Madsen 20 mm anti-aircraft cannon	Denmark	20×120mm
Type 97 automatic cannon	Japanese Empire	20×124mm
Mauser MG 213	Nazi Germany	20×135mm
Solothurn S-18/1000 anti-tank rifle	Switzerland	20×138mmB
Solothurn S-18/1100 anti-tank rifle	Switzerland
ST-5 20 mm AA gun	Switzerland
FlaK 30 and FlaK 38 single-barrel AA	Nazi Germany		Forerunner of Hispano-Suiza HS.820's post-war 20×139mm round
Flakvierling quadruple-barrel AA	Nazi Germany
MG C/30L aircraft gun	Nazi Germany
Rheinmetall KwK 30 and KwK 38	Nazi Germany
Cannone-Mitragliera da 20/65 modello 35 (Breda)	Kingdom of Italy
Cannone-Mitragliera da 20/77 (Scotti)	Kingdom of Italy
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)
Bofors m/40	Sweden	20×145mm R
Pansarvärnsgevär m/42	Sweden	20×180mm R	Recoilless rifle

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. A B following the case length such as the 20X138B, indicates the cartridge has a belt which is used for headspacing. An R indicates a rimmed cartridge such as the 20X145R. An RB indicates a rebated rim, one which is a smaller diameter than the case head such as the Oerlikon 20X110RB.