20 mm caliber is a specific size of popular autocannon ammunition. It is 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	20×42 mm
Denel NTW-20	South Africa	20×82 mm Mauser
Denel Vektor GA-1	South Africa
Vidhwansak	India
Anzio 20 mm rifle	United States	20×102 mm
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		[2]
Metallic RT-20	Croatia	20×110 mm Hispano
Yugoimport-SDPR M71/08	Serbia		Single barrel development of Zastava M55
Oerlikon 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

Historical weapons

Weapon	Country of origin	Cartridge	Notes
Becker Type M2 20 mm cannon	German Empire	20×70 mm RB
Oerlikon FF	Switzerland	20×72 mm RB
Type 99 cannon, model 1	Japan
Ikaria-Werke Berlin MG FF/M cannon	Nazi Germany	20×80 mm RB
Mauser MG 151/20	Nazi Germany	20×82 mm
Ho-5 cannon	Japanese Empire	20×94 mm
Berezin B-20	Soviet Union	20×99 mm R[3]
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	20.3×95 mm R	Bullet diameter 21 mm
Type 99 cannon, model 2	Japanese Empire	20×101 mm RB
Solothurn S-18/100	Switzerland	20×105 mm B
Bofors m/45	Sweden	20×110 mm
Bofors m/49	Sweden
Hispano-Suiza HS.404 and derivatives	Switzerland
Oerlikon F, FFL	Switzerland	20×110 mm RB
Polsten	Poland	20×110 mm RB
Colt Mk 12 cannon	United States	20×110 mm USN	Advanced derivative of the HS.404
Madsen 20 mm anti-aircraft cannon	Denmark	20×120 mm
Type 97 automatic cannon	Japanese Empire	20×124 mm
Mauser MG 213	Nazi Germany	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	Nazi Germany		Forerunner of Hispano-Suiza HS.820's post-war 20×139 mm 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)
Type 98 20 mm AA machine cannon and Type 4 20 mm twin AA machine cannon	Japan	20×142 mm
Bofors m/40	Sweden	20×145 mm R
Pansarvärnsgevär m/42	Sweden	20×180 mm R	Recoilless rifle

Naming conventions

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.

Common suffixes

• B e.g. 20×138B: the cartridge has a belt which is used for headspacing, i.e. it helps ensure the correct positioning within the gun's chamber.
• R e.g. 20×145R: a rimmed cartridge: the diameter of the rim forming the base is larger than that of the cartridge case itself.
• RB e.g. Oerlikon 20×110RB: rebated rim, one where the rim is a smaller diameter than the case head allowing the extractor to follow it into the chamber, facilitating advanced-primer ignition, a recoil-moderating system.