|Place of origin||USA|
|Used by||United States Navy Royal New Zealand Air Force|
|Designer||United States Navy|
|Case type||Rimless, bottleneck|
|Bullet diameter||19.9 mm (0.78 in)|
|Neck diameter||20.6 mm (0.81 in)|
|Base diameter||29 mm (1.1 in)|
|Rim diameter||29.5 mm (1.16 in)|
|Case length||109.5 mm (4.31 in)|
|Overall length||185 mm (7.3 in)|
|Maximum pressure||600 MPa (87,000 psi)|
The 20×110mm USN (also known as 20×110mm US Navy and 20mm Mk-100 Series) was developed by the US Navy after World War II for use in Mk 11 and Mk 12 autocannon. They are called the Mk 100 series as they range from Mk 101 to Mk 109.
It has the same rim diameter as the 20×102mm but a different length and contour. It is not interchangeable with any other 20×110mm cartridge.
Some of the available loads for the 20×110mm USN are as follows:
|Name||Pressure (PSI)||Projectile weight (grains)||Projectile Type||Muzzle Velocity (fps)||Identification|
|Cartridge, 20mm, High Pressure Test, Mk 101 Mod 0 (USN)||Not to exceed 72,500||1700||Inert||Not listed||Green or blue projectile with brown nose and 1/4 black letters reading "High Pressure Test Round"|
|Cartridge, 20mm, Low Pressure Test, Mk 102 Mod 0 (USN)||Not listed||1700||Inert||Not listed||Green or blue projectile with brown nose and 1/4 black letters reading "Low Pressure Test Round"|
|Cartridge, 20mm, High Explosive Incendiary, Mk 105 Mod 0 (USN)||Not to exceed 72,500||1700||Inert||3350||Green or blue projectile black lettering or blue projectile with brown nose and black lettering|
|Cartridge, 20mm, High Explosive Incendiary, Mk 106 Mod 0 & 1 (USN)||60000||1700 ±50||Impact detonating||3350||Unpainted fuze, red and yellow projectile|
|Cartridge, 20mm, Armor Piercing-Incendiary, Mk 107 Mod 0 (USN)||60000||1700 ±50||Armour Piercing Incendiary||3350||No fuze, Nose of projectile blue or brown with red band. Body of projectile black with white lettering.|
|Cartridge, 20mm, Armor Piercing-Tracer, Mk 108 Mod 0 (USN)||60000||1700 ±50||Armour Piercing Tracer||3350||No fuze. Hollow windshield. Brown or yellow nose, body of projectile black with white lettering.|
There is also a Cartridge, 20mm, Dummy, Mk 103 Mod 0 (USN) which is an inert round. It has an empty primer pocket and holes in the case or a plugged primer pocket. It may be empty or filled with inert material. The projectile is usually brass or bronze plated.