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Ho-301 40 mm cannon
Ho-301 40 mm projectile
A cutaway, showing the 40 mm caseless projectile.
Type Aircraft Autocannon
Place of origin Japan
Service history
Used by Japan
Wars Second World War
Weight 49 kg (108 lb)
Length 148 cm
Barrel length 78 cm

Cartridge 40 mm caseless
Caliber 40 mm (1.57 in)
Action API blowback
Rate of fire 475 rounds/min
Muzzle velocity 245 m/s (805 ft/s)
Effective range 150 meters (490 ft)
Feed system 10 round box

The Ho-301 was a Japanese 40 millimeter calibre autocannon that saw limited use during World War II, on Japanese Army Nakajima Ki-44 and Kawasaki Ki-45 KAI aircraft. It was unusual in using caseless ammunition. Although the effective range of the cannon was only 150 meters (490 ft), the Ho-301 was light and rapid-firing for its caliber.

The caseless design revolved around a specially designed projectile, that was in effect a small rocket. The round used an internal propellant chamber at the rear of the projectile containing a ten gram silk bag filled with smokeless powder. Drilled into the base plate of the round are twelve 3.8 millimeter diameter exhaust holes. The propellant chamber is sealed by a thin aluminum sealing cup, which covers the exhaust holes. When the primer is struck, the bag of propellant ignites, and the pressure rises until the aluminum cup bursts, allowing the exhaust gas to rush through the exhaust holes pushing the projectile forward. The propellant was exhausted before the projectile left the barrel.

Because of the low muzzle velocity (245 meters/second or about 804 ft/sec - equivalent to a moderately powerful air rifle)[1] the weapon was only suited to attacking bombers; even here the extremely low muzzle velocity made it difficult to achieve hits from any position besides directly astern the target aircraft and well within the range of defensive counterfire.

Ammunition specificationsEdit

  • Length of projectile (fuzed): 129 mm
  • Diameter of rotating band: 41 mm
  • Diameter of body: 39 mm
  • Wall thickness: 4 mm - 5 mm
  • Weight: 0.590 kg
  • Explosive content: 0.065 kg approx [2]
  • Fuze: Direct acting impact fuze

See alsoEdit


  • Williams, Anthony G (2000). Rapid Fire. ISBN 1-84037-435-7. 
  • TM 9-1985-5, Japanese Explosive Ordnance. Departments of the Army and the Airforce. 1953. 
  1. Williams & Gustin
  2. TM gives content as 2 ounces of H.E. filling plus 0.1 ounces of primer and 0.2 ounce gaine.

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