20 mm/85 (0.79") GAM-BO1
Updated 21 June 2008

This is the Oerlikon KAA cannon used in a manually-controlled mounting built by BMARC (British Manufacture and Research Company) under the designation A41/820.  Installed on many British warships after the Falklands War stimulated renewed interest in light machine guns.

"This cannon is essentially equivalent to the World War II Oerlikon 20 mm gun, with a higher muzzle velocity" - Norman Friedman in "The Naval Institute Guide to World Naval Weapon Systems 1991/92."


20 mm/85 (0.79") GAM-BO1 on Royal Navy Warship
Photograph copyrighted by Oerlikon Contraves AG


20 mm/85 (0.79") GAM-BO1 on Royal Navy Warship
Royal Navy Photograph


20 mm KAA Cannon
Photograph copyrighted by Rheinmetall AG

Gun Characteristics
Designation Cannon:  Oerlikon 20 mm/85 KAA
Mounting:  GAM-BO1
Ship Class Used On Modern British warships
Exported to many countries
Date Of Design N/A
Date In Service about 1985
Gun Weight 203 lbs. (92 kg) without ammunition
401 lbs. (182 kg) with 200 round belt
Gun Length oa N/A
Bore Length 55.1 in (1.400 m)
Rifling Length N/A
Grooves N/A
Lands N/A
Twist N/A
Chamber Volume N/A
Rate Of Fire Cyclic:  900 - 1000 rounds per minute
Type Fixed
Weight of Complete Round N/A
Projectile Types and Weights HE-I - 0.264 lbs. (0.120 kg)
Bursting Charge N/A
Projectile Length N/A
Length of Complete Round:  8.0 in (20.3 cm)
Propellant Charge N/A
Cartridge 20 x 128 mm (cartidge size is actually 128.7 mm)
Muzzle Velocity 3,440 fps (1,050 mps)
Working Pressure N/A
Approximate Barrel Life N/A
Ammunition stowage per gun
(on mount)
200 rounds
Elevation With 0.264 lbs. (0.120 kg) HE
Effective 2,200 yards (2,000 m)
Maximum 7,450 yards (6,800 m)
Aircraft 1,650 yards (1,500 m)
Mount / Turret Data
Designation GAM-BO1
Weight About 1,000 lbs. (454 kg)
Elevation -10 / +55 degrees
Elevation Rate Manual Elevation, only
Train 360 degrees
Train Rate Manual Training, only
Gun recoil N/A
Data from
"The Naval Institute Guide to World Naval Weapon Systems 1991/92" by Norman Friedman
"Jane's Ammunition Handbook:  Ninth Edition 2000-2001" edited by Terry J. Gander and Charles Q. Cutshaw
"Rapid Fire" by Anthony G. Williams
 Oerlikon Contraves AG Press Releases
Royal Navy Press Releases