The M61 operates on the Gatling principle – six 20-mm barrels are mounted on a geared rotor that is driven by a 20-hp electric motor. As the motor turns the rotor, the cam follower on the bolt of each rotating barrel follows a fixed cam path in the gun housing, opening and closing the bolt once per revolution. Firing only once per revolution reduces each barrel’s rate of fire to below that of most single-barrel revolver cannon. GE claims that this continuous rotary motion eliminates the impact loads on gun components and that sharing the thermal duty cycle among six barrels ‘significantly’ increases barrel life. The use of external power eliminates jamming due to a misfired round.
The 275 pound cannon have a muzzle velocity of 3,380 feet per second. The cannon has a selectable firing rate of 4,000 or 6,000 rounds per minute, and a total of 675 rounds of ammunition is carried. A drum assembly provides storage for the 20mm ammunition, and is directly linked to the ammunition conveyer system and the return conveyer system. An exit unit removes ammunition from the drum and an entrance unit returns spent cases, misfired rounds and cleared rounds to the drum. The complete ammunition cycle forms a closed loop from the ammunition drum to the gun and return.
The components that make up a complete round or cartridge used in the M61A1 gun are: a brass or steel cartridge case, an electric primer, propellant powder, and the projectile. The complete cartridge is approximately 6.625 Inches long and weighs roughly one-half pound. Three types of ammunition are currently available. The 20mm target practice cartridge (TP) is ball ammunition with a hollow projectile that does not contain filler. The 20mm armor piercing incendiary (API) projectile is charged with an incendiary composition that ignites on impact. The 20mm high explosive Incendiary (HE!) cartridge explodes with an incendiary effect after it has penetrated the target. The HEI cartridge is normally used against aircraft and light ground targets.