Pyrotechnic cocking charge
Encyclopedia
A pyrotechnic cocking charge is a device used to cock or re-cock a gas-operated automatic cannon, such as the Mauser BK-27
Mauser BK-27
The BK 27 is a caliber revolver cannon manufactured by Mauser of Germany...

 or the Gryazev-Shipunov GSh-6-23
Gryazev-Shipunov GSh-6-23
The Gryazev-Shipunov GSh-6-23 is a powerful, fast-firing six-barreled 23 mm Gatling gun used by some modern Soviet/Russian military aircraft....

. Such devices are not necessary in an externally powered cannon, such as the electrically driven M61 Vulcan
M61 Vulcan
The M61 Vulcan is a hydraulically or pneumatically driven, six-barreled, air-cooled, electrically fired Gatling-style rotary cannon which fires 20 mm rounds at an extremely high rate. The M61 and its derivatives have been the principal cannon armament of United States military fixed-wing aircraft...

, where the external power source is used to operate the mechanism of the weapon. The number of charges carried varies by design - the GSh-6-23
Gryazev-Shipunov GSh-6-23
The Gryazev-Shipunov GSh-6-23 is a powerful, fast-firing six-barreled 23 mm Gatling gun used by some modern Soviet/Russian military aircraft....

has a generous complement of ten, while others have as few as one. One of the benefits of an automatic cannon that isn't powered electrically is that they accelerate to their maximum rate of fire much more quickly. There is less "spin-up" time for the barrels than with an externally powered rotary cannon.
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