Wiktionary
n. A fully automatic projectile weapon similar to a machine gun, but firing shells instead of bullets and larger in calibre.
Wikipedia
An autocannon or automatic cannon is a large, fully automatic, rapid-fire projectile weapon that fires armour-piercing or explosive shells, as opposed to the bullet fired by a machine gun. Autocannons often have a larger calibre than a machine gun (e.g., 20 mm or greater), but are usually smaller than a field gun or other artillery. When used on its own, the word "autocannon" indicates a single-barrel weapon. When multiple rotating barrels are involved, the word "rotary" is added, and such a weapon is referred to as a " rotary autocannon." Modern autocannons are typically not single soldier-portable or stand-alone units, rather they are usually vehicle-mounted, plane-mounted, or boat-mounted, or even remote-operated as in some naval applications. As such, ammunition is typically fed from a belt to reduce reloading or for a faster rate of fire, but a magazine remains an option. They can use a variety of ammunition: common shells include high-explosive dual-purpose types ( HEDP), any variety of armour-piercing ( AP) types, such as composite rigid ( APCR) or discarding sabot types ( APDS).
Although capable of generating a high rate of fire, autocannons overheat if used for sustained fire, and are limited by the amount of ammunition that can be carried by the weapons systems mounting them. Both the US 25 mm Bushmaster and the British 30 mm Rarden have relatively slow rates of fire so as not to use ammunition too quickly. The rate of fire of a modern autocannon ranges from 90 rounds per minute (in the case of the British Rarden), to 2,500 rounds per minute with the GIAT 30. Systems with multiple barrels can have rates of fire of over 10,000 rounds per minute (the Russian GSh-6-23 for example). Such extremely high rates of fire are effectively employed by aircraft in air-to-air combat and close air support attacks on ground targets, where the target dwell time is short and weapons are typically operated in brief bursts.
Usage examples of "autocannon".
Without a pause, the autocannon were pushed out into the open, and opened up on the four spacecraft.
The Puma was eight-wheeled, well-armored for its size and heavily armed with a 20mm autocannon and a machine gun, but the close country made the drivers nervous.
And the flakpanzers, moving forward and risking their thin plating to hose their quadruple 20mm autocannon over the village, short bursts that hit like horizontal explosive hailstorms.
The turret with its 25mm autocannon flipped straight up, twirling end-over-end.
Great doors that appeared to be part of the three-story walls slid open, and wheeled autocannons were rolled out.
The turrets walked back and forth along the line of rubble that had been the northern edge of the village, 20mm shells exploding in white flashes, machine gun rounds flicking off stone with sparks and sharp ping sounds that carried even through the crash of autocannon fire.
Behind the flatbed were two more enormous eightwheeled transports, each bearing malcontents armed with autocannons.
Several more broke through the hangar walls, followed by Zentraedi shock troopers armed with laser rifles and autocannons.
Zentraedi infantry troops armed with control rods torn from ruined Battlepods dueled Excaliburs, swinging autocannons like baseball bats.
The Goliaths activated their twin autocannons, targeting two of the four cyborg warriors.
Blue Zentraedi radiation blasts were matched by the Veritechs' autocannons, which flung torrents of high-density armor-piercers at the enemy.
She shimmied into the access trunk of the Asp's turret, settled herself in the fighting chair, and placed her hands on the grips of the single multi-barreled autocannon which was the lightly armored vehicle's sole offensive weapon, then keyed her boom mike.
The VTs and the few remaining Hovertanks couldn't use Protoculture weapons either, of course, but there had only been time to retrofit a few of them with old-style autocannon and rocket pods.