M61 Vulcan (autocannon)
One of the first electrically-powered, rotating-barrel autocannon, firing 20mm ammunition, the M61 Vulcan was introduced on attack helicopters in the Vietnam War. It has six barrels but one actual firing chamber, the barrels being turned by an electric motor. It was also used as anti-aircraft artillery mounted on vehicles or fixed ground pedestals.
Turning the barrels allows them more time to cool, and also allows the ammunition and loading mechanisms to be separated from the firing mechanism. These allow for firing rates of 4000 or 6000 bullets per minute, far greater than conventional machine guns.
While a variant was carried by Arnold Schwarzenegger in the movie Predator, it is implausible that any human being could hold and fire a weapon with this much recoil, much less carry useful quantities of ammunication and electrical batteries. It is most commonly mounted on a swivel atop a pedestal.
While it worked well, it is being replaced by the lighter, cheaper and more reliable M134, a derivative that fires 7.62mm ammunition, or, when a more powerful round is needed, the 25mm M242 Bushmaster. The M242 is single-barrel, and there are even faster-firing autocannon such as the rotating-barrel 25mm GAU-12 and 30mm GAU-8.