A rotary gun is a type of machine gun which incorporates multiple barrels in a single rotating cluster to achieve a higher rate of sustained fire than single-barrel machine guns or cannons firing the same cartridge. In most rotary guns, a drive motor rotates the barrels within its housing, with a rotating firing pin assembly and rotary chamber.
Most rotary guns fire large cartridges usually employed in autocannons, and are generally mounted on aircraft and naval vessels. These weapons typically weigh at least a hundred kilograms, even without ammunition. The large size of these cartridges allow for the destruction of an aircraft with only a few hits. On naval vessels, they are most commonly employed as Close-In Weapon Systems, and are used to destroy incoming missiles or low-flying aircraft.
Smaller models, such as the M134, are often mounted on helicopters and light trucks used by special operations forces. Their extremely high rates of fire, and ability to fire for extended periods without stopping as frequently to allow cooling allow them to excel in these roles; door guns and mounted systems in helicopters require a high rate of fire to be practically used against human targets, due to the small window of opportunity for engagement, due to the helicopter's speed. This issue isn't as pressing for weapons mounted on ground vehicles, but the rate of fire provided by rotary machine guns in this role provides a much greater degree of firepower to be brought to bear against enemies by a single small element, which can be critical for special operations troops who may be greatly ounumbered and outgunned if compromised.
Against targets like the popular perception of zombies, it stands to reason that rotary guns like the M134 may be equally useful as against normal human targets, perhaps more so. The need to suppress the enemy no longer exists, which would translate to much smaller expenditures of ammunition. Even if the targets were not immediately killed, it is likely that they would be disabled, allowing the gunner and anyone under their protection to escape. Such usage would also allow the rapid saturation of a crowd by gunfire in an attempt to slow the approach of a massive number of zombies; it is an easy matter to consider a number of cases where rapidly and reliably halting a massive horde would be of much greater importance than saving ammunition or leisurely plinking at targets.
Despite the advantages rotary guns offer, they also present their own issues. Most rotary gun systems can fire thousands of rounds per minute, meaning that pulling the trigger even briefly could consume dozens or hundreds of rounds of ammunition. Such weapons are obviously only practical if vast quantities of ammunition are available. Their great size and bulk also precludes them from being easily carried by individuals on foot, with firing one from the offhand quite literally impossible, due to the recoil forces generated.