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A gun pod is a detachable pod or pack containing machine guns or automatic cannon and ancillaries, mounted externally on a vehicle such as a military aircraft which may or may not also have its own guns.
A gun pod typically contains one or more guns, a supply of ammunition, and, if necessary, a power source. Electrically powered cannon, such as the M61 Vulcan, may be powered from the aircraft's electrical system or by a ram-air turbine.
Gun pods increase a vehicle's firepower without occupying internal volume. When not required for a specific mission they can be omitted to save weight. On some vehicles they isolate delicate internal components such as radar from the weapon's recoil and gases, and for jet aircraft allow the weapons to be mounted away from the intakes of the engines, reducing problems of gun-gas ingestion, which may cause the engine to stall.
When designed to be suspension-mounted on a hardpoint on a typical post-WW II aircraft, gun pods are inherently less accurate than integral guns, or the type of "conformal" gun pods that are faired smoothly into or onto the nearby surfaces of an aircraft, because the "hardpoint" mounting is necessarily less rigid, so that the weapon's recoil produces more deflection. This problem is particularly acute with powerful cannon like the 30mm GPU-5 gun pod. Both hardpoint-mounted and conformal-mount gun pods also cause substantial drag on fast-moving vehicles such as fighter aircraft.
Gun pods are commonly carried on military helicopters, and are often fitted to light aircraft to equip them for counter-insurgency operations. Some air arms use gun pods for fighter bombers for use in strafing attacks. Since the Vietnam War, United States Air Force policy has been that the use of multi-million dollar aircraft for strafing is not economically justified, but the Soviet Union, and subsequently Russia, have remained proponents of strafing, and have continued to develop systems for this purpose. Soviet experience in Afghanistan in the 1980s led to an unusual innovation in the form of the SPPU series of gun pods, which have traversable barrels allowing them to continue to fire on a fixed target as the aircraft passes overhead.
In World War II the Third Reich's Luftwaffe made use of many different, and most often rigidly mounted, conformal and suspended-mount gun pod systems usually called Waffenbehälter (prefix of WB, literally 'weapon container') or Waffenträger (prefix of WT, literally 'weapon carrier'), and carrying anything from rifle caliber MG 81 machine guns, all the way up to the enormous Bordkanone anti-tank cannon based ordnance weapon series, ranging from 37 to 75mm in caliber, though the usual underwing conformal gun pods fitted to Bf 109 and Fw 190 single engined fighters used either the MG 151/20 or MK 108 in gun pod mounts.
Common Gun Pods 
GPU-2/A: Gun pod with M197 cannon.
GPU-5/A: Gun pod with GAU-13/A cannon.
M18/SUU-11/A: Gun pod with M134/GAU-2/A machine gun.
M12/SUU-16/A: Gun pod with M61A1 cannon.
M25/SUU-23/A: Gun pod with GAU-4/A cannon.
For more info see:
GUV-8700 Gun Pod (9A669), as above, plus 2x AGS-17 30mm automatic grenade launcher in the same pod.
UPK-23-250 Gun Pod GSh-23L twin barreled 23mm
SPPU-22 Depressible Gun Pod GSh-23L twin barreled 23mm
SPPU-6 Fully Mobile Gun Pod GSh-6-23 six barreled 23mm
SPPU-687 Depressible Gun Pod (9A-4273) GSh-30-1 high velocity 30mm (prototype)
In fiction 
Gunpods are a mainstay in many science fiction franchises, especially in anime mecha shows. In Macross, many transformable vehicles have gunpods that detach and can be held in the mecha's robotic hand. Such gunpods may look like gigantic assault rifles. In the video game Einhänder, the player's fighter was equipped with a manipulator arm capable of picking up gunpods from destroyed enemies.