Gatling sold his guns worldwide after the Civil War. He continued the development of the gun to increase its reliability. However, the invention of the automatic machinegun by Dr. Hiram Maxim in the 1880s doomed the Gatling to obsolescence. Gatling applied an electric motor to his design in the 1890s and achieved a 3,000 round/minute rate of fire from the gun, but the Maxim was on its way to dominance in the first of the Twentieth Century's Great Wars.
Fast forward to the 1950s. The .50 caliber Brownings on F-86 Sabre jets shot down cannon-armed Russian-built MiG-15 fighters at rates of over 10 to 1. But, the fliers wanted the punch of cannon because the .50 projectile was of limited internal capacity and the 20mm shell seemed to offer the best way to down an opponent. The problem was that jet-to-jet engagement times were very brief. Efforts to raise the cyclic rates of the M-3, M-24, and M-39 guns then in service or projected for service seemed to have reached a plateau of development at approximately 1,200 rounds/minute.
Someone in Army Ordnance remembered Gatling's experiments with the electric motor. Two .45-70 Gatlings were obtained and fitted with electric motors as a proof-of-concept of the design. Despite the jury-rigged nature of the hybrid, the guns did indeed fire at rates of 3,000 rounds/minute or more. Ordnance was astonished. By reverting to the rotary barrel concept they had solved the problems of high cyclic rate, overheating, and barrel wear. Electric drive also solved the problem of jams caused by faulty ammunition and made high reliability possible.
General Electric was given the go ahead to produce a 20mm rotary-barreled aircraft cannon for the "Century Series" of fighters beginning with the F-104. Hydraulic drive was selected for the first 20mm "Vulcan" guns because the electric motors tended to start too quickly and pull the ammunition links apart. Hydraulic drive guns were limited to 4,000 rounds/minute of linked ammunition. Fired brass and links were not dumped overboard as was the case with World War 2 fighters because the higher speed of the jets made the possibility of FOD (foreign object damage) to the aircraft too great. Instead, the brass and links were collected in bins that were dumped when the jet landed and rearmed.
The 20mm "Vulcan" came into its own with the adoption of the link-less feed concept. The ammunition was fed to the ammunition drum in links. A delinker was fitted to one end of the drum. The interior of the drum contained a screw-shaped feeder. A linker was fitted to the opposite end of the drum. The linker was connected to the delinker by link chutes. New ammunition was fed to the delinker where its links were removed and transported to the linker; loose rounds of ammunition were fed into the bottom of the drum. Fired cases exited the top of the drum where they were relinked with the links that had originally carried the live ammunition. Once the drum was loaded, the linker and delinker were removed and the feed chutes were connected to the gun. One feed chute contained live ammunition from the top of the drum for the gun's feeder, while the second feed chute allowed empty cases to return to the bottom end of the drum for storage.
When the pilot squeezed his trigger, an electric motor started the gun barrels spinning. The motor also drove internal gears which powered the feeder on the gun. An electric motor also powered the screw feeder in the drum. Rounds were admitted to the gun feeder by a solenoid-operated feed pawl. If the gun was mounted in the aircraft with the top barrel at 12 o'clock position, the feeder received the incoming ammunition at 4 o'clock, transported the round counter clockwise to the 12 o'clock where it was fired, and transported to the 6 o'clock position where it exited the gun feeder. (NOTE: clock positions are as seen from the rear of the gun.)
When the pilot released the trigger, the feed pawl blocked the flow of live rounds to the gun. The barrel inertia caused the barrels to continue spinning. This inertia ensured that any unfired rounds in the gun were fired and the empties were cleared from the gun. As the empties were cleared from the gun, the individual bolts moved to the rear of the gun into what is called the "clearing path."
When the pilot squeezed the trigger again, all the electrical drives and solenoids were energized. The bolts moved forward into the "feed path" and the cycle began again. The maximum rate of fire for the 20mm "Vulcan" M-61 gun is an astounding 6,000 rounds/minute (100 shots a second)! Cyclic rate is entirely dependent upon the speed of the electric motors. It wasn't long before GE began experimenting with the Gatling principle in other calibers and numbers of barrel combinations. Ideas included gun pods which were self-contained gun pods and modified versions of the M-61 for helicopters. The M-61 was modified for use in gun pods to arm fighters that were solely missile armed. (This was a stop gap until an internal gun could be fitted. Guns had been omitted from fighter designs of the mid-1950s in favor of missiles. Unfortunately, the Korean war-era MiG-17 (with guns) could out-match a Mach 2 F-4 "Phantom" with only missile armament.) The gun pod carried the gun, ammunition, controller, feeders, and drive motor for the pod. Some pods used the aircraft electrical system, while other pods were driven by the aircraft's slip stream.
The M-197 20mm gun was an M-61 cut down to three barrels for mounting in pods on AH-1G "Cobra" gunships or in the chin turret of the helo. A miniaturized version of the M-61 was developed in 7.62mm NATO. This was the GAU-2B/A (also known as the M-134 or Mini-gun). Minis were originally mounted in pods. They were hung on helicopters and attack aircraft for attacking infantry and soft targets with high volumes of fire. (One Mini-gun could put a 7.62mm bullet into every square foot of a football field.) GE also marketed prototypes of their electrified Gatling in .50 Browning machinegun caliber and 5.56mm M-16 rifle caliber; however, the designs did not go into service. The problem with the Mini-gun in a pod was it was limited by the amount of ammunition the pod could carry. Designs soon appeared that turned the Mini-gun into a turret-mounted gun (AH-1G "Cobra"), into a flexible gun mounted in the doors of UH-1 "Hueys" and HH-3 "Jolly Green Giants", and twin side mounts that replaced the four M-60C guns on the UH-1B/C gunship helicopters.
The main differences between the Mini-gun and its M-61 parent (other than caliber) are these:
* The Mini-gun uses linked 7.62mm NATO ammunition; the M-61 uses link-less 20mm.
* The Mini-gun dumps its links and brass overboard; the M-61 does not.
* The Mini-gun dumps 8 to 12 live rounds at cease fire as the barrels spin down; the M-61 fires all its live 20mm at spin down.
* The Mini-gun uses a feeder-delinker to strip linked ammunition from its links; the M-61 uses a link-less feeder.
* The Mini-gun was usually fitted with flash suppressors for the barrels; the M-61 was not.
* Both Mini-gun and M-61 use timing buttons on their barrel clusters and feeder-delinker/feeders to time them.