When we look at firearms throughout history, repeating firearms have been based on two different feed mechanisms:
- In the first category, we have manually powered feed mechanisms (i.e.) the firearm is loaded and cocked by the user pulling a lever somewhere on the firearm. In this category, we have mechanisms like bolt action, lever action, pump action etc., as well as revolver mechanisms such as single action revolver, double action revolver etc. Firearms in this category have a firing rate based on how fast a human can manipulate the reloading mechanism and then pull the trigger. In some cases (e.g. double action revolvers), the act of pulling the trigger drives the feed mechanism and also fires the firearm. The firearms in this category are generally older and date from 18th and middle 19th century.
- In the second category, we have firearms that use some of the energy from firing a cartridge to drive the feed and firing mechanism. In this category, we have semi-automatic and fully automatic firearms of various types, such as blowback operated, recoil operated, gas operated etc. Firearms in this category are generally from the late 19th, 20th and 21st century and include most modern pistols and rifles. The reloading/feed mechanism operates faster than what a user can accomplish manually and therefore, these firearms have a faster firing rate than firearms in the first category.
Firearms in the second category are very common in the 20th and 21st centuries. However, they generally have one weakness -- if a cartridge is faulty and does not fire properly, then the feed mechanism stops working and the user has to stop shooting until the faulty cartridge is removed. Firearms in the first category don't have this problem -- if a cartridge is bad, the user can usually pull the feed lever again and use the next cartridge in the magazine.
Since the latter part of the 20th century, we can add a third category for repeating firearms: using an externally powered source to drive the feed mechanism. In this category, we have modern chain guns. A chain gun uses an electric motor and a continuous chain to drive the feed and firing mechanisms. The chain is similar to that used in bicycles and motorcycles. Unlike weapons in the second category, a faulty cartridge will not stop the weapon from operating, since the mechanism is driven by external forces and the faulty cartridge will be merely ejected out.
It might be interesting to note that while there are quite a few chain driven guns made by different companies, the words "Chain Gun" are actually a registered trademark owned by Alliant Techsystems Inc.
Hughes EX-34 Chain gun. Click on the image to enlarge.
The above image shows a Hughes EX-34 chain gun and guns based on its operating system are found on helicopters, tanks, armored fighting vehicles etc.
Schematic of the Hughes chain drive system. Click on the image to enlarge.
The above image is from a patent document that shows the mechanism for the Hughes chain driven gun. The chain is powered by an electric motor that runs on 24 to 28 volts. The chain operates the ammunition belt feed, as well as powering the extraction mechanism and the firing mechanism.
MK 38 Machine gun. Public domain image.
The above picture shows the MK 38 machine gun system, which is used on US Navy ships since 1986. It has a range of 2700 meters, fires 25 mm. (1 inch) shells and is used as a defensive weapon to counter fast moving surface targets.
In chain driven weapons, the rate of fire can be adjusted as needed and most chain driven weapons have multiple firing speeds. For instance, the Bushmaster M242 can be fired in single shot, burst and fully automatic modes.
Chain driven mechanisms are generally used for larger caliber guns and auto-cannons and have a correspondingly bigger recoil than hand held firearms. Therefore, most chain guns are usually mounted on a vehicle, such as a ship, helicopter, tank etc.