General Purpose Machine Guns

In our last couple of posts, we looked at medium machine guns. In today's post, we will look at some developments in the General Purpose Machine Gun (GPMG) category (otherwise called Universal Machine Gun or UMG).

A general purpose machine gun is a weapon designed to use both magazines and belt feed, firing full sized rifle-cartridges, which is air-cooled and designed to be used either as an infantry support weapon (i.e. like a light machine gun), or as a vehicle mounted weapon (i.e. like a medium machine gun). Since it is air-cooled, a GPMG usually features quick-change barrels, so it can continue to fire on automatic mode for longer periods of time. A general purpose machine gun can be mounted on a bipod or a tripod, or even from a vehicle, such as a jeep or a helicopter.

The first GPMGs were something we'd just studied in the previous post. The MG 34 was the first general purpose machine gun. Carried by an infantry man with a drum magazine and mounted on a bipod, it served as a great infantry support weapon used for offensive operations. By switching the magazine with an ammunition belt and mounting it on a tripod, it became a very good medium machine gun. Mounted on a tank or a vehicle, it was an effective anti-aircraft and defensive machine gun.

Public domain image of a MG 34 machine gun. Click on the image to enlarge

The only problem with the MG 34 was that it was somewhat harder to manufacture, due to the fact that it needed some precision machining. Due to this, the Germans came out with the MG-42, which was easier to manufacture, more reliable, and as an extra bonus, had a higher rate of fire as well!

A MG-42 mouinted on a tripod. Click on the image to enlarge. Public domain image.

The MG-42 design was extremely successful and some variants (such as the MG3 and MG74 models) are still in service in some militaries around the world. It also influenced other countries to manufacture their own GPMGs based somewhat on the MG-42 design.

The US military was one of the first to pick up the idea of a GPMG from the Germans and started working on a design in the late 1940s. One of the requirements was that this gun had to be chambered to fire 7.62x51 mm. NATO ammunition and another requirement was that it should be capable of being fired accurately from the shoulder as well. After a number of trials, the final design was approved in 1957 and was called the M60 machine gun. The M60 served in various branches of the US military during the Vietnam war. It was carried by infantry units as a Squad Automatic Weapon (SAW) to support infantry operations and was also mounted on river patrol boats (PBRs), M113 Armored Personnel Carriers (APC) and as a door gun on helicopters.

US Marines with a M60 in Vietnam. Public domain image.

While the M60 saw service in Vietnam, some design flaws became obvious. For one, it had some jamming issues in muddy and humid conditions. It did better when used in static-defense or helicopter mounted roles, because it could be stored in cleaner conditions and regularly maintained. One more problem was the design of the barrel, which had a permanently attached bipod and this made barrel changes more difficult and took longer to accomplish. While some improvements to the design were made, the US military rejected the M60E3 and went with the Belgian FN MAG (which we will study in a second) and designated it as the M240 in US service.

Around the time that the US was developing the M60, the Fabrique Nationale company (FN) of Belgium was also developing their own GPMG, which they called the FN MAG (the letters MAG stood for Mitralleuse d'Appui Generale, which is French for "General Purpose Machine Gun"). Like the M60, the FN MAG is also designed to fire 7.62x51 mm. NATO cartridges. While it is heavier than the M60 and uses a more complicated gas operated system, it is more reliable compared to the M60. This is why it was also adopted by the US military as the M240 to replace the M60s in service. The M240 was first mounted on to tanks in 1977 and later adopted by other US military branches during the 1980s and 1990s.

US Marines firing a M240G mounted on a tripod. Click on the image to enlarge. Public domain image.

The FN MAG and the M240 continue to be used in service in many military forces around the world.

Other GPMGs include the Soviet PK series, the Heckler & Koch HK 21 etc.

Medium Machine Guns - II

In our last post, we looked at the early beginnings of medium machine guns. We will continue to study this class of machine gun in this post.

Before World War II, the German military came to the conclusion that static warfare tactics from World War I were obsolete and began to train using maneuver warfare concepts. Therefore, they needed a machine gun that could have a high rate of fire, but could be transported reasonably easily by hand, as well as used from a mobile platform. It was decided to design an air-cooled weapon with interchangeable barrels, so that it could fire rapidly for longer periods. The result was the MG 34 machine gun.

MG 34 Machine Gun. Click on the image to enlarge. Public domain image.

The MG 34 was used with great success by the German military in the early stages of World War II, but the rate of manufacture was slow because it needed some very precise machining to make it. Therefore, the German military held a new design contest for an improved machine gun and the result was the MG 42 machine gun.

Interestingly, the winning design was submitted by a company with no previous experience in firearms manufacturing, but they had experience in mass production technologies and knew how to produce stamped machine parts (they made metal lanterns!) The lead designer, Dr. Werner Gruner, actually attended a military machine gunner's course to familiarize himself with machine guns and talked to soldiers about what they wanted to see in a weapon. The resulting MG 42 design was much easier and faster to produce than the MG 34. It also had a much higher rate of fire (1200 to 1500 rounds/minute) than any other machine gun of that era. 

MG 42 on a tripod. Click on the image to enlarge. Public domain image.

Like the MG 34, the MG 42 could be carried by a single user and operated with a bipod in a light machine gun role. It could also be mounted on a tripod (such as in the image above) and used by a team in a medium machine gun role. When used in a medium machine gun role, the optimum team size was six: a commander who directed the team, a no. 1 gunner who carried the gun and fired it, a no. 2 gunner to carry the tripod (which weighed about twice as much as the gun!) and three others to carry spare barrels, ammunition, entrenching tools etc. The first three team members also carried pistols for protection, while the other three carried rifles. Often, the team consisted of only three members though, a gunner, loader and spotter.

The MG 42 was a very influential design and some variants continue to be used by military forces even in the present day. It also influenced other designs, such as the US M60 machine gun, the German MG 3 etc.

In our next couple of posts, we will study how the medium machine gun transitioned into the General Purpose Machine Gun class and the development of Squad Automatic Weapons (SAW)

Actions: Recoil Action: Muzzle Booster

A muzzle booster is a device that is sometimes used with some short recoil operated actions. It is used to add extra force to the recoil action. This helps boost reliability and increased rate of fire. The idea behind a muzzle booster is to tap some of the expanding propellant gases to add to the force acting on the recoiling parts.

A muzzle booster was first used on a Vickers machine gun of 1912. During World War II, it was used by the German MG-42 as well as the later MG-3 machine gun.

The basic example, as used on the Vickers machine gun consists of a flared cup attached to the end of the barrel. There is also an outer tube that surrounds the barrel as well. The outer tube is aligned very precisely so that it has an exit hole for the bullet to pass through after it emerges from the barrel. The outer tube also has other perforations to allow gases to escape.

When the bullet is first fired, it comes out of the barrel along with some burnt propellant gases. As it passes through the hole in the outer tube, it temporarily prevents gas from escaping out of the barrel. Therefore, the pressure in the outer tube rises and the gas moves backwards and pushes on the flared cup attached to the end of the barrel. This provides additional force to move the barrel backwards. The excess gas then escapes through some perforations on the outer tube. The animation below shows how this works (click on the image to view the animation).

Click on the image to view the animation.

Image licensed under Create Commons Attribution-Share Alike 2.5 Generic, 2.0 Generic and 1.0 Generic license

Created by user GraemeLeggett at en.wikipedia.org

Muzzle boosters are also used when attaching silencers (or, to give the correct name, "suppressors") to short recoil operated pistols (i.e.) most modern semi-automatic pistols. Since the barrel and bolt are the recoiling parts of a short recoil operation, adding a silencer to the barrel increases the barrel weight. The extra weight may interfere with the recoil operation since more force is now needed to push the barrel backwards fully. Using a muzzle booster provides the extra force to counteract the extra weight added. This muzzle booster is sometimes referred to as a Nielsen device.

The same idea is also used for some blank firing adapters (BFA). During ceremonial occasions and military demonstrations, users are provided with blank cartridges that only contain the propellant, but not the bullets. Blank cartridges have lesser power than normal cartridges that contain bullets. Therefore, firing a blank cartridge produces much less recoil force, which means that it may prevent a recoil operated action from working properly. By screwing on a special muzzle booster (the blank firing adapter) to the end of the barrel, the extra force required to cycle the action is generated.