Knowing through the pictures of gun is not enough. You have to understand it.The Japanese Nambu 96 and 99 light machine guns, the British Bren gun, the Czech ZB-26, the French Châtellerault 24/29, all of these things are really quintessential light machine guns, and you’ll know: they’re all magazine fed, rifle caliber, and they don’t have any particular features to really increase their rate of fire, their rate of sustained fire. Some of them will have quick change barrels, some do not. The quick change barrel is there to try and allow the gun to be pushed out of its envelope towards that of a medium or heavy gun, and because you generally don’t lose anything by having a quick-change barrel, it’s not that complicated to add to the gun and so it’s an easy addition to make without really changing the true nature of the light machine gun. Now, these would be phased out basically starting at the end of World War 2, as would the medium and heavy machine guns, because all of these started to come together into a single thing called a general purpose machine gun.
The Danes were the first to actually kind of codify the idea of a general purpose light machine gun, but the Germans were the first to really put it into serious service with the MG 34. And the idea here is to have a gun that can be used for, as the name implies, anything you would need to do with a machine gun, to consolidate all the different types of gun into a single design. That makes it logistically a lot easier to manufacture, to supply, to train, etc. So the four main roles that you’re trying to fill are going to be vehicle mounted machine guns, aircraft mounted machine guns and anti-aircraft machine guns, which have basically the same sort of defining characteristics, light machine guns, and heavy machine guns.
The way this was done by the GPMG concept was to have a gun with a fairly high rate of fire, a high rate of fire is essential to aircraft and anti-aircraft use, because you’re firing at fast-moving targets and need to make the most use of whatever opportunity you have on target, and then to have the guns equipped with both bipods and sophisticated tripod mounts. The idea being, you make the gun just light enough that it can be carried and fired by a single individual, from basically a permanently mounted bipod, which allows it to be used like a light machine gun, although they’re generally heavier than true light machine guns. This is a compromise solution, after all. And at the same time, you have a tripod mount (in the German case, the Lafette mount) That allows very fine-tuned adjustment of windage and elevation, and typically also has a recoil absorbing mechanism built into it to help increase the long-range accuracy of the gun so it’s not bouncing the tripod around.
The idea there is to be able to take a gun that’s still capable of being used as a portable light gun, but bolt it onto this tripod and be able to push it into service as a heavy machine gun for sustained fire. So what you’ll find with these guns: GPMGs are always going to be belt fed because they do need that belt, that long belt feed capability to truly have sustained fire, as well as to be capable of being used in vehicles or aircraft, where you don’t want to have to be accessing the gun to change a magazine every 20 or 30 rounds. They will fire full power rifle cartridges, which is essential to their role as a heavy machine gun, for sustained fire, at long ranges, and potentially at harder targets than just men in the open, and they will typically have some sort of a pretty effective cooling mechanism, and usually this takes the form of a quick-change barrel, could also be a heavy, or a fluted barrel, but you’ll see with guns like the MG 34 and the MG 42, they were specifically designed to have very easy quick-change barrels, and they were they were carried and issued with several spare barrels.
And so typically, doctrine on one of these guns would be when it’s being used from the Lafette mount in a supporting or heavy role, every time the belt has changed, so every 200 or 250 rounds, the barrel would also be changed, and if you had three barrels rotating through like this, it allowed the barrels to cool sufficiently that you could basically fire continuously.
Now, not dumping belts continuously, but as fast and as rapidly as is typically necessary in a combat environment. Now this concept turned out to be really successful and really popular and after World War 2, pretty much every country started looking at it and deciding to copy it, and probably the best example of copyin that concept is the Soviet PK and PKM machine gun.