[ELI5] How is an SMG a machine gun and an auto-rifle isn’t?

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SMGs of course are “submachineguns” because they’re generally full auto like other machine guns, despite firing pistol-caliber cartridges. However, my question is, why are “automatic rifles” not simply called a different form of machine gun? Surely every SMG and LMG isn’t smoothbore right? Why aren’t “assault rifles” just called “intermediate machine guns” or something of the sort when they’re fully automatic just like SMGs?

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– There is no consistency for firearms designation. Depending on the year, the inventor and the country names are interchangeable. The same SMG is called a *machine pistol* (Germany, with machine pistol having a different meaning in other countries) or a *machine carbine* (UK … while a carbine is something different in Germany).

– SMGs were developed during WW1, where the standard firearm was still a normal rifle, but stationary machine guns became an important part of warfare. So the SMG was not compared to a rifle, but to a machine gun and its ammunition.

– Automatic / Assault rifles however are not machine guns. Machine guns are designed to be heavy, relatively immobile (compared to normal weapons) and able to sustain heavy fire for a long time which puts, contrary to Hollywood movies, a heavy strain on the weapon. The assault rifle has *in theory* the ability to sustain full auto fire, but the usual usage are quick single shots.

– The main advantage of an assault rifle is the quick rate of fire, compared to contemporary WW2 weapons (assault rifles were developed in the later stages of WW2), large magazine, while still being lightweight and mobile compared to machine guns.

It is simply a matter of timelines and the country involved.

SYL

It mostly has to do with the invention of the term. At the time, armies had their “battle rifle” which every soldier used. Then The machine gun came out. It needed to be massive to safely handle the forces produced by a full sized rifle cartridge, as a result they needed 2-3 men to effectively operate them.

Soon the Thompson and similar machine guns came out. They used a significantly smaller pistol or “sub-caliber” round as compared to the larger machine guns in order to shrink the machine gun to a man portable size. Thus they were dubbed the “sub machinegun”.

Around the same time we were experimenting with other sizes of rounds. Thus the adoption of the term “medium machinegun” and “heavy machinegun.” However, the main soldier still used a semi automatic or bolt action “battle rifle”.
It wasn’t until after World War II had ended that we started arminf whole squads with fully automatic capable weapons. By then the naming conventions had been firmly established.

Ok. So assault rifle is typically used incorrectly or at the very least used disingenuously when talking about AR-15s and similar platforms. An M16 would be the “assault” rifle version of the AR15, which can shoot full auto via a toggle switch between safe, fire, and full auto. An “auto-rifle” isn’t a thing. I’m assuming you’re referring to semi-automatic rifles (which would include the infamous AR15). Semi-automatic guns fire a round every time to pull the trigger.

Like a lot of inconsistent naming conventions, it’s for historical reasons. When John T. Thompson invented the Tommy Gun, full-size machine guns were the only commonly-known automatic weapons. Fully automatic ‘machine pistols’ firing handgun cartridges had only just come into usage. So Thompson named his new weapon the “sub machine gun” to reflect the fact that it fired a smaller cartridge than full-size machine guns. And the name caught on (although the MP38 and follow-ups, widely regarded as SMGs, were of course called machine-pistols in German). In the second world war, most of powers still considered fully automatic weapons unsuitable for the rank and file (something, something they would waste all their ammo, something) and produced submachine guns (including Thompson’s own!) for officers. Quickly, however, they realized that in the fast-moving combat of the war, automatic fire small arms were really handy.

The term “assault rifle” for describing an intermediate cartridge full-auto capable rifle has a kind of peculiar history as well. Some forward thinkers in the German army realized early on that the bolt-action semi-auto rifle was not the way of the future, and looked into replacing it with an intermediate cartridge. As handy as a submachine gun like the MP38, with more stopping power, was the idea. However, the orthodox elements in the army wouldn’t except that as a replacement for infantry rifles, and the prototype had to be labeled “Maschinenkarabiner”, a “Machine Carbine”, ‘carbine’ being a term for a rifle-shaped weapon firing smaller cartridges. The production version was initially called the “Maschinenpistole 43” as if it were a follow-up to the MP38, even though it fired a bigger cartridge and wasn’t a pistol by anyone’s definition. It was only after the battlefield effectiveness of the weapon was re-named a more fitting appellation as the first “Sturmgewehr” or assault rifle, in part, still, to distinguish it from the normal rifle. The StG 44 was quickly recognized as a very good and useful design by the Soviets it was used against, and eventually developed the AK-47, simply called ‘Kalashnikov’s automatic,’ in Russian but widely recognized as a descendent of the German “assault rifle” in the west.

> However, my question is, why are “automatic rifles” not simply called a different form of machine gun?

An automatic rifle can fire full auto, or maybe burst, and single shot mode. A machine gun is full auto all the time, there are no other options.

> Surely every SMG and LMG isn’t smoothbore right?

That has nothing to do with it. They would all be rifled, you would only go with a smooth bore for some cannons.

> Why aren’t “assault rifles” just called “intermediate machine guns” or something of the sort when they’re fully automatic just like SMGs?

“Assault rifle” generally refers to an automatic rifle and not machine guns for the aforementioned reasons. A selectable fire mode and detachable 10 to 30 round magazine are typical of an assault rifle.

Of course the media will often misuse the term to refer to anything that looks military and scary, ignoring the selective fire option for fully automatic fire.