Assault RIFLE
Somewhere in the early part of WW II, when the Nazis were winning, Hitler gave an order not to waste resources on new rifle development. Work on future machineguns and submachine guns was allowed, even encouraged, but new rifles weren't "needed".
Some designers, however, did not follow that order. They worked on new rifle designs and concepts, but disguised their work by calling them MP's (MaschinPistolen -the German term for submachine guns - Machine Pistols)
By 43 they had come up with a magazine fed, selective fire rifle using a cartridge that was smaller than the standard 8mm rifle round (7.92x57mm), but larger and more powerful than the standard pistol round (9mm Parabellum). Some MP43s and MP 44s were sent to the Eastern Front for testing.
At a conference where Hitler was speaking with some East Front veterans, he asked them what they needed. Someone said "we want more of the new rifles!".
"WHAT NEW RIFLES???!!!!"
When it was explained to him, Hitler was, at first, furious that his order had been flouted. When the new rifle was demonstrated for him, he changed his mind, and became very enthusiastic. "That's what I want! That's what I need!" He named the new rifle the STURMGEWEHR.
Sturmgewehr is German for "Assault Rifle". It can also be translated as "Storm Rifle". This uses assault or storm in the military context, assaulting or storming an objective.
After WWII, the term "assault rifle" came into common use in the shooting community (in and out of the military). Based on the German Sturmgewehr, to be an assault rifle, it had to (generally) conform to those characteristics.
Magazine fed. SELECTIVE FIRE. Uses an "intermediate" power cartridge. It could have other things (straight line stock, pistol grip, bayonet lug, flash suppressor, etc.,) but without those three KEY things it was not an "assault rifle". It was something else.
An AR15 is not an assault rifle, because it is NOT selective fire. A G3 (select fire HK 91) is not an assault rifle, because it does not fire an intermediate power cartridge (full power 7.62mm NATO/.308 Win).
The main point of confusion is the English (primarily American) common use of the word "assault". An assault rifle is not (properly) any rifle used in an assault (though many people seem to think otherwise), it is a rifle that meets a certain set of physical characteristics, which make it a good weapon for a military assault.
The US military does not use the term "assault rifle" describing any of OUR weapons. US LAW does not use the term "assault rifle", as they are legally machine guns.
When the anti-gun bigots were whipping up hysteria about them, because of some nutcases using semi automatic rifles that looked like assault rifles in mass shootings, they cried "He used an Assault Rifle!!!!!"
We said, "No, its just a rifle. Semi Auto, NOT an assault rifle!"
They came back with "He used a semiautomatic assault rifle!!!!!!"
And they were sublimely convinced of the rectitude of their position. Even though we explained it, over and over...
However, "Semiautomatic Assault Rifle" is a mouthful to say, and didn't make for the "best" possible sound bites, so they invented a new term "Assault Weapon".
And, they defined it. They got it passed into law, and so their definition became the legal definition. And their definition was SEMI AUTOMATIC rifles, pistols and shotgun were "Assault Weapons" if they had certain features, which BTW are all cosmetic to the function of the gun. Bayonet lugs, Flash Hiders/Flash suppressors, forward handgrips, protruding pistol grips, folding stocks, etc. Also, they listed a large number of specific firearms by name/model number as "assault weapons" whether they had all, or even any of the listed features.
NOT ONE SINGLE ASSAULT RIFLE is covered under these laws, they apply ONLY to semi automatic firearms.
The similarity in terms is intentional, to further confusion about these things, which is beneficial to the lies that the anti gun bigots promote.