There is a lot of discussion about the AR-15 and assault rifles. Some of it is right, some wrong, and some depends upon where you go for the definition of "assault rifle." But none of it really explains the history of the weapon and how we got to this point in time. Perhaps the following will help.
The story really starts in WWI. At the time, the primary battle weapon was the bolt action rifle. Every major combatant used one, and most were based on a rifle originally created in Germany, the Mauser '98. The standard format was a bolt-action rifle with an internal magazine, firing a .30 caliber (or 8 mm) bullet that weighed about 200 grains, in a cartridge that ranged from 54mm long (Russian and French) to 63mm long (American). These had an effective range of 1,000 yards, and a typical infantryman could fire with accuracy to 300 yards. The guns were heavy, had a slow rate of fire, and the cartridges were heavy, too.
During the war the submachine gun was developed. That was a hand-held machine gun that fired pistol cartridges, usually 9mm or .45 caliber. They were very useful for very short range combat, particularly trench fighting. They had a high rate of fire. But they had very short range, 100 yards, and accuracy was measured in the 10s of yards. Cartridges were short and light, and a soldier could carry loads of externally loaded magazines.
How could a compromise be found between a slow rate of fire with a small number of heavy cartridges, with great range, and a high rate of fire with lots of small cartridges, at close range?
The first attempts were to create a light machine gun, a weapon that could be hand-held, with a high rate of fire, but using the rifle round. The best example of the Browning Automatic Rifle, or BAR. It used the American .30-06 rifle round. It was big, it was heavy, it was hard to use, and the magazines didn't carry a lot of rounds.
The first weapon that would now be classified as an "assault rifle" was not, as many believe, the German MP-43, used in WWII. I'll get to that later. It was the Russian Federal Avtomat.
The Avtomat used a smaller round that the Russian battle round, which was 7.62x54. It used a 6.5x50 round instead, a much lighter cartridge with a lot less recoil. That is one of the basic definitions of an assault rifle, one which uses a round that is reduced in bullet size, powder load, and cartridge size, vs. the standard bolt-action battle rifle.
While the Avtomat originally had an internal magazine fed by stripper clips, it was soon replaced by a 25-round external box magazine. That is another basic for an assault rifle.
Originally made for semi-automatic fire (it fired once each time the trigger was pulled), a selective-fire switch was added to allow full automatic fire (it kept shooting as long as the trigger was depressed). This is the final definition of an assault rifle in the firearms world. (Many media outlets define it applying just the first to standards, along with visual cues including pistol grip and flash suppressor).
The Avtomat was not a huge success, due to complexity of manufacture and the fact that it overheated quickly on full-auto fire. The Russians built just over 3,000 of them.
Fast forward now to WWII. The Germans developed what is now considered the grandaddy of modern assault rifles, the MP43, rechristened the Sturmgewehr 44 or StG 44.
The MP43 had a pistol grip, an external box magazine, and selective fire. It solved the bullet/cartridge/recoil problem by taking the standard German battle round, the 7.92x57, and shortening it to 7.92x33. That meant the bullet was the same diameter, but it was shorter, lighter (123 grains v. 181-198 grains) and it had less powder behind it. That meant a soldier could carry more rounds and the gun had less of a kick. Range was less than the battle rifle, but a lot more than a submachine gun. The Russians did the same thing with the AK-47, cutting their 7.62x54 to 7.62x39.
The United States' didn't adopt the assault rifle model at first. Instead, it added an external box magazine to the WWII Garand rifle, made slight changes to the cartridge for more ballistic efficiency, and created the M-14, a selective fire battle rifle. It was heavy and difficult to shoot, but had great range.
The M-16 was actually developed from the ArmaLite AR-15. The AR-15 solved the assault rifle problem with a return to the original Avtomat. Instead of cutting down the standard battle round, it went with a lighter bullet and cartridge. That gave the same weight and recoil advantage with increased range, at the cost of reduced bullet impact. The AR-15 fired a bullet the same diameter as the .22 rifle so many kids grew up with, plinking at cans and bottles on a rural fence post. But it was longer, and had a lot more powder behind it. The military version, the M-16, had a compromise cartridge, an external box magazine, and a selective fire switch. Today, the M-16 doesn't actually fire full-auto, but fires either one shot at a time or three round bursts. Anything more was inaccurate and a waste of ammunition.
So where does that leave us today? The AR-15 we see now has the compromise cartridge and an external box magazine, but is semi-automatic fire, only. So is it an assault rifle? Frankly, who cares what we call it. The more important question is, what is it and what are its uses?
First, the .223 cartridge is not useful for hunting anything the size of a deer or larger. In hunting the goal is a quick, humane kill, and the .223 just doesn't pack enough oomph. Here in Indiana, the minimum bullet size is .243 inches. The light .223 is easily effected by wind and has low penetration.
Second, a large box magazine is not only unnecessary, but in many cases illegal, for hunting. In Indiana, again, a hunter may have no more than 10 cartridges on hand at a time. And that makes sense. In a forest full of hunters, we want accurate individual shots, not some moron blasting away as fast as he can in the general direction of something that moved.
What about smaller game, like rabbits, squirrels, or other varmints? Well, that's really two different questions. Rabbits and squirrels are pretty small to be hit by a .223 round moving more than 3,000 feet per second. On a smaller animal, the hydrostatic shock of something moving that fast will pretty much blow it up. Longer range varmint shooting is often done with a .223 round, but in hyper-accurate bolt-action rifles, not semi-automatic box-magazine rifles like the AR-15.
So it's not a good hunting rifle. Sure, it can be used on smaller deer or larger varmints, but it's not ever the best tool for those jobs.
Is it a good self-defense weapon? Nope. Please allow me to explain. The .223 bullet is very small and moves very fast. At close range, like inside a house, before it starts to shed energy, it has very high penetration. Shoot it inside a house and it WILL go through dry wall into the next room, and unless you have X-ray vision, you can't see who or what is behind the wall. That's why a shotgun is the preferred self-defense long-warm. It is also long (though there are shorter carbine versions) and hard to swing around in confined spaces, which is why handguns work better for the purpose.
So what is it good for? It's good for a squad of soldiers laying down a high rate of fire at a greater range than a submachine gun. And it's good for an individual to put out a high rate of fire at the targets, be they paper or people. That's about it.
That's the AR-15 and how we got here. It is a highly-effective battle weapon, even absent full-auto fire, with little civilian use but target shooting or mass murder. That should be the basis of debate, not an argument about how to define the term "assault rifle."