In your orginal post you admited to 'being' naive about guns and having a simplistic opinion. Instead of debating you about your ideas in the beginning, I would instead like to educate you about guns. The reason is if you lack knowledge of something, then any opinion you have will most likely be highly flawed. For example you make the following erroneous statements as if they are fact in your opinion:
(NOTE: I'll list the facts, then explain below.)
... but being able to empty dozens of rounds in such a short amount of time and with such power is entirely unnecessary. ... Basically, any military grade weapon belongs in the military. But, I have no problem with shotguns, pistols, and bolt-action rifles in the possession of citizens.
1) The rounds an assault rifle (and their clones) fire are not high powered rounds.
2) Shotguns and bolt-action rifles tend to be far more destructive than military grade small arms.
3) Pistols are used in over 70% of murders and other crime because they are concealable and it is easy to get your hands on highly lethal ammo.
... more powerful than a pistol...
Up until WWII most armies fielded high-powered battle rifles that were basically the same as civilian hunting rifles. After WWII practically all armies realized they did not need high-powered battle rifles; but this new thing called the assault rifle. An AR fires a round that is in between a magnum pistol round and the low end of the high-powered rounds.
Now a few things you have to know about military rounds:
* They are, by international law, required to be FMJ. This basically means that the bullet has penetrating power, but no expansion properties. So if it hits a human or animal the bullet travels straight through the body causing minimal harm. The reason for this is the Geneva and Hague Conventions which seek to reduce the inhumanity of war. The thought being: 1) we don't have to kill as many young me and 2) if only wound a soldier then we take out three soldiers (two helping the guy get medical attention) as well as more enemy resources than if we just kill him.
* Most states do not allow hunters to hunt with FMJ/military-grade ammo because it is considered ineffective and unethical for hunting.
* The 5.56mm NATO round (.223 at your local gunshop) is actually a varmit sized round. If you replaced the military-grade bullet with a hunting bullet, all it would be good for is hunting small game from squirrel to coyotes. It is effective for deer...however you have to be an exceptionally good shot and hit vital organs. If not you're only going to wound the deer and it could die hours or days later after much unnecessary hunting.
* Even with the light 5.56mm (the round used in the M-16) round, when you put on it full-auto and squeeze the trigger; the recoil causes incredible barrel rise. So much so that after the third round you are shooting over people's heads. This is why it is ineffective and has been abandoned by most armies in their assault rifles.
* Machine guns (something different from assault rifles) tend to be heavier and use things like bipods and tripods to stabilize themselves making barrel rise not an issue.
* You asked if it was so ineffectual, why are they mass produced for the military? The answer is: they are not. Until 'Nam a M-16 had three modes: Safe, Semi and Auto. After 'Nam the military decided they wanted Auto changed from full auto to three round burst. This was done so that soldiers wouldn't be so wasteful with their ammo and have a chance of hitting something.
Let's talk about power.
Yes velocity is important. However, if your bullet is moving too fast it won't do much damage. What it comes down to is what type of bullet (projectile) the round is putting downrange. The first type of round I'll talk about is the Hollow Point.
The hollow point is a bullet that expands upon contact with a soft target (ie: flesh). This expansion causes the bullet to mushroom out and tear-up internal organs making death (without modern medical treatment) inevitable. Because it makes death inevitable...it is illegal to use in war. (NOTE: this is why we who know firearms find it amusing when ppl use the whole 'guns made only for killing' argument about semi-auto clones of military guns, it is factually wrong.) However, they are very useful for self-defense and hunting; times you want a quick kill.
There are different types of hollow points including the jacketed HP (JHP) which uses a metal casing around lead to add strength to the bullet. Some of these also allow the bullet to 'flower' instead of mushrooming where the bullet flattens in such a way that it tears into what looks like petals making it a mini-buzz saw inside the body. This helps it to more effectively destroy organs.
The next type are called Soft points. These expand similar to hollow points but not as effectively. There are two main reasons to use this over hollow points:
1) They are more effective at penetrating bigger game (ie: larger than a Whitetail deer).
2) If you have a military firearm chances are they are designed to work best with FMJ ammo...which you do not want to use for self-defense or hunting because of just how unlethal military grade ammo is. HP rounds can cause the gun not to feed correctly and jam. Soft points work almost as well as FMJ, and has some expansion properties you'd need in self-defense or hunting.
Finally, we have shotgun ammo. These come in three types of shells: buck shot, slug and shot.
Buck shot is basically large pellets used for self defense and some hunting. Each pellet is about .30 of an inch in diameter (this varies based on gauge and other factors). I have 9 pellets of 00 buck in every other shell in my shotgun I use for home defense.
Slugs are basically large balls of lead about .50 of an inch in diameter. They move slow, but can tear a hole through something or someone.
Shot are small lead pellets loaded in a shell. They are steel or lead and used for hunting birds and shooting clays.
Now put all this together and guess what? The guns you believe need to be regulated because of their power and high rate of fire...are shown to be the least effective firearms out there. The criminals also know this, this is why they don't use them. If you look at mass shootings you'll actually see mass shooters don't use assault rifles.
* Aurora: Holmes opened up with his shotgun first. Then switched to an AR-15 type rifle...that jammed because of the high-cap mag. Then he switched to his pistols. Most likely the most serious wounds were from the shotgun and pistol.
* VT: Cho used a .22 and 9mm pistol. He stalked his victims and took carefully aimed shots. That's why he was effective. When you look at shots fired and deaths...his is probably the most cruelly efficient spree.
* Columbine: the media refers to the weapons they used as 'assault weapons'. But they were mostly pistol caliber weapons...not intermediate caliber assault rifles.
Over 70% of murders are committed by pistols followed closely by shotguns. Far in the back of the park are rifles of any type.
In the end, what you are saying is poor public policy because it does not impact criminal behavior and what criminals choose to arm themselves with.