9mm is not the only caliber of pistol round. There are pistol calibers out there that have as much energy as a .223. It also doesnt say anything about ammo type. FMJ? Hollow-point? You mention large mags... it takes an unskilled, uncoordinated person maybe 2 seconds to swap a mag. Lets take the Nashville shooting as an example. The cops did an absolutely fanfuckingtastic job of getting in and taking the idiot out, but it still took them 12 minutes to arrive. Thats 720 seconds. Do you really think 30 seconds for 15 mag swaps will make a difference? If nobody is fighting back, and they have all the time in the world, the amount of ammo they can carry in a single magazine is meaningless.

Guns in war have not evolved to kill large amounts of people. Thats what we have bombs and missles for. They've evolved to protect the person wielding it. The higher the velocity, the further away you can be from your enemy. The more rounds in the magazine, the more you can spam towards the enemy in hopes of hitting them while not getting hit yourself, as you likely wont have time to take a steady shot.

The AR-15 is based upon the M16 platform originally designed for the US military 60 years ago. That means 60 years of parts; enough to maintain 8 million of them. The civilian version, the AR-15 can use many of the same parts. An extreme amount of parts surplus, along with it being a rather simple platform means they are cheap, easy to repair, and have garnered a bit of a cult following, especially among veterans. It has also lead to a huge aftermarket as the AR-15 is very simple, and easy to customize.

The AR-15 itself is a rather average gun. It performs as most other rifles do. It requires quite a bit of effort to keep it running though. Its very susceptible to dirt, grime, etc, and getting everything to run smoothly and reliably can take a lot of work.

The reason its become the target of so many? Its extremely common, as mentioned above, and has certain 'features' that stupid people really love to latch on to: Its black, it can have pointy bits on it so it looks 'aggressive', and it has AR right in the title! Too bad the idiots don't realize that it does not stand for 'assault rifle', but instead stands for 'Armalite Rifle', after the company that designed it. It has also become targetted because idiot politicians and the media need a simple object to hold up to the masses. They cant make money or get ratings or votes from images of mental health issues.. what are they going to show? People cant get angry over images of.. brain wave patterns or something. An evil looking black object though? McTrump himself, while incredibly stupid in all areas that matter, was intelligent enough to realize this. Create something simple, like a red hat with 4 letters on it, and the idiots will be drawn to it like moths to a flame. Same reason pop music is so popular. People like stupid, mindless and simple.

The many things the politicians, media and most people get wrong:

The definition of 'assault rifle'.
What makes something 'tactical' or 'assault-style'.

Pistol grips/braces: How do they in any way affect.. anything?

Bump stocks: Unreliable and useless, more of a hindrance than a help.

'High capacity magazines'. You mean normal capacity? It takes virtually no time to swap a mag.

Bullet vs Round.
Magazine vs Clip.

If these people are unwilling to understand and learn the subject they are speaking about, they need to just shut the fuck up.

What source?

Try closer to 750lbs.