I'm not a fan of "assault" rifles per se. (The definition used by the government to ban these a while back, before allowing the law to lapse didn't really address real assault rifles, but the civilian versions, and it went largely to cosmetics–flash suppressors, folding stocks, high-capacity magazines and such, and named names.)
If I were out in the boonies engaged in guerilla warfare, I'd want a good deer rifle. Better hardware, more powerful, more accurate. In a tight urban environment, I'd prefer a pump shotgun.
If you want a full-auto weapon, i.e., a machinegun, you can get one in many states, but it has to be registered with the feds, you have to have the checks, sometimes the approval of the local sheriff, and they will know who you are and where you live. I can't recall seeing a case where somebody who had one of those legally cut loose with it at the mall. And those, since they are no longer made for civilian use, cost a fortune. You can get a good used car for less than a nice Tommygun, and top-of the-line subguns are also spendy.
Do you need a hundred round magazine? A fifty? Twenty? Fifteen? Probably not. The average gun fight, according to the old FBI standards, was three feet, three shots, three seconds.
The video shows an Army pistoleer doing fast magazine changes. With a little bit of practice, you can drop and insert a fresh one faster than it takes to read this, so you don't really need a high-cap magazine unless you are facing the Chinese Army come ashore, and in that case, you are screwed anyhow.
My father's .22 lever action Marlin, as I recall, loaded like 19 rounds in a tube under the barrel. Not very sexy, and it took a while to reload once it ran dry, but outside of wars, more people have been killed with .22's than any other caliber.
If the end times came and I had to narrow my selection to one gun as I headed for the hills? A .22 rifle and all the ammo I could carry would be my first choice.
Where the real cure will come for deadly violence in America, if it ever does, will be to deal with the hand holding the weapon. As long as the hand is willing, a tool will be found. To educate people so that the idea of killing somebody else comes last, and not first? That's the trick.
I wish I knew how to pull that one off.