From a physics standpoint, it has everything in common with a hunting rifle, ammo count notwithstanding. The mechanics behind killing a 150lb human and a 150lb deer are pretty much identical.
Yes, again, modifications nothwithstanding. The picture I was commenting on was a 'normal' AR-15, aka a semi-automatic rifle that fires one of a myriad of calibers, probably a .223 or 5.56 round. AR-15 was chosen likely because they're very common, have kind of become a defacto weapon for modifications (making said mods very inexpensive due to economies of scale), and because they can field a round quite capable of killing people at hundreds of yards away when spraying on them from above, or on the case of this incident, when spraying at them down a crowded hallway.
Any given large caliber (greater than a .22) would have a similar enough stopping power to an AR-15 with equivalent ammunition that you'd be hard pressed to notice a difference on a corpse. As for your question regarding quantity of ammunition, here's a nice AR-556 sold by Academy Sports & Outdoors (you may have seen/heard their commercials, if you're in the US Southeast):
https://www.academy.com/shop/pdp/ruger-ar-556-556-semiautomatic-rifle#repChildCatid=1364736
And here's a nice mixed selection of rifles, pistols, and shotguns, all equally capable of taking the life of a human, and most look pretty black and scary to boot. I have no doubt that you could find a myriad of clip/magazine sizes available for any/all:
https://www.academy.com/shop/browse/outdoors/shooting/firearms/modern-sporting-rifles--1
To clarify my point, there's little difference between a hunting rifle which can carry more than x rounds, a hunting rifle which cannot carry more than x rounds but has an easily removable clip/magazine and a practiced shooter, and a 'military rifle' with the same limitations.
Now, there are options to limit accessibility to this, but you've got a severe uphill battle in the US to make something like that a 'thing', I'm talking
generations of time would be spent attempting to halt access to ammunition clips, weapons, etc.