Thanks for the help.

If they're banned since the 30's, how come I keep stumbling on YouTube content featuring them?

Banned isn't the right word. Heavily regulated (for an American) would be closer. To purchase a full-auto weapon, you need to undergo a background investigation including getting fingerprinted and pay a $200 tax. The same process is required for purchasing or creating suppressors, short barreled rifles or shotguns, calibers above .50, and explosive weapons like grenades, missiles, etc.

Manufacture of new legal-for-civilians machine guns was banned in the 1968 Gun Control Act, any legal ones you see on youtube or that you can rent at a range were manufactured before that bill. Because of the scarcity, they're worth at minimum tens of thousands of dollars which is a greater financial barrier than the $200 stamp, roughly $4500 when the 1934 NFA bill was passed.

No machine gun that's gone through the above process has been used in a crime by a civilian not in law enforcement, and only a handful of crimes have been comitted with the other items covered by the act.

There's your problem, "video games" are not necessarily representative of reality.

"Assault weapon" is a term invented by gun control activists to A) sound scary to drum up support and B) expand their bans to handguns.

"Assault Rifle" is an actual term, where they got the idea, and the source of this intentional confusion caused by MDA and Everytown. Assault Rifles are defined as "A select fire rifle in an intermediate calibre intended for infantry use." The bolded parts in the above definition mean the AR-15 is not in this catagory, as it is only semi-automatic (no select fire) and intended for civilian use, not infantry. The M4 and the M16 are both rifles that do fit the above definition, and the AR-15 is cosmetically similar, but the main function (the select/semi part) is different. In fact, civilians have not been able to own rifles that are select fire since 1986 (unless you have your Class III SOT, the permit required to own one, but for that you basically have to be building/selling them to mil and/or police).

Video game devs aren't necessarily known for being experts on guns, laws, etc, but to be fair to them, they don't need to be, because video games aren't real (sadly, as much as I would love to live in my Viva Pinata 1 garden I have had to come to terms with the imposibility of my dreams).

An assault rifle is full auto, or burst fire, a machine gun basically. That's also the case in every video game I've played. You can own them if you get a special federal license, it's expensive so there aren't many out there. Guys will set up businesses charging people $50 to shoot one for a few minutes. That's probably what you saw on YouTube. No mass shooting in recent history was done with an assault rifle.

An assault weapon is an imaginary legal term created during the Clinton administration so it could look like they were doing something about gun violence. The awb defines assault weapons using superficial cosmetic items like a bayonete mount, a pistol grip, a flash suppressor, etc. The same gun with 2 of these is legal, 3 of them and suddenly it's illegal despite no functional changes to the gun. Assault weapons and the assault weapon ban were idiotic ineffective political theater.

Mass shootings are usually carried out with a semi-auto rifle, which means it automatically reloads the chamber and is ready to fire another round as fast as you can pull the trigger. The most popular one is the ar-15. It's the standard semi-auto rifle, they're everywhere because they're cheap, common, and reliable. They show up in mass shootings because they're so common, not because they're necessarily dealer than any other semi auto rifle. The AR stands for "armalite rifle". It's the civilian version of the M-16 assault rifle.