Assault Weapon = "Any selective-fire rifle capable of chambering a medium-power cartridge"
The term "Assault Weapon" comes from the WWII German Sturmgewehr 44 (which translates literally into "Storm Rifle 44", but is usually translated into "Assault Rifle 44".
Sturmgewehr was meant to mean a medium-powered rifle for use by storm troops in the assault of an objective.
These "assault weapons" have been heavily regulated since the National Firearms Act in 1934.
The Gun Control Act of 1968 created a litany of other rules for the sale and transfer of firearms in general (not just so-called "Assault Weapons").
In 1986, there was a revision to the Gun Control Act of 1968 (the Firearm Owners Protection Act) which banned the manufacture and importation of selective-fire firearms, for sale to the public.
After 1986, no civilian could buy a newly-manufactured "machine gun". Existing "machine guns" could be bought and sold, as long as civilians complied with the laws in place since the National Firearms Act (from 1934).
What this did, was make the existing "machine guns" raise in value from $1,000 to $20,000 - $30,000, or more, which translates into billions of dollars.
Some people actually became millionaires, overnight. Your "average Joe" today, does not have the means with which to buy one of these, which effectively takes them off the market.
Because the police and military can still get them (and they continued making them for the police and military after 1986), they get stolen.
The people who steal them are what we call "criminals".
You can write all the laws you want, but there will still be criminals who won't follow them.
In this Great Age of the Anti Gun crowd, an "assault weapon"is now being identified as "any semi-auto rifle with a detachable magazine and a pistol grip".
Some of the other "scary" features are muzzle devices (brakes or flash hiders) and black plastic.
There are a ton of politicians trying to pass laws right now, who don't even know how firearms actually work. They just know that firearms are "bad".
They use terms like "gun violence", but they never say that someone mowed down by a drunk driver was a victim of "car violence".
They don't, because people would say "you're stupid!"