Okay, maybe not all semi-automatics can be described as assault rifles in your nomenclature, but I would have thought that weapons which can fire off multiple bullets in seconds can "assault" people very effectively.
Assault rifle is a technical category.
A semi automatic rifle, like my Ruger Mini-14, which fires .223 cal (5.56mm NATO standard round) when it has the manufacturer's five round clip in it is hardly an assault rifle. It is a semi automatic rifle.
In the early 80's, I purchased a number of banana clips, some 20 round and some 30 round. With the 30 round clip in it, you could assert that the rifle approaches the capability of an assault rifle
in capable hands. It then becomes a dick dance, but the definition is reasonably clear. My Mini-14 isn't one.
When I use it for hunting, however, having a banana clip in it is illegal. I normally only load 3 rounds anyway in the clip anyway, since most game runs off at the first shot.

Filling the air with lead by rapid firing in a semi automatic mode is an abysmal hunting method. It also wastes ammo.
FWIW, the definitions are useful to refer to when discussing this topic with Americans. The terminology matters.
Wikipedia has a decent summary.
Wiki said:
The term "assault weapon" in the context of civilian rifles has been attributed to gun-control activist Josh Sugarmann.
The term assault weapon refers to semi-automatic firearms (that is, firearms that, when fired, automatically extract the spent casing and load the next round into the chamber, ready to fire again) that were developed from earlier fully-automatic weapons.
By former U.S. law the legal term assault weapon included certain specific semi-automatic firearm models by name (e.g., Colt AR-15, H&K G36E, TEC-9, all AK-47s, and Uzis) and other semi-automatic firearms because they possess a minimum set of features from the following list of features:
Semi-automatic rifles able to accept detachable magazines and two or more of the following:
Large capacity ammunition magazines
Folding or telescoping stock
Conspicuous pistol grip
Bayonet mount
Flash suppressor, or threaded barrel designed to accommodate one
Grenade launcher
One can point to on my Mini-14 with a 30 round clip, and that's all. So no, not an assault weapon.
Semi-automatic pistols with detachable magazines and two or more of the following:
Magazine that attaches outside the pistol grip
Threaded barrel to attach barrel extender, flash suppressor, handgrip, or silencer
Barrel shroud that can be used as a hand-hold
Unloaded weight of 50 oz or more
A semi-automatic version of an automatic firearm
My Berreta 9mm is not an assault weapon.
Semi-automatic shotguns with two or more of the following:
Folding or telescoping stock
Pistol grip
Fixed capacity of more than 5 rounds
Detachable magazine
The gas operated shotgun I am considering buying is not an assault weapon. The pump action shotgun I own is obviously not.
Four more gun deaths today. How many more?
Why does that matter? People die every day.
I don't support the notion that because a nutcase may use a gun irresponsibly that you should restrict my rights, or that of the vast majority of citizens who are not nutcases. The reservation of rights except those afforded the government is a concept embedded in our legal framework. What was not embedded is a promise to bubble wrap life.
The point you were making about unstable people grabbing a gun and doing something foul points precisely to the problem, which is that a certain sort of people are the killing agents, and the root problem.
The old bumper sticker is "guns don't kill people, people kill people." (And some use guns to do so.)
DR