"assault weapon" isn't a valid description of anything. M4 isn't an "assault weapon" either.
Any weapon can be used in an assault. So I guess they are all "assault weapons" from a bb gun to a machine gun.
"Assault Weapon" is a bullshit, made up term to equate military rifles with sporting rifles so they can gun-grab at will.
The term "Assault Rifle" is a legitimate term of art.
It originated with the Sturmgewehr 44 (literally translates as "Storm Rifle" but means "Assault Rifle").
The problem the German Wehrmacht was trying to solve in creating the Strumgewehr 44 was the need for two different primary weapons. One for open-field combat, and another for close quarters combat (CQC) in cities and building-to-building.
Germany had the MP40:
Which fired 9mm Luger (the common 9mm pistol round used today) which is a small round, great for CQC. It was great for close quarters combat because it was small, light, easy to carry and maneuver in building-to-building combat, and was full-auto to allow suppressive fire without the need for machine guns.
The alternative primary weapon was the Karabiner 98k:
It was an excellent weapon for open-field combat at distances and very accurate because it fired one round at a time, but its bolt action, large cartage (big bullets), and length made it nearly useless in CQC.
The solution was to make a rifle that would serve both roles, so they wouldn't need mixed units. Everyone would carry the same weapon for both open-field and CQC. Germany developed a rifle that had a mid-sized round (not a pistol round, but also not a full rifle round), the ability to shoot one round at a time (semi-auto) and the ability to shoot full-auto for suppressive fire in CQC situations.
Behold, the Assault Rifle (Storm Rifle)....TADAAAAA:
And, who copied the Strumgewehr 44?
THE ******* SOVIETS:
Followed a few years later by the ArmaLite AR10 Rifle, which had too big a cartage for assault rifle purposes, so it was followed by the AR-15 chambered in 5.56:
The original AR15 had selective fire (both semi-auto and full-auto modes) which became the M16A1, but today's civilian variants are semi-auto only and cannot be converted to full-auto without a complete change of the receiver.
So, in sum, although the term "Assault Weapon" is a bullshit, made up term designed to empower gun-grabbers with scary-sounding words, the term "Assault Rifle" is a legitimate term for a military-grade weapon that most civilians cannot purchase.
Anybody who calls a civilian sporting rifle "assault" anything is a gun-grabbing POS who should be executed.