"Assault Weapon" is the term made-up by gun-control spin doctors.
"Assault Rifle" is a US military term for a fighting rifle in intermediate caliber (not pistol, not long action) capable of full-auto and/or burst fire.
AR-15 is (as you know) not an Assault Rifle.
M4 is an Assault Rifle.
They function differently, but to most folks, they appear exactly the same. This is how gun-control types inject fear, uncertainty and doubt into the debate.
The GCA banned the manufacture of transferable "machine guns" made after May '86.
The GCA, therefore reduces the supply-side of the equation for transferable full-autos. Transferable M-16s cost in excess of $10,000, plus the $200 excise tax to transfer them from one owner to the next.
An individual may legally own a full-auto capable weapon provided that they pass the strict NFA (National Firearms Act) requirements and that the weapon was made before May of '86.