Can you count the stars in the sky?
It is equilivent to how many assault guns are actually out there.
Even with trillions of assault weapons out there, how are you gonna get all of them and destroy them? Law can't destroy all of them, and very unrealistic, also there are too many people KNOW how to build one, how solve that problem? Law CAN'T take somebody's knowledge how to build one, even crooks too, there are plenty of crooks out there knowing how to build one, how can law prevent that?
I have worked in machine shop for 5 years, and have seen how they made parts for military assault weapons, tell ya, it is NOT hard to make! It was rather simple, you could turn any gun with built in recoil ability into machine gun! How? Just make a part where they just hold up a tooth to allow instant recoil powered by ammo! No batteries needed!
Lastly, never assume somebody's knowledge on firearms, assume itself can be deadly!
Lets say, you live on street with posted speed limit of 25MPH, and discovered a teenager drove his car on your street doing 90MPH, what are you gonna do? make your local law decrease speed limit to 20MPH from 25MPH is going to work and stop him from doing 90MPH?
Use your HEAD!