Here is my proposal, don't try to ban assault rifles
Create a law restricting muzzle velocity of all weapons. AR15 has muzzle velocity of 3300 ft/sec. This is what allows an AR15 to be so deadly. Its ammo meets international law: it is full metal jacket, it is not designed to flatten or break apart. But it does both of these because of the muzzle velocity. Which makes the AR15 highly destructive.
This would require a new cartridge. The current one has too much powder behind it. Reduce the powder, and the cartridge would be smaller (shorter). Ban the manufacture and sale of the original cartridge. Manufacturers would have to redesign the chamber and make adapters for existing AR15s to be able to fire the shorter round. Gun manufacturers may actually like this. I means increased sales.
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you won't listen to someone who is expert in designing or repairing guns before mandating retrofitting a firearm.
That is akin to passing a law regarding something medical and not bothering to discuss that law with doctors.
What actually makes the AR-15 deadly is that it fires a bullet. Robert Kennedy was killed by a .22lr from a revolver. A rifle only serves to give the powder charge a longer time to accelerate the bullet which in general only improves the effective range.
The specific magnitude of damage that a single bullet will cause is primarily determined by its energy. I majored in physics and have a detailed understanding of energy and dynamics. The kinetic energy of an object (E) is given by the formula E = 1/2 m*v*v, (v*v is v squared) where m is the mass and v is velocity.
The muzzle energy of a .223 Rem is 1100 - 1200 ft·lb. Comparing this to an AR-10, 2,550 ft-lb. Compare that also to the basic 9 millie which is 350 - 450 ft·lb. BTW the 9mm is one of two guns used at the deadliest mass shooting at a school, Virginia Tech. The other handgun was a .22.
Modifying an AR in .223 down to a smaller case for less powder would be expensive. Also, many people reload their own ammo by saving the brass and adding new bullets and powder.
The average deer hunting rifle fires with more muzzle energy than an AR-15.
I conclude that making this change will, at best, make the modified AR-15 about as deadly as the weapon used in the deadliest school shooting to date. 