Why? Clearly they are not the main problem when it comes to violence with guns. Why those particular weapons? Again, because they make the news? Because an AR-15 looks more frightening than a .357 Magnum Revolver? Why not just make ammunition more expensive? That way people are less likely to waste bullets. Like, let's see... make a 5.56 $250 a bullet. A 9mm $50 a bullet. The larger the size and greater damage the bullet could do, the higher the cost of the projectile. 50cal - $5000 a bullet. .357 Magnum $75 a bullet. .22 long rifle $7 a bullet. 12 gauge slug $88 per slug. 12 gauge buck shot $66, bird shot $55.
Just curious.... How many "100 round magazines" have you actually seen? I don't recall actually seeing these involved in mass shootings with AR-15's and the like. Seems like they would be heavy and would detract from the accuracy of the firearm. Or again... cause they look scary?