We need to make it more difficult for people to get these high rate of fire / high penetration weapons (aka "assault rifles"). Police forces all across the US have used the easy acces to these weapons as a pretense for an arms race. The police have used it as an excuse to carry their own assault weapons in every single patrol car. To carry out search warrants in full tactical gear with body armor. They've used it as an excuse to carry grenades and drive around in APCs.
Assault weapons are undermining our rights and are making the police more and more militaristic everyday. We need to get these weapons off the streets so the police no longer have an excuse to run around looking - and acting - like full blown militias all the time.