I know what an assault rifle is. I used to work in the firearm industry and own over 50 personal guns. The police have select-fire rifles built on the Armalite platform. They're assault rifles. The guns used by our department don't have an M designation because they're not US military spec or military issue. It's like the difference between an M9 and a 92FS.
They have riot shields, tear gas, etc in the cruisers. They carry the heavy body armor and the weapons back and forth because they are sized/kitted out to the officer's size and preferences and they share cruisers between shifts.