Last time I checked, cops on the street don't carry a rifle on their back and they only keep it in the car. Unless the cop was one of the response team members then he probably won't have a high-powered rifle with them.
Most of the time they have time to get there ar-15 out of the back of the trunk. Most squad cars in my town have the ar15 holstered near the front seat where a shotgun would normally go. I'm going to assume your department operates differently.
Edit: Also some countries like Norway and Switzerland have low murder rates and allow the ownership of guns.