Please show me where police carry grenades. Flash bangs and smoke, maybe. But actual grenades? Show me.
Do you understand that an "assault" rifle, more correctly termed a semi automatic, only fires as fast as you pull the trigger? Not to mention that you've got to aim.
You realize that if the governments succeed in taking away all semiautomatic guns, cops, the military, and all terrorists, drug dealers, gang bangers, smugglers, thugs, and just general bad dudes will continue to have them, sell them, buy them, and use them, right? And in the event that bad guys cant get their hands on them, they'll resort to using other means, like pipe bombs, fertilizer bombs, vehicles, airplanes, poison gas, I mean the list goes on and on. You're focus is on the wrong part of the problem. Getting rid of guns, any gun, isnt going to stop a bad guy from killing people.