The Police should not be able to have Machine guns. Why should the police be better armed than the Citizens, can we say Militarized Police force ?
To be fair, they should be able to be at least on par with actual criminals. That is, after all, their job. Nothing like facing off with criminals armed with an AK-47 when all you have is a .38 service revolver.
To be accurate, however, machine guns do not a better rifleman make! One of the reasons the U.S. went with three-round bursts for the M-16 is that full auto unnecessarily wastes a lot of ammo. Sure, there's a need for machine guns, as in hosing down convoys, as defense against enemy aircraft, and aboard aircraft while attacking things like convoys and tanks.
I think well-trained policemen armed with an AR-15s should be able to handle just about anything criminals throw at them. Thanks, flash-bangs, tear gas, standard sidearms, and tasers are really all they need.