Place a Large Caliber Gun on Police Helicopters to Disable Vehicles

I feel like this should have been done already.  How hard is it to put a 20mm cannon on a helicopter?  There is a really nice one that uses the same design as the new squad weapon replacement that has the reciprocating barrel that absorbs the recoil for ultra accurate shots complete with a FLIR firing camera.  I'd say 50 rounds would be enough for the five O, whaddaya say?  Next time someone wants to run from the cops, you think they'd get far, or even try knowing what would happen?  No.  Fire control computers are plenty capable these days of placing a round right through an engine block going 120mph.  Now, only question is is can we afford the new AuTi alloy bullets with the tungsten tips?