There's a smart way to do this. That would be to calculate the bullet's velocity and match up where the helicopter's pilot will be in accordance with when and where you fire (plus calculated wind intensity and gravity's effect on the bullet and helicopter).
But you are not that smart. Instead, you fire a couple shots off at the approaching helicopters. Nothing happens.
No, seriously. That was a bad idea.
You notice that the zomboamiceman (zombie-foam-policeman) is now right next to you. You shoot that sucker point blank. And the zombie goes down. And you feel quite proud of yourself!