Firing a func_tank when in a vehicle

I've been trying to get this to work for ages. I've made a brush helicopter and it needs weapons. How do I get a func_tank to fire while your flying?

I've managed to make a laser fire using ent_laser but I would rather have bullets fire instead.

Please help this is the 3rd forum I've asked now!

Cheers

Found out why It doesn't work...Its works on hl2 sp but not dm.