options/arena setup/environment/arena settings. Select the tab "weapons". change the "ammomult" variables.
Options>arena setup>environment>arena settings under Weapons> PlaneGunAmmoMultUnfortunately it looks like they changed it so the number limits displayed are in effect now. It says the highest it can go is 10 but I used to be able to set it to exactly 1000000 which made every plane have unlimited ammo.