PlaneGunAmmoMultiplier, or so it says in 'Arena Settings' in the help file. I have never adjusted that though so there might be a trick to it. There may be multiple listings of this in the arena settings due to primary and secondary weapons, I'm not sure.
I always set ProtectObjects to 0 so I could attack friendly targets and not have to worry about the field guns shooting off my sorry behind.
You know how to access the arena settings thru the clipboard I assume.