Do tank bullets inflict damage on other tanks and jets in Battlefield 3?
Solution 1:
The bullets from a tank's secondary position will inflict damage on most vehicles and all infantry.
Damage to aerial vehicles is fair, it's definitely worth shooting them to at least annoy them, and possibly disable them if they stick around long enough.
Damage to enemy armour is non-existant, and you're better off looking for enemy engineers repairing the tank, or scanning around for any sneaky engineers/supports who are looking for an easy kill on a distracted tank.