If your in battledress (and have the Vehicle(Battledress) or Vehicle (Heavy Armor) feats), use the characters BAB.
Otherwise, characters are required to have the Vehicle feat and P/Loader or P/Teamster skill to even think about hitting someone. Again, use the characters BAB for this, but the lower of the Arm or Characther Dex bonus.
Appendages do normal damage (1d4 + strength). When dealing with metal arms, the damage is both Lifeblood and Stamina.
As a house rule, I convert large Strength bonus back into dice (say each +5 = 1d10) for a more random damage results.