The BullDog Pistol is a backup weapon for when your other guns run out of ammunition. It has 15 clips and never runs out of ammo. You also automatically use this weapon whenever you pick up a heavy weapon and the heavy weapon runs out of ammo.
It is a Weapon Mod of:
Sic 'Em: Score 10 Hacks with this weapon
Man's Best Friend: Score 100 Hacks with this weapon
Junkyard Dog: Score 1,000 Hacks with this weapon