Using the craft skill with gunsmithing as your specialty, a character can modify firearms. Anything from recoil supression to amplified damage and the overall size and concealability of the weapon can be altered.
No firearm however can exceed a +2 mod to its damage output, if it does, the weapon has a short lifespan due to its frame not being able to withstand the increased recoil. Only one mod per firearm.
So a heavy pistol that has a base of 5 dmg, modified, could have a 7 dice pool for damage.
Special rounds:
currently there are very few types of rounds one can get outside of the military. Getting access to military grade rounds requires alot of dots in backgrounds, abilities, and so forth.
However, hollow point rounds are much easier to come by, and add a +1 to damage when fired.
So, you have a heavy pistol, base 5 dmg, modified to give it a 7 dice pool for damage, and are using hollow point rounds, the dice pool of damage is 8 base.
Firearms, regardless of hollow point or not, do BASHING DAMAGE to vampires halved rounded down. Unless it scores a headshot, then the damage is lethal.