Damage is net rolled. Crossbow rolls the Barghest's defense, your net hits (the number of hits more than the Barghest gets) are added to DV, and then the Barghest gets a soak roll to take some of that damage off. If it has any armour, the value of that armour is reduced based on your AP.
When declaring the attack, you should probably declare total DV before net hits, AP, and whether it's resisted with Ballistic or Impact.