Damage figured just like a firearm, so a normal swarm does 2d4 damage for 4 points. Swarms get +2 Parry for another 2 points, can't be Ganged Up on for +1 point, inflict damage automatically and bypass Armor for (now we start to wing it) about +6 points, take up a Burst template for +1 to +5 points, and have several immunities for say +10 points.