Why not make sure?
Double Tap (COMBAT)
Spend two shells to make shure it won’t move later.
Benefit: When you target a creature with a ranged attack, you can take a -2 penalty to the attack and spend two charges or cartrages for the attack. If you do, you take the number of dice the weapon does in damage (ie 8d6 fire damage) and divide the dice by 4, rounding up, and increase the dice damage value by this number. As an example a 8d6 fire damage weapon would do 10d6 fire damage.