You may have noticed that some damage has a plus after it, like so:
This plus is called "variable damage", and it shows that this weapon can do up to that much of damage, but does not always do that much. Some weapons rely completely on variable damage to be useful; others may not.
To determine Variable Damage, flip a coin for every 1 damage that Variable Damage offers and call each one. For every correct call, that much damage is added to the total amount.