We have a damage range of 50-150% or 0-200% right now anyway, which can be interpreted as "hitting the weak spots". Right now, the distribution for the damage range is even. Instead of a damage bonus, I would change the distribution to make the higher percentages more likely than the lower ones.

Example: for the 50-150% range, a result in the 50-100% range has a chance of 1/3 and a 100-150% result a 2/3 chance.

Edit: Also, this should be dependent on damage type. Explosions should not benefit, as you can't aim a granade at a weak spot.