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Those with the rapid shot skill have the ability to fire an extra missile
from a projectile weapon every round that they receive their full attacks. To
ensure that you get the most from this skill, be sure to always have extra
ammunition for your weapon in your inventory.
To ensure that scouts are not inferior to mercenaries in the ranged combat aspect, why not make it so that rapid shot -always- grant an extra shot? That would make it more like a specialization in all ranged weapon (excluding the accuracy) instead of just making the fast bows viable.
With most fast bows, it is highly likely that the merc would reach the fourth attack which he has naturally, while the scout gets his three + rapid. However, for slower and more damaging bows, the mercenary has an advantage. Because the scout does not receive his full three attacks, rapid shot will not proc, while the mercenary can reach the same number of attack without specialization, and even more than the scout with it...
A simple, minor change which would ensure the scouts dominance regarding ranged combat.