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For simplicity's sake, why not just have the use of an Atlatl increase the range of darts or javelins thrown with it to 80/320?
Sorry, I missed this earlier. I am playing around with different ideas to see how they turn out in actual play, hence the +damage -range set up.
I just went with my gut on this one, based on the rare bits I have read or seen it in use. I know 1) it uses large darts for ammo, 2) it adds quite a bit of force to those darts (though that can be represented just as easily by extra range as it can extra damage.) 3) One of my players might want to play a specific character who's focus is the Atlatl, and I want it to mean more than "you can throw these throwables slightly further", if that makes any sense.