I find all this stuff fascinating. Great thread! I want to comment on one little aspect though:
One of the main benefits of the general design of a javelin (I don't know the specifics of Gallic javelin design) is that it can be made of different materials to fine-tune both balance and weight for given effects. This adaptability is almost unique among projectile weapons -- consider such disparate examples as the weighted atlatl spear, the Roman plumbata, and the modern sports javelin.
Technically, gravity would only help if one were throwing a spear downhill. But its mass is a big help, easing the power transfer from the thrower's arm into the projectile itself and reducing the chance of injury for powerful and/or expert throwers.
-Glee
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