In the museums it tends to be pretty easy to tell the javelin-heads from the spear-heads - the former tend to be all slim and narrow. Even the barbed ones. And we all know what the factor "lots of momentum" does when paired with "really small area", don't we ? I understand many historical armour-piercing arrow/-head designs were intentionally made heavy, trading effective range for sheer momentum and penetrative capability.

I've read javelins tend to fly so slow, and are so easy to see coming, that people with room to maneuver (ie. specifically *not* heavy infantry in combat order...) tend to have little trouble dodging them though. Apparently javelineer skirmisher duels tended to be exercises in frustration, with very little results on either side. Javelins against close-order troops, on the other hand, were just nasty...