So, in essence, an adamantine axe would still be worse than a sword - the mass difference is negligible, and the axe can't stab.
in real life most longswords were weighted towards the hilt, while the axes were weighted towards the head, this meant that if both weighed the same the axe would have more force in a swing. a falcion though was designed with weight towards the head, which meant it performed very much like an axe, it also wasn't good at stabbing as well, however there was a weapon called a falcata, which performed like a falcion but was also good at stabbing as well.
in game a harder material should hold a sharper edge, and thus improve the penetration value (i would guess), thus with long swords being falcatas, i would say that adamantine axes would perform very similar to adamantine long swords, which if hardness is taken into account would make them better than steel ones. just look at ceramic knives over cheap steel ones, the ceramic cuts much easier than the less sharp steel ones, and thus the weight difference is more than made up.
That's a shortsword, with weight 30 vs the axe's 80(actually volume, as noted by Toady). The longsword is closer to it, at 70. Still, in adamantine's case, having an axe twice as large is like having a weapon consisting of two blocks of styrofoam instead of one - the weight difference is too small. The larger contact area also means less penetrating force, so less useful against armor.
I'll take high-quality steel instead of adamantine for all my dwarves' normal weapons, thank you. Arrows and armor, on the other hand, are perfect for the metal, especially when I mod in superheavy arbalests with a greater maximum fire velocity.
suit yourself, steel hammers would be better, and steel is most likely much easier to get.
Also, the SHOOT_FORCE and SHOOT_MAXVEL properties are interesting. It might be cool to play around with weapons with a relatively low MAXVEL that need heavy projectiles to get the most out of the shot -- I think slings would fall into this category, since lead was the favored material for sling bullets (Xenophon apparently said lead bullets went twice as far as stone ones).
SO what's the ideal metal for ballista arrow heads?
it's edged, so adamantine, unless hardness doesn't play into it. even more so if the weight comes from the log used, fully steel or lead would increase range if that is in effect (i don't think it is though).