Arrowheads

>ammo is dried clay balls

Cheapest projectiles too, though seeing how round balls have shit for aerodynamics, probably the worst one as well.

plain old extra nasty barbs

go fuck yourself

>Arrows wrapped in oiled cloth for setting things on fire

An alternative is to use a pyrotechnic mixture (saltpetre, coal and sulphur, with much more sulphur and much less saltpetre than for gunpowder) that's soaked into a fibre wrapping (cord, fabric, some such) and allowed to dry. This appears to have done the job better and with far less fuss in real life (and thus what was used once it has been invented), but in fantasy the difference may mostly one of style. Or a way to make alchemical knowledge more battle relevant I guess.

youtube.com/watch?v=uL4vnolCwLI

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Fuck man that'd be horrifying to get hit with.

Skip the connecting cord and these harpoon arrows would probably be a good poison delivery system. Probably a bit longer points though so you can fit more poison on there. If you're stuck with old IRL grade poisons though the effect will probably be rather slow to depend on it in a combat situation, so building your arrow for that instead of doing structural damage straight up is probably for the hit and run or assassination situation.

>when you want to gun but you also want to arrow and you want your "man-packed siege weapon"-itch scratched

Notice how the guys shooting fire bolts down in the right corner aren't all armed with crossbows.