So, advanced [on average comparable to the end of 19th century Europe/America, though diverging in specifics due to magic] magical civilization has a problem. They have unique magic guns, that work on a principle where magic is used to accelerate any solid projectiles inside[well, any solid projectiles except for metals, as explained later]. Magic propagates slightly slower than 500 m/s, making that the top speed of projectile fired by such weapons.
But magic and metals don't work well together. While non-reactive metals (Gold, Platinum, Silver, Copper) can be used to reflect magic, all the metals are unaffected by direct magic manipulation and reactive metals have an even distributive effect on magic within a few centimetres around them, so you don't really want to make metal bullets, because those wouldn't work.
Luckily, at least in the form of compounds (such as oxides, sulphides, etc.) of metals don't have this annoying problem for magic guns, the problem only manifests if the structure of metals is present, so at least in that way, there are multiple possible materials to use.
Things I think need to be taken into account:
- Unlike with explosive firearms, magic firearms don't need propellant.
- The magic gun continuously delivers force and creates something like an acceleration field inside the barrel, so it's far easier to accelerate long ammunition than with conventional gunpowder-powered ammunition (since the force that ammo is subjected to grows at almost the same rate as mass of the ammo does).
- How would one make this ammunition work with rifling?
So, with these restrictions, what will ammunition for handguns, long guns and cannons look like?