Steel, cast iron, etc. won't be great as a projectile because (1) it's not dense enough (2) it's not soft enough. (1) is important for ballistic properties and

Steel, cast iron, etc. won't be great as a projectile because (1) it's not dense enough (2) it's not soft enough. (1) is important for ballistic properties and carrying energy to the target (2) is important because you want to be able to fire the weapon more than once without destroying the barrel's rifling. Now you might argue that some kind of sabot or patch could work to mitigate this. But you'll still have a density problem. Would have to test it to find out how badly it impacts performance.