What crime, and the severity of such, would depend heavily on the state. In Florida it could fall under a number of laws, including Throwing a deadly missile (FS 790.13), Assault (if missed and non-deadly weapon, FS 784.011), Aggravated Assault (Missed and Deadly, FS 784.021), Simple Battery (Hit and Non-deadly, FS 784.03), or Aggravated Battery (Hit and Deadly, FS 784.045)
Assault, and Simple Battery are misdemeanors in the state of Florida. Aggravated Assault, Aggravated Battery, and Throwing a deadly missile are all felonies. There are probably some other laws that can apply as well. These all require intent as well. Without intent you may get negligence and littering, but that's about it from a criminal standpoint.