The verb 'atirar' is commonly used to express the meaning of the English verb "to fire", like in the sentences: Policial atirou em suposto traficante. Jovem atirou acidentalmente em sua irmã. But it is ok if you use to mean "to throw" like in the example: O garoto atirou uma pedra no rio. However, the meaning of the verb "to throw" is better expressed by the verb "jogar" in portuguese spoken language. For example: O menino jogou uma pedra no rio. For written language, the verb "arremessar" sounds more adequate, since it is a little more formal. If you use "jogar", "arremessar" or "atirar" to express the meaning of "to throw", you're not going to make any mistake. However, if you want to express the meaning of "to fire", you have to use the verb "atirar". :)