At the end of chapter 13, why did Ishmael have no problem shooting his gun?

The army returns to their village base at nightfall. While sleeping Beah has a nightmare that as he is picking Josiah up from the tree trunk, a gunman accosts him and points his rifle at Beah’s head. Beah awakens from his nightmare firing his gun inside the tent, emptying the magazine. He notes that from that point on, he had no problem shooting his gun.