What happened when the sniper threw the revolver down?

The answer to this question can be found in the final paragraphs of this excellent short story. The sniper has just used his revolver to shoot and kill the enemy sniper. He watches the enemy's body fall from the roof to the ground, and the sniper is momentarily revolted by his violence, the war's violence, etc.

Weakened by his wound and the long summer day of fasting and watching on the roof, he revolted from the sight of the shattered mass of his dead enemy. His teeth chattered, he began to gibber to himself, cursing the war, cursing himself, cursing everybody.

The sniper then looks down at the gun in his hand, swears, and angrily throws the gun down at his feet. The gun's impact with the ground causes the weapon to fire off another round that passes very closely to the sniper's head. This serves to jar the sniper back to his senses, and he laughs to himself for having acted so fearfully.

He was frightened back to his senses by the shock. His nerves steadied. The cloud of fear scattered from his mind and he laughed.

Next, the sniper decides to pick up the revolver and return back to his commanding officer to report that everything is quiet; however, the sniper decides to check and see if he knows who the other sniper was. He's horrified to find out the other sniper was his own brother.