The most noticeable technique in "The Sniper" is the surprise ending. This also goes by the name of the "O. Henry ending" since O. Henry was known to use surprise endings in his short fiction. In the story, the sniper is a Republican soldier fighting in the Irish Civil War. In the process of engaging with an enemy sniper, he kills a woman and another soldier in an armored car. The sniper tricks his enemy and is able to kill him. When he goes to the street to see his enemy's dead body, he looks "into his brother's face." This revelation is certainly a surprise, especially if it is taken literally. Even figuratively speaking, the sniper could feel like he had killed his brother since civil war is often called a battle of brother against brother. 

The other term that describes this revelation is anagnorisis. This is a Greek term that means "recognition." A famous example of this is in Oedipus Rex when the main character, Oedipus, realizes he's fulfilled a prophecy by killing his father and marrying his mother. The sniper has a dramatic moment of recognition when he realizes he's killed his brother.