Early in the movie, when Issac is removing blocks from the wall to make an opening to look through with his scope, the blocks fall and damage his hand, specifically his right index, or "trigger finger". That finger is shown bloody and misshapen, possibly broken, for a vast majority of the remainder of the movie. At the end, he is able to take a shot at the enemy sniper using that same finger and it is shown to be uninjured. Precise trigger control is critically important to making accurate shots, particularly for a sniper. Making a long range shot like that with an injured or broken finger is highly unlikely.
