When you reach the shooting range in
Chapter 14, you will gain access to challenge
4-C which includes multiple lines of pirates. Simply use the rifle (
) when you see them line up and position yourself on the left, right, or center so you have a clear shot down the line and aim for the head (will also hit the barrels if they are up).
Jump to time
15:28 in this video for a full breakdown of 4-C.