Getting a head shot depends on the enemy you are fighting. If it is an enemy with a 9mm, he will stop shooting a few times to reload. You can take him out at this point. An enemy with a machine gun will just keep moving -- just kill him. Big enemies will not move that much, depending on what happens to them. If you shoot him and miss, he will run to a different location, stop, and shoot at you. It is easy to take him out with a perfect shot to the head.
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