Once you reach the area where your "buddies" stand on either side of a door and tell you to find another way around and flank the machine gunner (there will be an open safe in the wall on the left side of the door) look over to the right of the door and you will see a caged area with two stair cases, go down the stairs and in to the shooting range, I suggest you clear out ALL the enemies as they will pop out as you progress through the range (once you reach the room with the moving target no more enemies will pop up inside the range) you can then go back to the beginning and take out the targets, there should be 12 total targets that pop up, the chivo didn't pop for me until I shot the moving target, so if it doesn't pop for you, try that...
as a side note once you reach the end of the range the next enemy that will pop up is to the left side once you reach the top of the stairs
Any problems please refer to the video below. Thanks to Maka91Productions for the video
Hope that helps good luck!!!