This is found only by loading extras -> Shooting range
I found that if you are having trouble with this one, go into the game settings and switch everything from the highest resolution to the lowest to lower the game's speed a few hairs. Anyway, Simply open the door (not the cage in front of you) with the E button and then the second one inside. Walk to the gun near the targets and pick it up (E again) and don't fire it. Head to the table to your right and grab two boxes of ammo so your count now reads 15/30. Now I found the best place to shoot from is to wedge yourself in the right corner of the center box. The furthest right target is out of your reach there but you'll have plenty to shoot at instead. There's a natural lull un the targets going up. That's when you should hit R for reload. Aim for the red centers as they give +3 to points and don't miss and just hit the board like in the head or outside the ring. this takes a point away. If you've no you failed, hit ESC and load up the range again. It took me about 10 tries to get the rhythm down. then I over-scored at 35!