I wasted a ridiculous amount of time running around in circles, reloading earlier saves, and restarting this mission; thinking it was glitching.
In reality, I was missing one randomly-located crate (even more random than the one next to the sniper). So, hopefully this video helps!SIDE NOTE: You don't need the Teufelsfeuer to earn this achievement, but it is undeniably badass, and it might make this a bit easier! If you want it, it's located in a crate on the floor of the gun range (building to your left when you first entered this area).
There are three soldiers firing them on the gun range, one soldier walking along a cat walk above the area, and two soldiers in a room on the upper level.
This video was originally posted via YouTube by gamerfourlife99. I'm not badass enough to create content such as Sniper Elite, and do not claim the fame deserved by the people who do. In short, I can take no credit for the video, its content, etc.