If you by chance played Resistance 1 and/or 2, you will be very familiar with how the Auger works, it isn't much of a complicated weapon, especially in this game as they teach you how to use it and force you to pick it up.
All you do when you have ammo for it is hide behind an object close to enemies/be close to enemies who are behind objects, hold
to look down your scope and you should see their body highlighted even though there's an object in the way, so all you do is just shoot at them, and by your 25th kill you should easily have it.
This is also an easy method to just survive in the game, because it saves you sticking your head out in the open to kill.