From the Menu go to Academy -> Tutorial and proceed until you get to the Firing Range, after you have completed the Firing Range (after using the Frag Grenades) turn around and walk back to the entrance of the Armory, (if facing the door, turn around) You should now see 4 Sniper Rifles for you to pick them up, once picked up the Achievement should pop up!
The Achievement is worth 5G
If you can't find the Sniper Rifle here's a video