Here is a very useful video guide that I found on Youtube. Follow exactly what he does in the video and the achievement is yours.
All credit/copyrights goes to John Stewart and his channel for this video guide. *UPDATE - October 24th, 2014*
"Just wanted to add that there is a gewehr rifle in the locked storeroom under the observation deck. I got access to that room once the officer was dead. I set traps, entered the SN and then made a manual save. Shoot the red light, then 14 targets. I tried with the carcano but the reload was causing me to miss targets." - vetteindreams77