–SAT Sniper, missing Lucy and showing that even the best weapon is only as steady as the hand that holds it
The Accuracy International Arctic Warfare Sniper Rifle is an 7.62x51mm NATO rifle. It is noted both for its implied accuracy and its overall reliability in both normal and extreme environmental conditions.
In the series
The rifle is used by the S.A.T. sniper in Episode 4, when he attempted to shoot Lucy, but misses.
Arguably, this near-miss (possibly caused by heightened nerves when facing a killing machine like Lucy) saves Nana's life, taken as it was just prior to Lucy finishing her. Distracted, Lucy also does not notice when Nana then disables Lucy's vectors, leaving her open to a seemingly suicidal punch landed on her by a vengeful Kurama. Since many even more powerful weapons in Elfen Lied were perhaps not even a brief distraction to Lucy, happenstance causes it to have a huge upgrade in-series.
Since the SAT in theory would have to deploy at a moment's notice to any section of Japan, from the salt air of Kamakura to the sometimes-frigid areas of Hokkaido, this weapon would make great sense for a soldier to use. Add in the Diclonius, and it becomes a must.