Upotte's portrayal of the Finnish RK-95 cold weather rifle, a derivative of the AK-47/AKM.

RK-95 is known exclusively by the name sako. She is fairly tall, looks older than she is, blonde haired, red eyed and has very long elflike ears. 

Sako is a self-worshiping sadist, who throughout the manga bullies, tortures and even attempts to kill other members of her class. This... Unique behavior has made her quite popular in fandom circles. 
sako rk 95 (upotte!!)