Tsukumogami: Teito Jouhoubu Tokken Chohouin 007-shiki

The story of the novel series revolves around female middle school student Chiyoko, who is part of her school's naginata club. In her house sleeps a single revolver. That gun is actually able to transform into human form, and in the past, he was created to be an assassin for the imperial city's information bureau. Chiyoko learns of the revolver, whose name is Zerozero Nanashiki (007-shki), and she teams up with him to defeat the halberd tsukumogami Reishiki (0-shiki), who has been killing people for the past few hundred years.

(Source: Anime News Network)