Colt M1911 .45 Semi-Automatic Pistol
The Colt M1911 .45 Semi-Automatic Pistol was originally given to Yoshimi Yahagi as her weapon. When she met up with her boyfriend, Yoji Kuramoto, she gave it to him as she thought he would protect them better with her weapon than his(which was a knife). After a brief fight between the two, Mitsuko Souma got her hands on the weapon and murdered the couple.
After Hiroki's death, Kazuo Kiriyama would take the gun to aid in his massacre of his classmates.