Didn't mention the number of hands provided; spoilt for choice.
Here's the wiki synopsis of the OVA
"While each piece of media takes place in its own universe, they each focus around Black Rock Shooter, a mysterious black haired girl who possesses a burning blue eye and a powerful cannon that can shoot rocks at high speed.
The OVA focuses on a girl named Mato Kuroi, who befriends another girl, Yomi Takanashi, upon entering school. As the two start to drift apart, with Yomi becoming jealous of Mato's friendship with Yuu Kotari, Yomi suddenly disappears. As Mato searches for her, she is taken to a strange world where she meets the mysterious Black Rock Shooter, who merges with her and helps her search for Yomi, fighting off the evil Dead Master who possessed her.
The television series uses the same characters from the OVA in a similar but slightly different storyline. As Mato enters middle school and becomes friends with Yomi, she soon faces personal troubles and the influence caused by another world where Black Rock Shooter fights other girls."
Mato using her black blade as a side arm other than the Rock Canon which was featured more extensively in the previous entry.