Magicannon Girl Yongou-chan

Manaka is good at archery, but she hates it because her father forced her into it and the other members of the high school archery club hate her for being better than them.
While she’s feeling down, she gets caught up in the battle between a gun-wielding woman and a tank, after which she succeeds the woman’s magicannon, a device that lets her turn into Yongou, the personification of a WW2-era tank.
With her newfound meaning of life, Manaka and the new friends she meets take on the Nine Old Guns from an organisation that may be striving for world peace or world destruction.

Related: http://www.mangago.me/read-manga/narue-no-sekai/