Aoharu × Machine Gun (2012)

Blurb:
Hotaru Tachibana is a girl (though all too frequently mistaken for a boy!) with a deep sense of justice. When a classmate informs Hotaru that she was bilked out of her cash by a shady host at a host club, Hotaru immediately rushes to confront the villain only to discover that the con man in question is her new neighbor! Worse, he proposes to settle their feud with firearms! Okay, it turns out that they’re just toys, but when Hotaru is soundly defeated, she finds herself sucked into the world of survival games. Is this new world one she can actually escape??

Source: www.anisearch.com/manga/19921