Aoharu X Machinegun. Vol. 1 - Naoe

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??