Some weak guy finds himself in the center of gunfights between students after entering high school. In both series the story makes equally little sense, but if you did liked one, the other should prove interesting too.
Your average high school boy gets involved with gun welding high school girls and has no idea what's going on. Brief explanations from the girls doesn't give much enlightenment in that matter either. But the boy is not the only one who stays in the dark, you, the reader are given little more information on what is actually going on.
Sounds interesting? No? Too bad. But if the answer to the previous question would be yes, than you ought to read Parabellum and the other series with the long title.