Uchinukareta Senjou wa, Soko de Kieteiro

For a hundred years, East and West have waged a ferocious war with everything from tanks to witchcraft. On this savage battlefield, one young soldier from the East uses a bullet he's never seen before and kills an enemy officer with it. After this, nothing will ever be the same for the boy, Rain Lantz. Fate soon leads him to the mysterious bullet's maker, a young girl named Air, who warns him that anyone struck by the daemon's bullet he fired will disappear from the world as if they had never existed! Armed with the staggering knowledge that every shot he takes will change history and destiny, where will Rain aim next?!

(Source: Yen Press)