Buso Renkin Vol 8

Buso Renkin Vol 8 by Nobuhiro Watsuki. A Shonen manga rated Older Teen, published in English by Viz Media in 2007.

High schooler Kazuki Muto dies trying to save a girl from a monster, then wakes up with an alchemical core in place of his heart that transforms into a weapon. Now he's drafted into a secret war against man-eating homunculi. From the creator of Rurouni Kenshin.