Project ARMS Vol 6

From the VIZ Media edition

Ryo, Hayato, Takeshi, and Kei are Arms - kids implanted with nanotech limb enhancements that cause them to transform into grotesquely deformed and out of control superweapons. As they attempt to uncover the mysteries of their past, the latest group of superagents to come after them are the chillingly cruel Red Caps with their most deadly strategy - the vicious "snark hunt." Desperate and chased to the brink of death, Hayato Shingu undergoes a transformation of sublime proportions.

Source: Anime News Network Encyclopedia