From MangaHelpers: 1x1t5l

Kyouji is a cop in Japan until one terrible day, when his hesitation to fire his gun allows a spree killer to murder 8 people and injure more. When Kyouji is recovered from his physical injuries, he moves to Los Angeles. He is barely there and settled before he is present at a shop where a man is murdered by a gun-toting gang. This time, when faced with violence, he manages to save the man's young son, Christian Sanchez.

The gang is determined to hunt down the child, but Kyouji is just as determined to protect him. He goes to a sketchy local Japanese man, Hateruma Takeru, to acquire the weapons and knowledge he needs to protect Chris. Maybe this was why his life was spared on that deadly day in Japan! [tethydust]