Shotgun Boy Manga

A brief description of the manhwa Shotgun Boy:

While running for his life in the woods, bullying victim Gyuhwan stumbles upon a shotgun and just enough shells for his classmates. It’s a recipe for disaster. But fate takes an unexpected turn when he returns to find his classmates under attack by brain-eating creatures. Now, the only thing that stands in the way of their ravenous feast is a boy with a shotgun.