Yeopchongsonyeon

While running for his life in the woods, bullying victim Gyu-Hwan 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.
(Source: WEBTOONS)
Note: Chapter count includes a prologue and one extra.