Boom, Headshot

Ethan Cole dies and is reincarnated into a popular fantasy comic he loved, two years before its main story begins. Now living as Kaien Arclight, he uses his knowledge of the original story and his unique ability to summon and enhance firearms to protect the characters he cared about—while secretly becoming Gluttony, a member of the mysterious Veiled Crown. But the more Kaien interferes, the more the world begins to change. — Description from novelcodex.org