I Awakened an Invincible SSS-Rank Gatling Gun

Ten years after monsters overran the world, Ethan has no serum, no rank, and only a sister who risked her life to buy him one chance. When a military heir spills that serum, Ethan slashes his own hand to awaken it - and an SSS-rank Doomsday Gatling answers, turning every sneer on the training ground into a countdown.