If You Are a Mortal, You Use Barrett to Snipe the Demon King?

My name is Lin Mu, and I traveled to a Dai Yue dynasty where demon kings were rampant. At the beginning, there is a pile of dead people, surrounded by monster soldiers eating people. Good news, I have awakened the [Fire Weapon System]. Bad news, only 30 rounds. I used this sand eagle to defeat all the demon soldiers who were chasing me. Until the second-level demon king came and pinched me like a chicken. The bullets were gone, but I knew that its eyes were its weak point. Bang! The demon king's right eye exploded! Lord Mengchuan happened to be passing by and saved my life. He gave me the demon pill. The system said that by turning in the demon pill, you can unlock stronger weapons. Barrett, Gatling... I feel like I can open up my arsenal in this demon king's world.