The Scarab Gun is the child of the BFG9000. In the factories where they clean feces out of flushed water, they found 3 Plasma Rifles, they tried to shoot it and it blew up the capital of EH1200M (Some country where Forerunners lived, they had Russian accents). The three Plasma Rifles skyrocketed (literally) and travelled through space. They came to earth, suprisingly, they did not burn in re-entry (That is why Scarab Guns do not overheat), they landed on buildings in Africa, New Momlikeabossa to be exact.