He was a child soldier in his homeland of Galzburg before leaving and joining the Roger Pirates. After leaving the crew, he was imprisoned in Impel Down.
Bullet ate the Gasha Gasha no Mi, a Paramecia-type Devil Fruit that allows him to manipulate and merge objects like iron and weapons at will. With this fruit's power, he can make a large variety of mechanical creations.
(Source: One Piece Wiki)
