As a way to increase the operating speed of the gas blow back gun, it is absolutely necessary to reduce the weight of the sliding part that moves with gas pressure. The heaviest Hi-CAPA slide-related part is the part that forms the piston. By making the piston parts with a completely different material, it is possible to achieve the most effective weight reduction. By applying a bold lightening process that disregards strength, further weight reduction is achieved. It is an item that is purely obsessed only with weight reduction.