“GBB” type, flat hopup rubber, 4 hardness set (50°/60°/70°/80°)   Initially designed for the TAC-41 “GBB” hopup, this hopup rubber can fit inside most of airsoft guns using “GBB” hopup rubber. Transparent silicone for easier installation, this kit includes 4 hardness:   – blue color, 50° – green color, 60° – yellow color, 70° – red color, 80°