UAC High Speed Recoil Spring For Hi-capa was specially designed and come with two type of spring in the package. They are both combine feature of a fast respond action, short recoil cycle but gentle recoil to let you have a better control of your gun. It shorten the time for your gun back to the orginal aiming point and ready to make the second shot. *PS: Please note: installing parts / assemble /fitting airsoft gun will need intermediate knowledge; for expert or airsoft gunsmith. Please refer to your airsoft gunsmith for proper installation / fitting. Fitting may be needed as the condition of each airsoft gun might be different.