3 different sizes of O-Ring for our B01 Dynamic Blowback Housing which can perfect fit for most common used nozzle on the market. These O-Ring is anti-oil and reduce the fiction during cycling. *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. Regular maintenance is needed for all airsoft guns in order to keep it in good working condition and reduce wear out issue on parts.