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Genuine KJW rubber gas routers to fit all types of KC-02 magazines and HPA adaptors.
Part #122 as shown in the diagram.
These genuine routers are best, they don't cause any cycling problems by interfering with the nozzle movement.
These occasionally get torn during use which leads to gas leaks causing inconsistent power output. The back edge of the square port rips on both sides as the nozzle catches it if it's dry.
Just wipe them with silicone grease before each game day and it will greatly reduce the risk of tearing them, standard practice.