This 6063 aluminium hop-up chamber is made by precision CNC machines.  It fits M4(T.Marui) series GBB airsoft gun, and it is specially made for professional shooter and competition needs, because:
1) High precision dimension increases shooting efficiency and reliability;
2) Smooth surface finishing improves smooth loading BB;
3) The large O-ring is modified for holding the hop-up chamber as a composite unit, and it also keeps the hop-up chamber and inner barrel to coaxial with the upper receiver;
4)  Dimension is modified for increasing hop-up performance and shooting accuracy.  Therefore, it has difference dimension from the original T.Marui hop-up chamber.