YouTube- 007 Airsoft - SRC Gen III M4 Gearbox Review
Shimming was done properly, in fact, exactly how I would have done them. Without the cylinder and spring assembly, the gears can spin freely even with a tightly closed gear box. There is only very minor play for the each gear axle between the corresponding bearings.
Back side of cylinder head is lined with a rubber pad to absorb impact from piston head