When I owned several USGI M14's back in the mid-80's, we discovered that all the gas cylinder assemblys were loose as heck. The USAMTU shooters in Camp Perry solved this rattling problem by drill / tapping / screwing the front band on to the gas cylinder. The US Marines welded (tack'ed) the front band on. Lots cheaper. Then both units would shim the gas cylinder in place.
This rapidly tightened up your groups by reducing the vibrations and vibration pattern into a more predictable and consistent tuning fork-like node/anti-node resemblance.
Most of the Norinco gas cylinders are pretty tight to begin with. This advantage of buying a Nork M14 is quite surprising. BOTH my M14's were very tight needing maybe a 3 thou shim. My SA and Winchester M14's needed between 5 to 15 thou of shims. Wowsers !
Enjoy your Norinco M14 !!
Cheers,
Barney