[Metalsynth]

First thing that comes to mind is hop-up disaliglignement.

Try taking out the inner barrel and check if the sleeve is aligned properly. I've seen that happen to a friend of mine and I repaired that issue just by twisting the end of the inner barrel ( field repair ) but since you've got time for it, I recommend taking out the whole thing and checking with light how the sleeve reacts when the ajuster pushes down on the sleeve.