What you are calling a "shim" is a barrel collar, and it should be a bit loose on the barrel. I haven't seen one that isn't.
I haven't taken a lot of K11 rifles apart, but all that I have, the "barrel collar" is loose and stopped from moving back on the barrel by a flange, coming up against the front of the fore end
There is also a "barrel collar" that is held in place with a set screw, but it's not a shim. I take it you mean the small silver colored bit, directly behind the front site that slides back and forth on the barrel?