The shims are for the barrel-to-receiver fit. The action itself isn’t adjustable. It’s steel with large pins and seems fairly solid. I know it can be bent if you install the barrel with zero shims, meaning the chamber is too far into the receiver and the bolt has to be forced closed (puts too much stress on the linkage). When adjusted properly it should close very easily and not “snap” into place. Basically, just enough that it stays closed when firing and not more than that. PWS has a spec that the bolt will retain a shim (closed on that shim it cannot be removed w/out opening the action) and it shouldn’t be any tighter than necessary to do that. It’s suggested that you start w/ 4 shims on the barrel, I do believe. That is, if you’re assembling it yourself and not buying a complete rifle, in which case PWS tunes it properly.