Headspace "cartridge rim-to-recoil shield clearance", barrel-to-cylinder gap, and cylinder endshake are all related. Change one, and the other two will also change.

At the time the revolver is fitted, the extractor hub is ground to provide headspace within specifications. The cylinder endshake is regulated to specification, and the barrel is fitted to provide b-c gap to specification.

Any after-market changes (adding endshake washers, replacing barrel, fitting replacement cylinder or extractor) must be done properly to make the revolver function correctly.