I don't know for sure about the 642 Pro Series, but I carry a 442 Pro Series which is also cut for moonclips. The 442 does not have the internal lock and it works just fine with or without the moonclips. When the cylinder is machined for the clips, a ridge is left around the outside which allows the rims of the cartridges to have something to bear against for headspacing. If you don't use moonclips, the gun works just like any other normal revolver.