Forgive my ignorance here.... Will someone please explain what moon clips are and their function? Is there an advantage to using them in any way? I'm about to buy my first revolver and would like to know a few things before I purchase...I've checked out a few and so far,I'm pretty set on a S&W 640 stainless... One of the 640's I've checked into takes moon clips...The other slightly less expensive one doesn't...I have several semi autos but I'm not that familiar yet with wheel guns...Any info appreciated...