@M3Stuart - TK Custom lists several options for moon clips for the 642, dependent on the brass you're using. Expensive, but not as expensive as you quote. They appear to be $6.50 ea.
Revolver Moon Clips, 45 ACP Full Moon Clips and Speed Loaders - TK Custom
@IraIII - While the 625, 646, 940 and now the 929 and 986 can be shot without moon clips, it is not advisable unless you are just fooling around at the range, or it's a dire emergency and you have no other choice.
These are all chambered for rimless pistol rounds, so they headspace on the moon clip. Without a moon clip they have to rely on the shoulder in the chamber and headspace on the case mouth. Look at the specs for case length of a given cartridge and then measure a piece of once-fired or new brass. You will find the manufacturers make their brass shorter than max length. Max length is where the shoulder in the chamber will be cut so seldom does the case mouth actually come in contact with that shoulder. When the hammer hits a round, it drives it forward in the chamber. It may go off, it may not. So you may ask, "How do these calibers function in a semi-auto, as they rely on the shoulder in the chamber and headspace on the case mouth?" That's theoretical - in point of fact most semi-autos headspace off the extractor, which holds the cartridge against the breech face as it is being struck by the firing pin.
The other downside to using a pistol caliber in a revolver without moon clips, is extraction. Since there is no rim for the extractor star to push against, empties (those that don't fall out from gravity) must be poked out one at a time with a rod. Not exactly conducive to fast shooting. It was disingenuous of the S&W rep to say that it was, "...designed to work without moon clips." That simply means they didn't bore the cylinder straight through.
@epj - re: Bore diameter. I've seen this question come up often on many forums. There was one video of Jerry Miculek, taken at the SHOT show in Smith's booth, where he categorically states that the 929 (and one would then assume the 986) has a bore sized for .355" bullets. There was a lot of concern about this in the competition community. Why spend >$1K on a gun that is not made to be optimally accurate (i.e. a .355" bullet going through a .357" bore).
HTH
Adios,
Pizza Bob