Let's clear a few things up. First, equating your experience with moon clip use with a .45 ACP revolver to using them with an 8-shot .38/.357 caliber gun is comparing apples to oranges.
With six large holes and a cartridge that is short and round-nosed, the .45 practically loads itself, and cartridge wobble in the clip actually facilitates the loading process.
OTOH, with a cylinder of the same diameter, but now with eight smaller holes, any cartridge wobble in the clip will be working against you in trying to accomplish a quick reload. Added to that is that the longer the cartridge the more that wobble is exaggerated - this is why people competing with 8-shot 627/327's generally use short cartridge variations like .38 Long Colt and .38 Short Colt.
Secondly, as some have already pointed out, the dimensions of the groove in front of the rim, on a rimmed revolver cartridge, is left up to the manufacturer - there are no SAAMI specs like there are for the extractor groove on rimless cartridges. This complicates getting the proper cartridge to clip fit - which for the 8-shot guns you want as little cartridge wobble as possible. This means trial & error fitment tests with different headstamps and moon clips. Yes, most clip manufacturers can give you initial guidance on what clips work best with what brands of brass - but that in no way assures an optimum fit - it is a starting point at best.
Once you have found that optimum combination you now need to lay in a supply of the correct clips and brass and keep them segregated. (All this is pretty much irrelevant to .45 ACP guns as they will work well with most anything).
Third: In just about any thread that involves moon clips, inevitably somebody brings up the use of the RIMZ polymer moon clips and how great they are as they do not require the use of tools. All well and good, but RIMZ clips are only available for certain guns and not always what you would expect. Need a polymer clip for your 646? How about your 610? No problem. How about any .38/.357 N or L frame? Sorry, not available. I will throw in here that RIMZ is making clips for the 929 and they are stellar. RIMZ got their start making clips for the 25/325/625 family of guns and they have a couple variations to fit those guns.
Fourth: That leaves us using metal clips for the 627/327 guns, which means we need the proper tool(s) to optimize our enjoyment of the gun. Certainly at the top of that list is the BMT tool - the only tool you'll ever need to load and unload your clips. IN the alternative, there is the Original Moon Clip Tool (and the copies and clones out there). Slower, as it is a one cartridge at a time proposition, but it works well and has the added advantage of being usable for various calibers by simply changing the arbor, whereas the BMT is cartridge/gun specific. The Original Moon Clip Tool bills itself as both a loader & unloader, and, indeed, unloading can be accomplished with the use of the loops on the end of the tool. I have found that it is more expedient to purchase, separately, one of the screwdriver handle type unloaders, from sources like Brownells or Midway - or make one from some copper pipe or a piece of golf club shaft. There are other tools out there, but these account for the majority in use by competition shooters.
Which brings me to my fifth and final point (big sigh) - all of the above is moot if your gun is just a range toy - where the speed with which you reload is irrelevant. Then moon clips are just a convenience. Sit at home and load up your clips in front of the TV, saving you range time. However, if you endeavor to compete in any gun game that requires loading "on the clock" then moon clips are the way to go.
A lot of the above can already be found in some of the individual posts above - I just endeavored to put them in one cohesive post.
Adios,
Pizza Bob
