The extractor groove diameter on cartridge cases is the most important part of fitment into a moonclip

There is no SAAMI specifications for the extractor groove and each manufacturer makes them as they want. The following list gives you a great example. This is why the diameter size of the brass is critical to proper loading, unloading and fitment. Cartridge diameter too big and moonclip too small causes the moonclip to bend. The cartridge should rotate within their respective place and wobble slightly when all cartridges are loaded.

Brand of Ammo/Brass .38/357 Groove Diameter
Armscor .365
Starline .366
Fiocchi .368
W-W Brass .372
PMC Brass .372
Rem Golden Saber Nkl .374
Speer Blazer Brass .374
Remington Brass .374
Federal Brass .374
Hornady Nkl .375
This photo will give you an idea of what we mean.