How does a single action revolver (blackhawk) hold a rimless cartridge like 45 acp, 30 carbine or 9mm?

If these cartridges are rimless, how come the cartridge doesnt fall through the cylinder bore? If a single action revolver can hold a rimless cartridge, why cant a double action revolver? I realize the extraction is easier with a single action but I'm refering only to the cylinder holding the round.
Update: Yes I know about moon clips. So the moon clip is needed strictly for the extraction of the spent cases and not to hold the round in place? I did not know there is a ridge inside the bore that the cartridge "Head spaces" on. I have heard that term before but didnt know what it means.
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