Historically, most revolvers used .45, .357, 0r .38 caliber ammunition. The .9mm round gained popularity along with the .9mm automatics in the past 50 years or so. Today, several high-quality revolver options use .9mm ammunition.
Rimmed vs. Rimless Ammunition
Revolvers traditionally fire rimmed cartridges, which headspace on the case rim. Headspace refers to the distance between the breech face and the point in the chamber on which the cartridge case seats, stopping its forward movement.
The revolver rim also functions as the extractor flange, providing a surface for the extractor star to impinge against for unloading. Rimless pistol cartridges, such as the 9mm Luger, headspace on the case mouth, and the rim is the same diameter as the case body.
To load and unload rimless ammunition in a revolver cylinder, the use of full- and half-moon clips is generally necessary. Moon clips are ring- or star-like accessories that snap into the extracting grooves of rimless pistol cartridges. Instead of a revolver rim, the weapons use the grooves.