On a rimless case, the rim is the same diameter as the base of the case; it is known as an extractor groove. Since there is no rim projecting past the edge of the case, the cartridge must headspace on the case neck, for a straight walled case, or on the shoulder of the case for a bottlenecked case; the extractor groove serves only for extraction. The lack of a projecting rim makes rimless cases feed very smoothly from box magazines, and they are primarily used in firearms that feed from a box magazine. Rimless cases are not well suited to break-open and revolver actions, though they can be used with appropriate modifications, such as a spring-loaded extractor or, in a revolver, a moon clip.
Rimless straight walled cases are problematic in applications such as magnum revolvers and break-open single shot firearms, where headspacing off the case mouth prevents an aggressive crimp to hold the bullet in place against the heavy recoil of firing.
Examples of rimless handgun cartridges include the 9 mm Para, .40 S&W, and .45 ACP. Rimless rifle examples include the .308 Winchester, .223 Remington, and .30-06 Springfield.