The Base of the Case
The case head is the solid base of a cartridge case, and the rim at its edge is the feature the firearm's extractor grips to pull the case from the chamber. The head also carries the primer pocket and headstamp. Because the rim style governs how a round feeds, seats, and extracts, its geometry is held to tight dimensional standards.
Rimmed Cases
A rimmed case has a rim noticeably wider than the case body, forming a protruding flange. That flange gives an extractor an easy, positive grip and lets the case hang against the rear of the chamber. Rimmed designs are classic in revolvers and many break-action and lever guns, though the protruding rim can complicate stacking rounds in a box magazine.
Rimless Cases
A rimless case is not truly without a rim; its rim is machined to the same diameter as the body, with an extractor groove cut just ahead of it. The extractor snaps into that groove. This flush profile stacks cleanly in magazines, which is why rimless cases dominate semi-automatic pistols and rifles like the 9x19 and .308.
Rebated Rim Cases
A rebated rim is narrower than the case body, so the body bulges wider than the rim itself. This lets a large-bodied cartridge use a bolt face or extractor sized for a smaller rim. The trade-off is that the small rim can make reliable feeding and extraction more demanding to engineer.
Belted and Semi-Rimmed Variants
Two intermediate styles exist. A belted case adds a raised band just ahead of the extractor groove, historically used on high-pressure magnum rounds to control headspace. A semi-rimmed case has a rim only slightly wider than the body plus an extractor groove, blending traits of rimmed and rimless designs.
Why Head and Rim Are Standardized
The head and rim are the mechanical interface between cartridge and mechanism, so the bolt face, extractor, and ejector are all cut to match a specified rim diameter and groove. Holding these to defined tolerances is what lets ammunition from many makers function in the same firearm without hand-fitting.
Worked Example
Contrast the .38 Special, a rimmed revolver cartridge, with the 9x19, a rimless pistol cartridge. The .38 Special's wide flange rests on the rear of a revolver cylinder and is grasped by a star extractor that pushes all cases out at once. The 9x19's flush rim and machined groove let rounds stack in a magazine, and the pistol's extractor rides in that groove to strip and eject each case.
A Common Misconception
Many people read "rimless" as meaning the case has no rim at all. It does have a rim; it is simply cut flush with the body and paired with an extractor groove. The name describes the absence of a protruding flange, not the absence of a rim.