A rim is an external flange that is machined, cast, molded, stamped, or pressed around the bottom of a firearms cartridge. Thus, rimmed cartridges are sometimes called "flanged" cartridges. Almost all cartridges feature an extractor or headspacing rim, in spite of the fact that some cartridges are known as "rimless cartridges". The rim may serve a number of purposes, including providing a lip for the extractor to engage, and sometimes serving to headspace the cartridge.

There are various types of firearms rims in use in modern ammunition. The main types are categorized as rimmed, rimless, semi-rimmed, rebated, and belted. These describe the size of the rim in relation to the base of the case.

The rimmed cartridge, sometimes called flanged cartridge, is the oldest of the types and has a rim that is significantly larger in diameter than the base of the cartridge. Rimmed cartridges use the rim to hold the (usually straight sided) cartridge in the chamber of the firearm, with the rim serving to hold the cartridge at the proper depth in the chamber—this function is called "headspacing". Because the rimmed cartridge headspaces on the rim, the case length is of less importance than with rimless cartridges. Rimmed cartridges with straight walls, such as shotgun shells, allow various lengths of the same cartridge to be chambered in the firearm; if the round protrudes into the rifling of the barrel past the length of the chamber, the rifling can act as a fluted chamber to ease extraction.

This allows some firearms chambered for similar rimmed cartridges to safely chamber and fire shorter cartridges, such as using .38 Special cartridges in a .357 Magnum revolver, as these are the same diameter despite the nomenclature. Rimmed cartridges are well suited for certain types of actions, such as revolvers and break-action firearms, where the rim helps hold the cartridge in position.

Rimmed cartridges generally do not work quite as well in firearms that feed from a box magazine, since the magazine must be carefully loaded so that the rim from each successive case is loaded ahead of the round beneath it, so the round will not snag on the rim of the cartridge below it as the bolt strips it out of the magazine. However, box magazine firearms firing rimmed cases have seen extensive use; the famous Lee–Enfield rifle used by the UK, and the Russian Mosin-Nagant rifle which uses an interrupter to help prevent the snags, were both used from before World War I until after World War II. Semi-automatic handguns have been chambered in rimmed cartridges as well, for example a LAR Grizzly or Desert Eagle in .357 Magnum or .44 Magnum. The .22 Long Rifle is extremely popular in semi-automatics.

Rimmed cartridges work with belt-fed machine guns that use a two stage pull out – push through feeding operation, notably the Maxim gun, Vickers, M1919 Browning and M2HB. Push through links are possible with rimmed cartridges using specially designed belt links, but not as reliable as using push through links with rimless ammunition.

Rimfire cartridges also use the rim to contain the priming compound to ignite the cartridge instead of a centrally mounted primer, as is commonly used in centerfire cartridges.

Under the metric cartridge designation system, a capitalized "R" added at the end of the designation denotes a rimmed cartridge. For example, "7.62×54mmR" is a rimmed cartridge, while "7.62×51mm" is a rimless cartridge. Under imperial or US customary designations, there is typically no distinction between rimmed and unrimmed cartridges, unless one is referring to a rimmed version of a cartridge which is typically rimless, such as the .45 Auto Rim, a special rimmed version of the .45 ACP ("Automatic Colt Pistol" aka ".45 Auto"), intended for use in M1917 service revolvers.