The .303 British or 7.7×56mmR, is a .303-inch (7.7 mm) calibre rimmed tapered rifle cartridge. The .303 inch bore diameter is measured between rifling lands as is the common practice in Europe which follows the traditional black powder convention.
Left to right: .303 British, 6.5×50mmSR Arisaka and .30-06 Springfield ballistic tip ammunition
Longitudinal section of Mk VI ammunition 1904, showing the round nose bullet
Longitudinal section of Mk VII ammunition circa 1915, showing the "tail heavy" design
.303 British Cartridge (Mk VII), manufactured by CAC in 1945
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.
Rimmed .357 Magnum revolver ammunition
Rimmed vs Rimless cartridges
Rimless 9mm Parabellum pistol cartridges