The term belted magnum^{[1][better source needed]} or belted case refers to any cartridge, but generally a rifle cartridge, with a shell casing that has a pronounced "belt" around its base that continues 2–4 mm (0.079–0.157 in) past the extractor groove.

This design originated with the British gunmaker Holland & Holland for the purpose of headspacing certain more powerful cartridges. Non-shouldered (non-"bottlenecked") magnum rifle cartridges especially could be pushed too far into the chamber and thus cause catastrophic failure of the gun when fired with excessive headspace. The addition of the belt to the casing prevented over-insertion, while allowing smoother feeding from a box magazine compared to a rimmed cartridge. An example of an American adaption of this practice is seen in cartridges like the .458 Winchester Magnum, a straight-walled cartridge with a belt.

Many subsequent cartridges of "magnum" nomenclature were based on the original .375 H&H Magnum cartridge, so over time the belt became something of a standardized attribute, expected as part of a "magnum" cartridge. Many cartridge designs of the last century include this belt, but do not really require it.

Since the beginning of the 21st century, there has been an ever growing trend toward designing beltless magnum cartridges, virtually all of which are heavily shouldered designs that obviate the original motivation for a belt. The last belted magnum to be introduced in the market was Weatherby's 6.5-300 Weatherby Magnum, commercially available since 2016.^[2]

Nevertheless, the most popular magnum cartridge remain those with a belted case, such as the .300 Winchester Magnum, the .375 Holland & Holland Magnum and the 7mm Remington Magnum.^[3]