Overbore cartridges are those with a relatively large case volume or case capacity, coupled with a relatively small diameter bullet.
The case volume or case capacity and barrel bore area can be mathematically related to obtain a case volume to bore area ratio in metric or imperial units.
where:
The higher the O<sub>ratio</sub> result, the more overbore a cartridge will be. As the ratio is expressed in units of length, relatively high O<sub>ratio</sub> is a good predictor of suitability for relatively long barreled guns.
O<sub>ratio</sub> is also used to predict barrel life in cartridges of the same caliber, but not of different calibres, since the ratio is an extensive quantity that does not correlate to temperature or pressure (e.g. a .50 caliber straight-wall cartridge (.500 Bushwhacker) may have the same overbore as a necked down .17 caliber bottleneck cartridge (.17 Remington)).
The bore cross section areas "Q" used in the calculations were taken from the appropriate C.I.P. data sheets.
The intermediate cartridges .30 Carbine, 7.92ÃÂ33mm Kurz, 7.62ÃÂ39mm, 7.62ÃÂ45mm, 5.45ÃÂ39mm, .223 Remington/5.56ÃÂ45mm NATO and 5.8ÃÂ42mm stand out as having relatively low sub 8 O<sub>ratio's</sub>.