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.

= \frac

where: *

Volume_

= the cartridge case internal volume or case capacity (in ml or (for non-metric users) grains of water) *

Area_

= barrel bore cross section area (in cm² or in²) The higher the O_ratio result, the more overbore a cartridge will be. As the ratio is expressed in units of length, relatively high O_ratio is a good predictor of suitability for relatively long barreled guns. O_ratio 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 cal straight cartridge may have the same overbore as a highly necked down .17 cal cartridge).

Comparative index for various rifle cartridges

{, class="wikitable sortable" , - ! , Chambering , , Case capacity (ml / cm³) , , A_bore (cm²) , , O_ratio (cm) , - , .30 Carbine , , 1.36 , , 0.4740 , , 2.87 , - , 7.92×33mm Kurz , , 2.22 , , 0.5178 , , 4.29 , - , 7.62×39mm , , 2.31 , , 0.4799 , , 4.81 , - , 7.62×45mm , , 2.79 , , 0.4749 , , 5.87 , - , 5.45×39mm , , 1.75 , , 0.2399 , , 7.29 , - , .223 Remington / 5.56×45mm NATO , , 1.87 , , 0.2503 , , 7.47 , - , .303 British , , 3.64 , , 0.4807 , , 7.57 , - , 9.3×62mm , , 5.06 , , 0.6632 , , 7.63 , - , 7.65×53mm Mauser , , 3.70 , , 0.4836 , , 7.65 , - , .308 Winchester /

7.62×51mm NATO The 7.62×51mm NATO (official NATO nomenclature 7.62 NATO) is a rimless, straight walled, bottlenecked, centerfire rifle cartridge. It is a standard for small arms among NATO countries. First developed in the 1950s, the cartridge had first be ...

, , 3.64 , , 0.4751 , , 7.66 , - , 7.92×57mm Mauser , , 4.09 , , 0.5178 , , 7.90 , - , 7.5×54mm French , , 3.76 , , 0.4717 , , 7.97 , - ,

5.8×42mm The 5.8×42mm / DBP87 ( zh, p=Dàn, Bùqiāng, Pŭtōng, 87, s=弹，步枪，普通 87, ) is a military bottlenecked intermediate cartridge developed in the People's Republic of China. There is limited information on this cartridge, although ...

, , 2.11 , , 0.2642 , , 7.99 , - ,

8mm Lebel 8 mm may refer to: Film technology *8 mm film, a motion picture film format ** Super 8 film ** Single-8 film * 8 mm video format, three related videocassette formats Firearms * 8 mm caliber, firearmm cartridges ** 7.92×57mm Mauser, designated ...

, , 4.28 , , 0.5240 , , 8.16 , - , 6mm Norma BR , , 2.45 , , 0.2952 , , 8.30 , - ,

9.3×64mm Brenneke The 9.3×64mm Brenneke (designated as the 9,3 x 64 Brenneke by the C.I.P.) is a rimless bottlenecked centerfire rifle cartridge designed in 1927 by German gunmaker Wilhelm Brenneke. It is suitable for hunting medium to large game animals in A ...

, , 5.71 , , 0.6632 , , 8.61 , - , 7.62×54mmR , , 4.16 , , 0.4799 , , 8.67 , - , .375 Holland & Holland Magnum , , 6.18 , , 0.7016 , , 8.81 , - , 6.5×47mm Lapua , , 3.11 , , 0.3459 , , 8.99 , - , 7.5×55mm Swiss GP 11 , , 4.22 , , 0.4633 , , 9.11 , - , .30-06 Springfield , , 4.42 , , 0.4755 , , 9.30 , - , 7×57mm , , 3.90 , , 0.4041 , , 9.65 , - , 6.5mm Creedmoor , , 3.40 , , 0.3466 , , 9.81 , - , .300 Winchester Short Magnum , , 4.99 , , 0.4752 , , 10.50 , - ,

8×68mm S The 8×68mm S Rim (firearms), rebated rim bottlenecked centerfire rifle Cartridge (firearms), cartridge was developed in the 1930s by August Schüler of the ''August Schüler Waffenfabrik, Suhl, Germany'' as a magnum hunting cartridge that would ...

, , 5.58 , , 0.5178 , , 10.78 , - , 6.5×55mm , , 3.75 , , 0.3436 , , 10.91 , - , 7×64mm , , 4.48 , , 0.4029 , , 11.12 , - , .243 Winchester , , 3.51 , , 0.2925 , , 12.00 , - , .338 Norma Magnum , , 6.95 , , 0.5689 , , 12.22 , - , .300 Winchester Magnum , , 5.93 , , 0.4732 , , 12.53 , - , .338 Lapua Magnum , , 7.40 , , 0.5686 , , 13.01 , - , 7mm Remington Magnum , , 5.31 , , 0.4039 , , 13.15 , - , .300 Norma Magnum , , 6.75 , , 0.4751 , , 14.21 , - , .50 BMG , , 18.97 , , 1.2963 , , 14.63 , - , .300 Remington Ultra Magnum , , 7.14 , , 0.4740 , , 15.06 , - , .300 Lapua Magnum , , 7.34 , , 0.4751 , , 15.45 , - , .257 Weatherby Magnum , , 5.45 , , 0.3307 , , 16.48 , - , .30-378 Weatherby Magnum , , 8.64 , , 0.4748 , , 18.20 , - , 6.5-300 Weatherby Magnum , , 6.36 , , 0.3452 , , 18.42 , - , 14.5×114mm , , 42.53 , , 1.6513 , , 25.76 The bore cross section areas "Q" used in the calculations were taken from the appropriate C.I.P. data sheets. The

intermediate cartridge An intermediate cartridge is a rifle/ carbine cartridge that has significantly greater power than a pistol cartridge but still has a reduced muzzle energy compared to fully powered cartridges (such as the .303 British, 7.62×54mmR, 7.65×53 ...

s .30 Carbine, 7.92×33mm Kurz, 7.62×39mm, 7.62×45mm, 5.45×39mm, .223 Remington/ 5.56×45mm NATO and

stand out as having relatively low sub 8 O_ratio's.

References

“Overbore” Cartridges Defined by Formula Can a Formula Provide a Useful Index Ranking of Overbore Cartridges? http://www.accurateshooter.com
Firearms Firearm terminology Ammunition Ballistics