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 The .30 Carbine (7.62×33mm) is a rimless carbine/rifle cartridge used in the M1 carbine introduced in the 1940s. It is a light rifle round designed to be fired from the M1 carbine's 18-inch (458 mm) barrel. History Shortly before World W ...

, , 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 The .223 Remington (designated as the 223 Remington by the SAAMI and 223 Rem by the CIP) is a rimless, bottlenecked rifle cartridge. It was developed in 1957 by Remington Arms and Fairchild Industries for the U.S. Continental Army Command ...

, , 1.87 , , 0.2503 , , 7.47 , - ,

.303 British The .303 British (designated as the 303 British by the C.I.P. and SAAMI) or 7.7×56mmR, is a calibre rimmed rifle cartridge. The .303 inch bore diameter is measured between rifling lands as is the common practice in Europe which follows th ...

, , 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 The .308 Winchester is a smokeless powder rimless bottlenecked rifle cartridge widely used for hunting, target shooting, police, military, and personal protection applications globally. It is similar but not identical to the 7.62×51mm NATO ...

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

8mm Lebel 8 mm or 8mm may refer to: ;Film technology *8 mm film, a photographic cine film format principally intended for domestic use. The term may also refer to later variants: ** Super 8 mm film ** Single-8 film ** 8 mm video format, a type of video reco ...

, , 4.28 , , 0.5240 , , 8.16 , - ,

6mm Norma BR This is a list of firearm cartridges which have bullets of a caliber between and . *''Length'' refers to the cartridge case length *''OAL'' refers to the overall length of the cartridge Measurements are in millimeters then inches, i.e. ''mm ...

, , 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 Afr ...

, , 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 The .30-06 Springfield cartridge (pronounced "thirty- aught-six" ), 7.62×63mm in metric notation, and called the .30 Gov't '06 by Winchester, was introduced to the United States Army in 1906 and later standardized; it remained in military use ...

, , 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 300 Winchester Short Magnum (also known as 300 WSM) is a .30 caliber rebated rim bottlenecked centerfire short magnum cartridge that was introduced in 2001 by Winchester. Specifications The overall cartridge length is 72.6 mm. The cartr ...

, , 4.99 , , 0.4752 , , 10.50 , - ,

8×68mm S The 8×68mm S rebated rim bottlenecked centerfire rifle cartridge was developed in the 1930s by August Schüler of the ''August Schüler Waffenfabrik, Suhl, Germany'' as a magnum hunting cartridge that would just fit and function in standard-siz ...

, , 5.58 , , 0.5178 , , 10.78 , - , 6.5×55mm , , 3.75 , , 0.3436 , , 10.91 , - ,

7×64mm The 7×64mm (also unofficially known as the 7×64mm Brenneke, though its designer's name was never officially added as a part of the cartridge name) is a rimless bottlenecked centerfire cartridge developed for hunting. As is customary in Euro ...

, , 4.48 , , 0.4029 , , 11.12 , - , .243 Winchester , , 3.51 , , 0.2925 , , 12.00 , - ,

.338 Norma Magnum The .338 Norma Magnum is a cartridge first introduced in 2008 and came into production in 2009, designed by Norma of Sweden. Design history The .338 Norma Magnum was originally developed as a long-range sport shooting wildcat cartridge by the A ...

, , 6.95 , , 0.5689 , , 12.22 , - ,

.300 Winchester Magnum The .300 Winchester Magnum (also known as .300 Win Mag or .300 WM) (7.62×67mmB, 7.62x66BR) is a belted, bottlenecked magnum rifle cartridge that was introduced by the Winchester Repeating Arms Company in 1963. The .300 Winchester Magnum is a m ...

, , 5.93 , , 0.4732 , , 12.53 , - ,

.338 Lapua Magnum The .338 Lapua Magnum (8.6×70 mm or 8.58×70 mm) is a rimless, bottlenecked, centerfire rifle cartridge. It was developed during the 1980s as a high-powered, long-range cartridge for military snipers. It was used in the War in Afghanistan and ...

, , 7.40 , , 0.5686 , , 13.01 , - ,

7mm Remington Magnum The 7mm Remington Magnum rifle cartridge was introduced as a commercially available round in 1962, along with the new Remington Model 700 bolt-action rifle. It is a member of the belted magnum family that is directly derived from the venerable ...

, , 5.31 , , 0.4039 , , 13.15 , - ,

.300 Norma Magnum The .300 Norma Magnum, also known as .300 NM or "300 Norma" for short, is a centerfire magnum rifle cartridge developed by Swedish ammunition manufacturer Norma Precision. The .300 Norma Magnum uses a .338 Norma Magnum case necked down to the ...

, , 6.75 , , 0.4751 , , 14.21 , - , .50 BMG , , 18.97 , , 1.2963 , , 14.63 , - ,

.300 Remington Ultra Magnum The .300 Remington Ultra Magnum, also known as the .300 Ultra Mag, 7.62×72mm or .300 RUM is a 7.62 mm (.308 inch) rifle cartridge introduced by Remington Arms in 1999. The .300 Remington Ultra Magnum is one of the largest commercially ava ...

, , 7.14 , , 0.4740 , , 15.06 , - ,

.300 Lapua Magnum The .300 Lapua Magnum (7.62×70mm) is a specialized rimless bottlenecked centerfire cartridge developed for long-range rifles. The commercially successful .338 Lapua Magnum cartridge has functioned as the parent case for the .300 Lapua Magnum, ...

, , 7.34 , , 0.4751 , , 15.45 , - ,

.257 Weatherby Magnum The .257 Weatherby Magnum is a .257 caliber (6.53 mm) belted bottlenecked cartridge. It is one of the original standard length magnums developed by shortening the .375 H&H Magnum case to approx. . Of the cartridges developed by Roy Weatherb ...

, , 5.45 , , 0.3307 , , 16.48 , - ,

.30-378 Weatherby Magnum The .30-378 Weatherby Magnum is a .30 caliber, belted, bottle-necked rifle cartridge. The cartridge was developed in response to a US Army military contract in 1959. While still unreleased to the public, the cartridge went on to set world records ...

, , 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, 7.92 ...

, 7.92×33mm Kurz, 7.62×39mm, 7.62×45mm, 5.45×39mm,

/ 5.56×45mm NATO and 5.8×42mm 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