Overbore
Overbore is a term used to describe firearms cartridges 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:
- Volumecase = the cartridge case internal volume or case capacity (in ml or grains of water)
- Abore = barrel bore cross section area (in cm2 or in2)
The higher the Oratio result, the more overbore a cartridge will be. A relatively low Oratio seems to be a good predictor of favorable barrel life and suitability for relatively short barreled weapons. Many magnum cartridges feature a relatively high Oratio.
Comparative index for various rifle cartridges
| Chambering | Case capacity (ml / cm3) | Abore (cm2) | Oratio |
|---|---|---|---|
| 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 | 1.87 | 0.2503 | 7.47 |
| .303 British | 3.64 | 0.4807 | 7.57 |
| 7.65×53mm Mauser | 3.70 | 0.4836 | 7.65 |
| .308 Winchester | 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 | 4.28 | 0.5240 | 8.16 |
| 6mm Norma BR | 2.45 | 0.2952 | 8.30 |
| 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 |
| 6.5×55mm | 3.75 | 0.3436 | 10.91 |
| 7×64mm | 4.48 | 0.4029 | 11.12 |
| .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 |
| .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 cartridges 7.92×33mm Kurz, 7.62×39mm, 7.62×45mm, 5.45×39mm, .223 Remington/5.56×45mm NATO and 5.8×42mm stand out as having relatively low sub 8 Oratio's.