Overbore

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 Oratio result, the more overbore a cartridge will be. As the ratio is expressed in units of length, relatively high Oratio is a good predictor of suitability for relatively long barreled guns.

Oratio 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[]

Chambering Case capacity (ml / cm3) Abore (cm2) Oratio (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 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 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
9.3×64mm Brenneke 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 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 cartridges .30 Carbine, 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.

References[]