Overbore Cartridges and Barrel Life
Overbore Index is a calculation accounting for the total case capacity compared to the bore area. The Overbore Index can be used to estimate how quickly a cartridge will burn out a barrel when compared to others. In general, a higher overbore index will correlate to a lower barrel life, however it is neither perfect nor all-encompassing of every variable in cartridge design. Some other factors to consider when estimating your barrel life:
- Material composition of the barrel. This will make more difference than anything else on the cartridge. You will always shoot out a 416 stainless barrel quicker than a 4150 barrel (assuming they're chambered in the same cartridge)
- Whether the cartridge is bottle-necked or straight wall. Bottle-necked cartridges such as 5.56 NATO generate higher pressures and therefore burn through barrels quicker when compared to straight walled cartridges such as .50 Beowulf.
- Pressures of the ammo you're actually firing. Overbore index accounts for the total case capacity, so if you're firing lighter loads your barrel will last longer.
- Material composition of the projectile you're firing. Bullets with a bimetal jacket, tracers, incendiaries, and some armor-piercing bullets are rougher on the bore compared to a premium copper jacketed projectile.