Q: I’ve often come across the term “overbore capacity” in various shooting magazines, but find it difficult to comprehend exactly what it means. After reading PO Ackley’s handbooks, I assume that it refers to an extremely large cartridge that holds more powder than the bore size can handle efficiently. Ackley also suggests maximum capacities for each calibre.
A: Actually, a well known ballistician, Homer Powley, links bore capacity with expansion ratio (ER) which is the ratio between the total volume of the bore and the volume of the case. ER is the number of times the gas will expand by the time the bullet reaches the muzzle. A high expansion ratio gives an indication of good barrel life; a low ratio means short barrel life. Low ratios generally result in the highest velocities, but all too often a slight gain in velocity is accompanied by a serious reduction in barrel life. Calibres which are excessively overbore often suffer extremely short barrel life. They include the 7mm STW and the .30-378 (around 600 rounds).
These ultra-high velocity cartridges are chosen for their flatter trajectories and their ability to shoot at long range. In actual fact, there’s little advantage to cartridges which offer only 100 to 150fps over the 7mm Rem Mag and the .300 Weatherby Magnum.
In the real world, six major factors affect the burning of the powder:
- Type of powder
- Bore and groove dimensions
- Rate of twist
- Sectional density of the projectile
- Maximum allowable pressure
- Cartridge design (shape).
An efficient design will deliver optimal ballistic performance for the calibre, give good case life and long barrel life. Other advantages are: flexibility in load development, lessrecoil, minimal muzzle blast, and often better accuracy.