The .14/222 Eichelberger Mag wildcat would have to qualify as overbore capacity. Diagram courtesy of "Cartridges of the World".

When is a rifle cartridge overbore capacity?

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.

Norm Rossiter

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:

  1. Type of powder
  2. Bore and groove dimensions
  3. Rate of twist
  4. Sectional density of the projectile
  5. Maximum allowable pressure
  6. 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, less recoil, minimal muzzle blast and often better accuracy.




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Nick Harvey

The late Nick Harvey (1931-2024) was one of the world's most experienced and knowledgeable gun writers, a true legend of the business. He wrote about firearms and hunting for about 70 years, published many books and uncounted articles, and travelled the world to hunt and shoot. His reloading manuals are highly sought after, and his knowledge of the subject was unmatched. He was Sporting Shooter's Technical Editor for almost 50 years. His work lives on here as part of his legacy to us all.