Safe chamber pressures in British and American sporting rifles


Q: My uncle gave me a BSA sporting rifle in .30-06 calibre. While this rifle appears to be very strong and rugged, I noticed that the barrel is stamped “18 tons per square inch”. 

If this marking refers to British long tons of 2240lb, that translates into a pressure of only a little over 40,000psi, or a lot less than the 55,000psi and more for which American rifles of this calibre are rated. 

Is this rifle safe for use with American factory ammo and my handloads?

Craig Waters

A: Have no doubts about the strength of your BSA. The difference between the mean service pressure of 18 tons per square inch and the 50,000 to 60,000psi more standard for the .30-06 in America is due to a difference in pressure measurement. 

In the USA, powder pressures are obtained by the ‘radial’ method, in which a gas piston through the side of the chamber compresses a copper crusher, from which the peak pressure is calculated. 

The British, on the other hand, use a ‘base’ method whereby a suitable copper crusher is contained in a special breechblock. The cartridge case is oiled so that it may move back bodily and compress the crusher. The greater mass of the base piston effects slightly the amount it compresses the crusher. 

The pressures shown by base crushers are about 90% of those obtained by radial copper crushers. So, in fact, the 18 tons marked on your rifle would be about 20 tons or 45,000psi by American measuring methods. 

By the same token, British ammo makers rarely ever loaded cartridges above 19 tons (47,500psi) since they reckoned that most of their big-game ammo would be used in the tropics where prolonged exposure to high temperatures will raise firing pressures considerably. 

In any event, you need feel no concern about using any modern factory ammo in that BSA of yours. The Mauser-type BSA has one of the strongest, safest actions ever made.

 

 

 


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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.

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