British System of Chamber Pressure Measurement

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    Ever wonder why your rifle is stamped 18.5 Tons per [] “ ?  That’s Tons per square inch of course, but do the math, that’s 41,440 psi, and falls short of the maximum pressure rating for the .303 British cartridge. The Ton is the British Long Ton 2,240 lbs.  
I’ve been puzzled about this for many years, and never found an explanation.  Surely, the action must be proofed well beyond 41,440 psi?  

I bought a copy of the April 1967 issue of “American Rifleman” off eBay as it contained an article on bedding the No. 4 rifles.  But also in this issue was an article on the British method of measuring chamber pressure.  Below are a few photos from the article.  

As most of us know, a oiled proof round was used.  The case was dipped in light oil up to the neck and this allowed the cartridge case to act like a piston pushing on a steel pad (in place of the bolt) which in turn acted on a cylindrical copper “crusher”.  The compressed copper was then measured with a micrometer and the “decrement” (amount of length reduced after firing the cartridge in the pressure test gun) used in a look up tarage table to get the chamber pressure.  These tables were made in the late 1800’s.  

The tarage tables were created to approximate an “average” pressure and the British system produced a lower average with the base crusher pressure than the US method of using a radial crusher (from a hole drilled transverse into the chamber); both methods gave measurements under the actual peak pressure, but there was no way to measure the peak pressure in those days.  The explanation given relates to the very rapid impulse of the oiled cartridge case on the steel pad which then acts on the copper crusher.  These copper crushers had their origins in measuring static, or near static, pressures in steam boilers.  But in a rifle cartridge pressure test, this occurs so quickly that dynamics of the measuring system are involved and it’s own inertia reacts some of this load, hence the copper crusher cannot measure the actual peak, but rather more aligned with the average impulse.  

So, it appears we still do not have the information to directly relate the base copper crusher method of pressure measurement to the CUP, SAAMI and CIP methods that give the maximum pressure of the .303 British cartridge (45,000, 49,000 and 52,939 psi respectively).  

It is also noteworthy that it was stated in the article that a dry cartridge produces about 3/4 of the bolt thrust that an oiled cartridge case produces.  A good reason to degrease and dry the chamber before firing the rifle.   

Also, the 1954 British Law to mark each action with the actual service pressure of the cartridge for which it is intended indicates that this marking, 18.5 tons per sq in is not the proof pressure.  This value was apparently obtained from an oiled service cartridge (does not say which mark of cartridge) using the base copper crusher method.  The article approximates this method of measuring gives about 4/5 of the peak pressure, which would then translate the 18.5 tons per sq in to 51,800 psi peak pressure, which is close to CIP max pressure of the .303 British, the CIP method directly measures the peak pressures of the propellant gases in the chamber.  

Incidentally, I came across some information stating the Mk 6 ball cartridge produced 18 tons per sq in and the Mk 7 ball cartridge (Cordite loaded) produced 19.5 tons per sq in which equates to 43,680 psi. This of course cannot be actual peak chamber pressure and likely derived from the same method of base copper crusher method.  