Long version Frank
Wallace H. Coxe, in “Smokeless Shotgun Powders: Their Development, Composition and Ballistic Characteristics” published by E.I. du Pont de Nemours & Co., 1927, “Measurement of Pressures”
The common method of taking pressures in small arms in this country is known as the Radial Pressure system. A housing is built around the barrel, and a hole drilled through the housing and barrel into the chamber at a distance of 1 inch from the breech and at right angles to the axis of the bore. The hole is then bushed and drilled to a uniform diameter of 0.2250 inch. Then a piston is made the length of the piston hole and 0.2250 inch in diameter. Next the piston hole is lapped to permit the piston to fit snugly without either sticking or getting out of alignment.
In firing the gauge, the piston is inserted and seated, then a lead crusher cylinder is placed on the head of the piston and held firmly in place by a screw and anvil attachment built into the housing. When the cartridge is fired, a portion of the same gas pressure that pushes the bullet through the barrel drives the piston against the lead cylinder and compresses it.
The length of the lead crusher cylinder after compression is naturally less than before the shot was fired and the difference between the original length and the length after compression therefore represents the amount of pressure which has acted upon the lead. Thee exact pressure is read from a table giving a pressure reading for every remaining length reading and commonly called a Tarage Table.
Pressures that are determined at ballistic laboratories are merely relative values and are not absolute values.
NOTE: The Tarage Table conversion may be for pressure expressed as Tons / Sq. Inch or Pounds/ Sq. Inch (PSI).
September 1931
American Rifleman. “Standard Shotgun Pressure Barrels”, by Merton A. Robinson, Ballistics Engineer, Winchester Repeating Arms Co. The article described in detail the crusher method of pressure testing, with images of pressure barrels then in use by Peters, Hercules, Western, Remington, Winchester, Du Pont and Federal. Although the barrels were of somewhat different designs, they were ALL crusher barrels, without evidence of piezoelectric transducers. (Courtesy of Larry Brown)
Major Sir Gerald Burrard published an estimated LUP (Long) Tons/ Sq. Inch (TSI) to piezoelectric transducer Pounds/ Sq. Inch (PSI) conversion formula in the Third edition of
The Modern Shotgun, 1955, Vol. 2 derived from simultaneous crusher and transducer pressure readings in a test barrel. He observed the under-reporting of pressure by (crusher) LUP compared to piezo transducer gauges:
“The calibration of lead crushers by means of the piezoelectric gauge suggests that lead crusher pressures are somewhat on the low side; 2 Tons per square inch being about 2.5 with the piezoelectric gauge…”
Estimated Long Tons Per Square Inch Lead Crusher Pressure (Cp) conversion to PSI (pound force per square inch) (Cp x 1.5) - .5 = TSI, TSI X 2240 = PSI. Using this formula:
3 Tons/sq. inch by LUP (crushers) = 8,938 PSI (pounds/ sq. inch) by transducer
3 1/4 Tons = 9,682 psi
3 1/2 Tons = 10,427 psi
3 3/4 Tons = 11,480 psi (The SAAMI maximum for 2 3/4” 12g is 11,500 psi)
4 Tons = 11,917 psi
“Eley Shooter's Diary 2005” - “The later transducer system uplifts the (PSI by LUP) values by approx. 14%.”
From the
CIP site: confirmation that 850 BAR = 12,328 PSI by crushers (LUP) is actually 960 BAR = 13,924 PSI by transducers (13% higher)
http://www.chircuprodimpex.ro/produs...ammunition.pdf
“These cartridges, if they are to be fired from standard proof shotguns (ie those proved to 960 bar [transducer] or, previously, 850 bar [crusher]), or magnum proof shotguns (ie those proved to 1370 bar [transducer] or, previously, 1200 bar [crusher]) must not exceed the maximum admissible service pressure of 74 MPa (new units “megapascals”, in place of the old 740 bar) specified by the CIP
and measured by means of a piezo-electric transducer.”
SHORT VERSION
If the tarage table conversion is expressed as psi - add 10-14% for modern piezoelectric transducer measurement
If the tarage table conversion is expressed as (long) tons - use Burrard's formula