The long version
British Crusher Gauge illustration
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA29&dqTarage Table
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA37&dqWallace 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). Coxe clearly converted psi to tons simply by dividing by 2240 for some of his published pressure curves.
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 Lead Crusher (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:
“A most interesting outcome of pressure measurements on shotgun cartridges using the piezoelectric gauge has been the comparison of the maximum pressure readings with those of the lead crusher gauge. By mounting the “Two Gauges” on opposite sides of the same gun at one inch from the breech face “Simultaneous” readings on the same cartridge were possible. As had been known for some time, the crusher readings were less than those given by the piezoelectric gauge due to the time lag in the plastic deformation of the lead-the piezoelectric gauge piston only moves about 1/100,000 inch when subjected to 2.5 tons per sq in and is thus free from this error. Over the ranges of pressures usually encountered at the one inch position in shotguns the readings given by the two gauges are related by the equation;
P = 1.5L - 0.5 tons per sq inch X 2240
Where;
P = the maximum pressure (PSI) indicated by the piezoelectric gauge
L = the maximum pressure (TONS) indicated by the lead crusher gauge”