The 1936 locked room mystery I described yesterday started with a woman who had been shot dead while standing by the open door of a domestic coal furnace. Robert Williams Wood was asked to examine the 'bullet' recovered, which was very small and made of copper.
His findings were reported in the Proceedings of The Royal Society of London:
"...though its form resembled nothing with which I was familiar, I surmised it was probably part of a dynamite cap or detonator used for exploding the dynamite charges in the mines, which, by some carelessness on the part of a miner, had been delivered intact along with the coal."
The question was how the thin copper casing of the detonator could be transformed into a solid high-velocity projectile. The detonator was a cylinder about 40mm (1.5") long and 5mm (1/4") in diameter filled with mercury fulminate. Wood noted that the head of the cylinder was formed into a shallow cup.
It seemed probable that the solid pellet of copper, recovered during the autopsy, had been formed in some way by the concave head of the detonator by the enormous instantaneous pressure developed by the fulminate.
To test the theory, Wood suspended a detonator above a five-gallon earthenware jar full of water, pointing the head downwards. On firing, the jar was shattered into pieces. A copper projectile recovered from the remains of the jar matched exactly the one found at the scene.
Wood estimated that the projectile entered the water with three times the speed of a rifle bullet – much faster than any ordinary shrapnel from an explosion. The factor was that the force of the blast had reshaped the copper into an aerodynamic slug. Wood had discovered the explosively-formed projectile or EFP, which effectively translates a high percentage of the energy from the explosion into kinetic energy.
His diagram at left shows how he believed the EFP was formed. In 1940 Hubert Schardin – a German scientist, and like Wood noted as a photographic pioneer – took flash x-ray photographs which confirmed the deformation pattern. He gave his name, along with Hungarian Colonel Mizsnay, to the Mizsnay-Schardin effect which describes how blast travels perpendicular to a sheet of explosive. EFPs are called Mizsnay-Schardin projectiles (as are some other devices).
Such projectiles are complex. A 1987 studyfrom Germany notes:
Very small variations in the charge configuration often cause radical changes in the projectile form. In tests, the occurrence of small deviations in liner material quality – deviations that lay completely within the limits of the material specifications – had the effect that previously perfect EFPs now broke into fragments.
Arriving at a design is a matter of much trial and error (or these days, intensive 3-D modelling on supercomputers), but once a design is obtained, EFP liners of the correct shape can be turned out by the hundred with fairly modest facilties. And an effective EFP can retain 80% of its velocity and punch through armor out to a thousand charge diameters - that's about a hundred yards for one the size of a coffee can.
The chance of a detonator just happening to be the right configuration were minute, but that was exactly what happened. As soon as the coal went into the furnace, the detonator fired its tiny projectile, and the victim happened to be in exactly the wrong place.
EFPs have been used for off-route mines against armor for many years, but judging from Wood's original study they would also make lethal anti-personnel weapons.
