The development of the particle analysis method for FDR detection and identification involved consideration of how the particles are formed and of their physical and chemical nature.170171

At the present time the exact mechanism(s) of formation of the particles can only be deduced from considerable practical experience and limited experimental work, most of which, if not all, has been concerned with the elemental content of FDR particles.

An extensive and very valuable study of the nature of FDR particles was conducted at the Aerospace Corporation, California, in the mid-1970s. The vast majority of this chapter is based on its findings. From firings involving a wide range of handguns and ammunition, extensive statistics were gathered about the size, shape, and elemental composition of discharge particles. On the basis of these observations, several hypotheses have been suggested concerning the formation of the particles.

Detectable FDR is mostly particulate in nature. Unjacketed lead bullets produce residue in which greater than 70% of the particles are lead. Coated bullets give the same result, except that a substantial proportion of the lead particles contain copper from the coating material. With jacketed or semi-jacketed bullets the proportion of lead particles in the residue is greatly reduced. It was concluded that most of the lead in the residue comes from the bullet rather than from the primer. This has subsequently been confirmed by experiments involving the use of radioactive tracers.172

The stability of the smaller lead particles under the SEM electron beam suggests that they consist of metallic lead rather than lead oxides. Considering that most of the lead particles originate from the bullet, coupled with their stability under the electron beam, leads to the conclusion that the vast majority consist of metallic lead.

Particles due to the discharge of a firearm can be loosely divided into two types: those originating from the bullet, and those originating from the primer. This is a broad general classification only and is not absolute. Particles classified as primer particles because of their elemental content are unlikely to contain metallic lead because they were initially compounds and are unlikely to be reduced to the metal in the oxidizing environment of the primer ignition.

The formation of particles is thought to proceed in the following manner. The hot, high-pressure gases from the burning of the propellant (and primer) act initially on the exposed lead at the base of the bullet. The bullet then passes though the barrel and is subjected to strong frictional heating. This causes some of the bullet and bullet jacket material to be melted and vaporized as well as small fragments to be stripped from the bullet by the rifling. The metal vapors from the bullet mix, to some extent, with the vapors from the inorganic compounds of the primer and are emitted from the firearm through the muzzle and other gaps. They then condense into particles, some of which could be deposited on to the skin and clothing surfaces of the firer.

Discharge particles can be broadly classified into bullet particles and primer particles. This is not unexpected considering that while vapors are miscible, most inorganic compounds in the liquid and solid state cannot dissolve metals, and vice versa. Upon solidification the metals would be expected to separate from the compounds and form separate particles.

Copper is found in both bullet and primer discharge particles if the bullet is coated or jacketed with copper or copper-containing alloy. Discharge residue from ammunition with bare lead bullets shows a very marked decrease in the number of copper-containing particles compared to ammunition with coated or jacketed bullets. This is strong evidence that the copper originates primarily from the bullet coating or jacket rather than from the primer cup assembly or the cartridge case. To explain why copper is found in both bullet and primer discharge particles it is assumed that some proportion of the metallic vapors become oxidized by oxygen or sulfur from the vaporized primer mixture and possibly by atmospheric oxygen outside the gun. Thus the primer (compound) particles contain a contribution from the bullet.

Experiments were conducted involving the addition of tracer compounds to the propellant, followed by an examination of the discharge particles to determine if any of the particles and which type contained the tracer.173 The tracer was found in primer particles only, which supports the proposition that they should be found dissolved in the oxides and salts originating from the primer and not in the bullet particles. A similar experiment involving the coating of bullets with metals not normally associated with firearms ammunition, followed by examination of the discharge particles, supported the proposition that the bullet material makes a contribution to the primer particles.

It was also observed that the number of discharge residue particles on the firing hand decreased markedly as the bullet velocity increased. One possible explanation for this is that the greatly increased suction in the wake of the faster bullet causes more particles to be sucked out of the muzzle leaving fewer to exit from other vents and to be deposited on the hand. This explanation is supported by the observation that the number of bullet particles deposited is reduced slightly as a function of velocity than is the number of primer particles. Primer particles are on average larger and are produced farther away from the bullet than the bullet particles.

Very little is currently known about the formation and nature of organic discharge residue as the main thrust, until relatively recently, has been directed toward the inorganic content of discharge residue.

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