Deposition

There is evidence to suggest that the FDR on the firer's hand is blasted on to the hand during the firing process and that residue settling from the atmosphere does not appear to contribute. Copious amounts of FDR issue from the muzzle but play a secondary role in hand deposits. The hand deposits are mainly emitted from openings around the breech and ejection port in self loading firearms and from the flash gap between the cylinder and barrel in revolvers.174 Such residues can also be deposited on to the face, head hair, and clothing of the firer.

Many factors are thought to influence the amount and nature of FDR deposited on the firer. It should also be borne in mind that there can be a considerable variation in the amount recovered for analysis depending on the sampling method and on how efficiently it is used. Such factors include:

Type of gun—A low success rate for FDR detection has been observed for casework involving .22" caliber revolvers and rifles, and for shotguns which are usually closed breech weapons, some rifles, namely, bolt action, require manual extraction of the spent cartridge case; consequently the design of the firearm can have an influence on the quantity of FDR deposited.

Mechanical condition of the gun—A gun in poor mechanical condition is likely to have larger gaps in the firing mechanism, thereby allowing more FDR to escape.

Cleanliness of the gun—Contact with a "clean" gun is likely to produce less FDR than contact with a "dirty" gun of the same type.

Type of ammunition—Jacketed bullets produce substantially fewer FDR particles than unjacketed bullets. Primer type (size, composition, burning characteristics) can influence the number of primer particles produced. The temperature and pressure achieved by the burning of the propellant determines the power and velocity of the bullet which can influence the number of particles deposited.

Direction and force of air currents (wind)—Tests involving similar firings with the same gun and ammunition batch, one series conducted indoors and the other outdoors, produced substantially less FDR on the firer in the case of the outdoor tests. This was thought to be due to the effect of the wind. Other climatic factors such as rain, humidity, and temperature could also play a part. Firing location and duration of exposure—Firing from confined spaces, for example, doorways, small rooms, vehicle interiors, will tend to expose the firer to a more residue-laden environment for a longer than normal period of time; consequently, it will take the FDR longer to disperse and the chances of FDR from the muzzle being deposited are increased. Nature of surface—Skin condition (dry, moist, natural oils, amount of hair) and nature of clothing (smooth or rough) are thought to influence the quantity deposited.

Other factors include the position of the gun relative to the firer; that is, whether the arm is outstretched or closer to firer at time of discharge, whether firing is single or double handed, whether the firer is sitting, standing, kneeling, or lying down will all affect the surface area available for deposition of FDR. Subsequent manipulation of the gun such as unloading, reloading, cleaning, and picking up spent cartridge cases can yield more FDR than the actual shooting.

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