Bullet lubricant, bullet/case mouth sealant and gases, which have squeezed past the bullet on its passage through the bore, leave the outside of the bullet coated with a layer of black sooty material. As the bullet passes through an object, whether it be cloth, skin or some solid object, this black material is deposited on the periphery of the bullet entry hole. This black ring is often referred to as the 'bullet wipe' and, where no discharge residues are present, it is a very useful identifier of the bullet entry hole.
The quantity of material left on the ' bullet wipe' mark is dependent on the quantity of carbonaceous material picked up by the bullet during its passage through the bore. Assuming that the first shot fired was through a clean unfouled barrel, the bullet wipe mark should be easily distinguishable from the second bullet which had been fired through the barrel fouled by the first bullet. In this instance, the first bullet wipe mark will contain less carbonaceous material and grease than the second and will appear much fainter than the one from the second shot. In cases where multiple shots have been fired, it is thus possible to determine with a reasonable degree of accuracy which was the first shot to strike the target.
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