Backspatter

Backspatter is the ejection of blood and tissue from a gunshot wound of entrance. While blood and tissue are often ejected from exit wounds, this is not the case for entrance wounds. The occurrence and degree of backspatter depends on the anatomical location of the wound, the range and the caliber of the weapon. A contact wound of the head from a large-caliber weapon is more likely to produce backspatter than a distant wound of the torso from a small-caliber weapon. Backspatter is important because the resultant stains may be found on the weapon, the shooter and objects in the vicinity.

There are three possible etiologies for backspatter in the case of head wounds: expansion of gas trapped subcutaneously; intra-cranial pressure generated by the temporary cavity and tail splashing. The last phenomena refers to backward streaming of blood and tissue along the lateral surfaces of the bullet. This may represent an early stage of the temporary cavity effect. The effects of expanding gas subcutaneously is only relevant in close range wounds while the other two etiologies of backspatter are independent of range.

Karger et al. studied backspatter using live calves shot with 9 x 19-mm pistol.21-22 The calves were shot in the head at ranges of: tight contact; loose contact; 5 cm, and 10 cm. The resultant backspatter was divided into macro-backspatter (stain diameter of >0.5 mm) and microbackspatter (stain diameter 0.5 mm or less). There was macrospatter after every shot with the maximum distance traveled varying from 72 to 119 cm. The vast majority of stains were between 0 and 50 cm. The direction of the exiting droplets was at every possible angle resulting, overall, in a 180 degree semi-circle spray. For individual shots, the distribution of the droplets is usually uneven and asymmetrical.

In the case of microbackspatter, there was microspatter after each shot with the maximum distance traveled 69 cm. The vast majority of stains were between 0 and 40 cm. Microspatter stains tended to be more numerous than macrospatter. The stains produced were exclusively circular or slightly oval in contrast to macrostains that showed variations ranging from circular to exclamation mark forms. Just like macrospatter, the direction of the exiting droplets was at every possible angle resulting in a 180 degree semi-circle spray, though the distribution of the individual droplets was uneven and asymmetrical.

The authors felt that the number of droplets and the maximum distance these droplets would travel would be greater for humans because of the anatomical differences between calf heads and human heads. They also speculated that the maximum shooting distance that would result in backspatter in gunshot wounds of the head is likely more than 10 cm, the maximum range that they shot the calves.

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