Backspatter from experimental close-range shots to the head. I. Macrobackspatter.

Backspatter is the ejection of biological material from a gunshot entrance wound against the line of fire. This phenomenon was investigated experimentally in transverse gunshots to the heads of calves (n = 9) using two types of 9 mm Parabellum ammunition from shooting distances of 0-10 cm. The resulting bloodstains were documented on white paper placed horizontally 60 cm below the impact site. In this report the analysis was restricted to stains with a diameter > 0.5 mm. Backspatter was documented after every gunshot. The number of stains varied from 31-324 per gunshot and appeared to be independent of the shooting distance. The maximum distance droplets travelled varied from 72-119 cm. The majority of droplets accumulated between 0 and 50 cm. The number of droplets and the distances travelled should be higher in man for anatomical reasons. The direction a single droplet can take comprises every possible angle between the most tangential ones to the skin surface. This resulted in a semi-circle of 180 degrees covered with stains. Skin ruptures of the entrance wound were not observed. The succession of events was documented on high speed film and started with the recoil of the firearm, immediately followed by a blow-out effect of the skin. Large droplets exited approximately 0.7-4 ms after the bullet impacted the skin. The calculated minimum initial velocity of these droplets was 13-61 m/s. Backspatter from gunshots to the head likely is caused by the hot gases expanding subcutaneously and by cavitation-related intracranial overpressure and tail splashing. In three out of nine gunshots, secondary backspatter additionally occurred as a result of droplets produced by a stream of blood from the entrance wound impacting the paper surface.