Effect of muzzle gas on forward blood spatter from a gunshot: Experiments with a supersonic de Laval nozzle

Abstract

For bloodstain pattern analysis (BPA), interpreting statistically reliable data on a crime scene resulting from gunshots is a great challenge. This is due to various uncertainties, including blood rheology, hematocrit, coagulation, surrounding atmospheric conditions, victim's peculiarities, gun types, geometries, etc. In addition, muzzle (propellant) gases that follow the bullet may influence the aerodynamics of blood spatter in the cases of short-range shooting. We studied the muzzle gas effect on forward blood spatter. Muzzle gas can penetrate the wound channel and be ejected from the bullet exit hole affecting the forward blood spatter. Experiments with blood atomization by a gas flow issued from a supersonic de Laval converging–diverging nozzle are conducted. Defibrinated sheep blood was enclosed in a thin solid cylinder, which was filled by a supersonic air flow ejected from a de Laval nozzle, mimicking the muzzle gas flow through a wound channel. The mass flow rate of the supersonic air stream was varied by controlling the upstream chamber pressure. It was found that the number counts of the forward blood spatter from the muzzle gas blasting peaked at relatively shorter distances from the exit hole compared to the one that would be caused by a bullet. The effects of the muzzle gas and bullet could cause the formation of a bimodal spatter distribution on the floor behind the exit hole. These findings imply that atomization events owing to muzzle gas cause coarser atomization than that of a bullet, which could facilitate BPA in distinguishing certain homicides from staged suicides.