The ‘triple contrast’ method in experimental wound ballistics and backspatter analysis

Abstract

In practical forensic casework, backspatter recovered from shooters' hands can be an indicator of self-inflicted gunshot wounds to the head. In such cases, backspatter retrieved from inside the barrel indicates that the weapon found at the death scene was involved in causing the injury to the head. However, systematic research on the aspects conditioning presence, amount and specific patterns of backspatter is lacking so far. Herein, a new concept of backspatter investigation is presented, comprising staining technique, weapon and target medium: the 'triple contrast method' was developed, tested and is introduced for experimental backspatter analysis. First, mixtures of various proportions of acrylic paint for optical detection, barium sulphate for radiocontrast imaging in computed tomography and fresh human blood for PCR-based DNA profiling were generated (triple mixture) and tested for DNA quantification and short tandem repeat (STR) typing success. All tested mixtures yielded sufficient DNA that produced full STR profiles suitable for forensic identification. Then, for backspatter analysis, sealed foil bags containing the triple mixture were attached to plastic bottles filled with 10% ballistic gelatine and covered by a 2-3-mm layer of silicone. To simulate backspatter, close contact shots were fired at these models. Endoscopy of the barrel inside revealed coloured backspatter containing typable DNA and radiographic imaging showed a contrasted bullet path in the gelatine. Cross sections of the gelatine core exhibited cracks and fissures stained by the acrylic paint facilitating wound ballistic analysis.

Study of backspatter using high-speed video of experimental gunshots

“…All these approaches have particular advantages, but the common disadvantage was that the targets are not transparent and are dissimilar to human heads. The investigation of staining inside firearm barrels initiated using nontransparent silicone covered hemispheres [34], boxes [24] or polyethylene bottles [35]. With introduction of the "reference cube" with 12 cm edge lengths and a weight of 1.7 kg [10], a cheap transparent gelatin target model was available for reproducible ballistic experiments without ethical issues.…”

Nothing but hot air? – On the molecular ballistic analysis of backspatter generated by and the hazard potential of blank guns

“…It was donated by two informed and consenting adult volunteers, neither of whom were involved in the loading or shooting process at the shooting site, nor sample collection, nor weapon cleaning. EDTA blood of each donor was mixed with the CT-contrast agent Barilux® (Sanochemia Diagnostics, Germany) in a ratio of 1:1 to obtain a “double-contrast” mixture as described recently [15], representing a modification of a previously established multi-component contrast mixture [16].…”

“…The generation of backspatter by conventional firearms has been known for a long time [13] and its molecular ballistic analyzability has been proven not only in real cases e.g. of multiple homicide [14], but also experimentally [15, 16]. In the present study, we are first to evaluate the possibility of STR profiling from backspatter traces generated by blank cartridge gunshots.…”

On the effect of shooting distance, ballistic model construction, doping and weapon type on the simultaneous analysis of DNA and RNA from backspatter recovered from inside surfaces of firearms

“…Acquisition of samples, doping and construction of ballistic models were performed as described elsewhere [3][4][5]. For experimental shootings, sampling procedure, RNA/DNA coextraction, quantification, integrity assessment and STR profiling, reverse transcription (RT) and qPCR see Figure 1.…”

“…Traces of backspatter may consolidate on and be recovered from the shooter and the shooter"s surroundings but also from inside surfaces of the firearm. However, such criminal acts involving shooting firearms at biological targets cannot be planned or controlled, therefore experimental shootings have to rely on standardized ballistic models [1,2] and recently, a new modelling method was presented that allows for the economic molecular analysis of DNA from backspatter with simultaneous investigation of wound ballistic phenomena [3]. Herein, we aim to extend this method"s versatility to include the analysis of mtDNA and RNA (project B).…”