DNA-free does not mean RNA-free-The unwanted persistence of RNA

Contact shots to the head often provoke a transfer of biological traces into firearm barrels, which are not visible at endoscopic inspection. STR-PCR can amplify these latent traces and assign them to the victim. Via RNA-DNA-co-extraction also miRNA can be detected, which allow a conclusion to be drawn about the body fluid or tissue. Molecular genetic analysis of experimental stains in firearm barrels requires the guarantee that the barrel is initially free of any nucleic acid. Twelve shots were fired to so-called "reference cubes" (10 % gelatine, 12 cm edge length, embedded paint-blood-pad) using three current handguns: from 20 and 30 cm distance, four at close range (1-2.5 cm) and six contact shots. After endoscopic examination and swabbing of the barrels, a previously described mechanical and chemical cleaning using DNAExitusPlus™ was performed. The inner surface of the barrel was thoroughly wiped off using moistened forensic swabs, which were submitted to RNA-DNA-co-extraction. The combined thorough mechanical cleaning with Ballistol® and the application of DNAExitusPlus™ eliminated any profilable DNA in all samples. However, in 10 of 12 samples RNA concentrations between 0.11 - 0.79 ng/μl were measured. Furthermore, in 9 of 12 samples blood-specific miRNA (miR-451a) was detected. Summarizing, none of the experimentally contaminated barrels was RNA-free despite the performed cleaning procedure. Further investigation showed, that even "professional" cleaning by a gunsmith did not remove RNA.