The revolver halo as a forensic marker: Raman spectroscopic evidence of primer-driven gunshot residue deposition

The halo phenomenon in revolvers consists of a circular deposit of gunshot residues (GSR) formed around the fired chamber after firearm discharge. Despite its mention in forensic practice, its scientific basis and chemical origin have not been previously established. In this study, the occurrence, composition, and origin of revolver halos were investigated using Raman spectroscopy. Controlled firing experiments were conducted using four revolvers and four types of ammunition. Halos were observed in 15 out of 16 (94% of the shots), supporting their reliability as indicators for identifying the fired chamber. Raman analysis revealed that the halo is primarily composed of inorganic primer-derived compounds, including lead oxides and soot, demonstrating that the primer-rather than the propellant-is the main contributor to halo formation. Comparable spectral profiles were obtained for complete cartridges and primer-only firings, reinforcing the dominant role of primer residues. The results support the use of the revolver halo as a scientifically grounded tool in forensic firearm identification.