The deceleration of bullets in gelatine — A study based on high-speed video analysis

The velocity of a projectile is of crucial importance to calculate its kinetic energy. The study was performed to investigate if a measurement of the velocity profile of a bullet in gelatine was feasible using high-speed video. More than 50 records captured with a SA-X2 Photron camera at 40,000 fps (40k) and 10 μs exposure time were analysed frame by frame. A measurement accuracy of one pixel could be realised. Records of free flying bullets with known velocity served as a control. In further experimental shots from distance the "12 cm reference cube" was used as target model. 18 full metal jacketed bullets and 20 deforming bullets in forensically relevant calibres were fired. In addition 11 expanding bullets were studied with a close-up setting of the camera and 81,000 fps (81k) at 2.5 μs exposure time. A quasi-constant deceleration of the FMJ bullets in gelatine with a relative loss of velocity of 8.4% (.32 auto) and 13.2% (9 mm Luger) was observed. Expanding bullets showed a much higher deceleration and a relative loss of about 56% for the copper hollow point bullet and 73% for the .32 auto Gold Dot. The targeted analysis of the .32 auto Gold Dot based on 81k videos revealed an abrupt drop of the bullet velocity within the first 75 μs, the time needed for full expansion. The deformation of the Action-4 (9 mm Luger) was completely achieved within the first 2 cm of gelatine. After the deformation of the bullet the final deceleration was approximately constant. This could be demonstrated successfully by separating the deformation process experimentally from the final deceleration of the deformed bullet.