2019
Michael Haag
Nashville, TN
One of the primary factors that greatly affects the deflection of a bullet is the angle of impact with the surface it strikes. One way to visualize this is by imagining a projectile perforating a piece of drywall orthogonally. This situation will induce minimal deflection, however, as the angle of impact decreases, the observed deflection of the bullet from the original path increases. At a point, depending on the speed and mass of the bullet, the hardness of the material structure and the angle of impact, the bullet will cease perforation, and instead, ricochet from the surface. One of the problems for the reconstruction of the trajectory, is that at such shallow angles, when deflection can quickly change the part of the bullet, a secondary, or downrange impact is no longer the proper point to use in order to establish the original trajectory of the bullet. This means that a trajectory rod placed in the usual manner, from primary impact to secondary, may not be representative of the original path of the bullet, and therefore not point accurately to the location of the firearm when discharged.
At angles of impact close to 30 degrees and less, the mark known as the “”lead in mark”” will become apparent as a parabolic shape. This is created by the side of the projectile as it rubs and depresses the impact material. This mark can be used in determining direction of travel, but it can also play an important role in the relabel determination of the initial trajectory of the projectile. Because there is potential for significant deflection of projectiles while perforating materials at shallow angles, this lead in mark can be used in association with a trajectory rod to reliably demonstrate the original, undeflected path of the bullet and therefore aid in determining the location of the firearm at the time of discharge. The use of the end of a trajectory rod with the lead in mark area, has also been referred to as the “”rocker point”” method.
This paper will review the past documentation and history of this simple technique, demonstrate when it is applicable, describe the methodology behind it, and demonstrate its reliability from experiments trials.