Mechanism of injury
Some understanding of the factors that influence wounding capacity and pattern of injury may help the imaging assessment. The main contributing factors are the type of firearm, projectile characteristics and the inherent characteristics of the wounded tissues.
Type of firearm
The type of firearm influences the velocity of the projectile and given that e = m x v2, projectile velocity is a major contributor to the energy imparted to tissues and hence, to wounding capacity.
Projectile characteristics
- Deformation : encased (jacketed) bullets used in higher velocity firearms tend to deform (“mushroom”) on impact. A deformed bullet decelerates faster, hence increasing local tissue damage
- Tumbling: when a bullet hits its target, it loses its directional stability and is able to rotate around its short axis; for this reason, non-deformed bullets may sometimes be seen pointing towards the entry wound
- Fragmentation: generally occurs on impact of a non-jacketed or semi-jacketed bullet. As well as giving some clue to the bullet type/characteristics, fragmentation often leads to a telltale distribution of fragments spreading along the bullet course. Fragmentation gives important information about the mechanism and direction of injury.
Tissue characteristics
Bullet injury is more severe in friable organs such as liver and brain due to temporary cavitation at a distance from the bullet path. Both dense (e.g.bone) and loose tissues (e.g. subcutaneous fat) are more resistant to damage. Bone can significantly alter behaviour of the projectile and its wounding capacity by slowing it down and changing its path