You have just finished resuscitating a gunshot victim who has a wound to his chest. The bullet remains in the patient. The detective comes and asks, “Doc, what caliber is the bullet? We have a suspect and need to match his weapon to the bullet.”

Can you tell the caliber of the bullet based on wound size or radiographs?

The simple answer is no. The size of any gunshot wound, entrance or exit, is primarily determined by five variables: the size, shape, configuration, and velocity of the bullet at the instant of its impact with the tissue, plus the physical characteristics of the impacted tissue itself. Importantly, because of the elasticity of skin, the size of the entrance wound will not coincide with the caliber of the bullet. The wound may be smaller, larger, or the same size as the bullet. When a bullet hits the skin surface, it causes indentation before perforation. Following perforation, elasticity causes the skin to recoil, and the resulting round, circular defect is not the same size as the diameter of the bullet.

Figure 1. Radiograph of retained projectile.
Image Credit: William Smock, MD, FACEP

With regard to radiographs, several factors affect the estimation of bullet caliber. Magnification while taking the radiographs can distort the size of the bullet on the radiograph. The bullet size on a radiograph will increase as the distance from the film to the X-ray source decreases. To be accurate, the radiograph must be taken exactly 73 inches from the bullet. As the distance from the X-ray plate to the bullet increases, so does this magnification effect. There are two ways an X-ray can be used to directly estimate bullet caliber. One is to take two radiographs at 90 degrees to each other to estimate the depth of the bullet in the body. A number of bullets of different caliber are then placed alongside the body and at a suitable position and then imaged to compare the bullet caliber. This is rarely done in clinical practice. The other method is to use a micrometer to compare the size of the bullet’s shadow on the X-ray to several bullets of known caliber. Whatever bullet is closest in size to the radiographic image must be the largest caliber that the bullet in the body can be. Once again, this is only an estimate and can only exclude other bullets and not precisely determine caliber. There are other complex radiographic protocols and formulas that have been used, but none have been admitted into court.