Someone shouts, “Shots fired!” But who fired? And from where? To answer these questions, the investigators begin by identifying the trajectory or path of the bullet.

To track the bullet’s trajectory, a line is drawn connecting two points along the bullet’s known path. These two points could be an entry wound (A) and exit wound (B) on a victim (Figure 1) or a bullet hole in a car window and an entry wound on someone inside the car (Figure 2).

Cartridge casings were found at two different recent crime scenes. Refer to photographs labeled A–L on the following page. Three suspects were apprehended. Police test-fired firearms belonging to the suspects and compared firing pin marks and other marks on the cartridge casings with those marks on the crime-scene spent cartridge casings.

By the end of this activity, you will be able to:

Measure the mass, length, and diameter (caliber) of both perfect (undamaged) spent projectiles and deformed spent projectiles.

Count the number of lands and grooves, and determine whether the projectile has a right or left twist.

Describe the characteristics of both deformed and perfect spent projectiles, including size, mass, composition, and any unique markings.

Compare and contrast spent projectiles to determine whether any could have been fired from the same firearm.

By the end of this activity, you will be able to:

Calculate the time it takes for a bullet to reach a target, given the velocity of the projectile and the distance to the target.

Calculate the distance that a projectile drops over time due to gravity.

Determine the adjustment required for the projectile to hit a target’s bull’s-eye to compensate for the force of gravity.