ABSTRACT
Impression marks are caused by the action of the mechanisms of a firearm during the firing process. When an automatic pistol is discharged, the shooter pulls the trigger until its hammer mechanism strikes the firing pin. In turn, the firing pin strikes the cartridge’s primer, initiating the burning of the smokeless powder charge. The smokeless powder burns, producing gas and pressure, which pushes the bullet out of the casing and through the pistol’s barrel. After passing through the length of the barrel, the bullet exits the pistol’s muzzle accompanied by the remnants of the burning smokeless powder, primer residues, and flame or muzzle flash. Simultaneously, the pistol’s slide is forced backward, which causes the spent casing to exit the pistol’s ejection port. Figure 37.1 shows the basic components of an automatic pistol. Figure 37.2 depicts a discharging pistol.
