In the laboratory, the forensic experts subject the evidence they have collected to closer scrutiny. A large part of their work concerns ballistics – i.e. the examination of firearms and ammunition. If the perpetrator has used a firearm, one or more projectiles and bullet cases can usually be found at the scene of the crime. When the cartridge is fired, the case is first pulled back in the weapon and subsequently ejected. The next cartridge then moves into the barrel from the magazine. The ammunition collected at the crime scene is examined in more detail in the laboratory using optical magnification instruments, sometimes even special comparison macroscopes.