Firearms have been used since the settlement of
  North America for many different reasons. They  
  were used by colonials to hunt game animals such as
  deer and turkey. They were also used as protection,
  from both wild animals and enemies. Today, guns
  are used for a lot of the same reasons. They are used 
  for hunting, for protection, as well as for sport. The
 focus of this presentation will be in semi-automatic
  handguns, and the physics behind the recoil, the 
  automatic reloading, and the cartridge itself.

      There are a few different types of handguns, with
  many different calibers shot through them. Some 
  common calibers are .22LR, 9mm, .40 S&W(Smith
j  and Wesson), and .45 ACP(Automatic Colt Pistol).
  Semi-automatic handguns normally have a
  removable magazine that feeds the ammunition into
  the breech. The recoil from firing the bullet is then
  used to move the slide and replace the bullet with a
  new one from the magazine. In this presentation the
  focus will be on a Colt 1911, produced in 1917,
  chambered in .45 ACP.

Left to Right: .22LR, 9mm, .40S&W, .45 ACP
  Top Picture source