A handgun’s caliber is not a reliable indicator of its overall size. Caliber refers only to the diameter of the gun barrel and of the bullet itself.
Even though a .45 caliber handgun has a slightly larger barrel than a .40 caliber, the handgun itself can still be smaller depending on the size of the chamber and the grip. In America, caliber is measured as percentages of an inch. For instance, a .40 caliber handgun has a barrel that is 40/100 of an inch. Both .40 and .45 caliber handguns are commonly used in law enforcement. Theoretically a .45 caliber handgun has more stopping power, but again, because of the other variables, such as the length of the bullet that caliber does not measure, this is not always the case.