The term ".45 caliber" refers to a specific measurement of the diameter of a bullet, which is approximately 0.45 inches. This caliber is commonly used in firearms, particularly handguns and some rifles. The most well-known .45 caliber cartridge is the .45 ACP (Automatic Colt Pistol), developed in the early 20th century for military use. Firearms that use .45 caliber ammunition are often favored for their stopping power and effectiveness in self-defense situations. The larger bullet size typically results in greater energy transfer upon impact, making it a popular choice among law enforcement and civilian shooters alike.