The term "0.45 caliber" refers to the diameter of a bullet or the bore of a firearm, measuring 0.45 inches (11.43 mm). This caliber is commonly associated with handguns and rifles, particularly the .45 ACP (Automatic Colt Pistol) cartridge, which is popular for self-defense and target shooting due to its stopping power. Firearms that use 0.45 caliber ammunition are known for their larger bullet size, which can deliver significant energy upon impact. The M1911 pistol, a well-known firearm that uses the .45 ACP cartridge, has been widely used by military and law enforcement agencies since its introduction in the early 20th century.