Caliber is not a finger bullet, but refers to the inner diameter of a gun or barrel - a rifled weapon refers to the distance between two opposing lines. Caliber is usually measured in millimeters, with guns below 20 mm and cannon above 20 mm.
Although the calibers of bullets, artillery shells and guns are corresponding, the true diameter of the warhead is not equal to the caliber, but slightly larger than the caliber. This makes it easier to insert the warhead under high chamber pressure. Rifling, so that the warhead will rotate at high speed when it rushes out of the gun barrel, which will help overcome air resistance and maintain flight stability, allowing the warhead to fly farther and hit more accurately.