口径常用在表示管状火器与弹药，或者是其他圆柱形武器，譬如火箭或者是飞弹的直径大小上。口径大小的差异往往也代表武器或者是弹药的重量与能够产生的威力。

口径在管状火器，像是枪械或是火炮方面，代表的是枪管或是炮管内部的直径大小。譬如5.56公厘步枪代表枪管内部的直径是5.56公厘。口径和倍径两个数字是标示火炮型态的重要数据，像是40公厘/70或者是40公厘/L70的表示方式，前者代表火炮直径（40公厘），后者代表炮管长度是口径的70倍（倍径）。

In guns, particularly firearms, caliber or calibre is the approximate internal diameter of the barrel, or the diameter of the projectile it fires, in hundredths or sometimes thousandths of an inch. For example, a 45 caliber firearm has a barrel diameter of .45 of an inch. Barrel diameters can also be expressed using metric dimensions, as in "9mm pistol." When the barrel diameter is given in inches, the abbreviation "cal" (for "caliber") can be used. For example, a small-bore rifle with a diameter of 0.22 inches can be referred to as .22 or a .22 cal; however, the decimal point is generally dropped when spoken, making it a "twenty-two caliber" or a "two-two caliber rifle".