A .22 caliber rifle fires a bullet with a 22/100 inch diameter. A 9 mm pistol fires a bullet with a nine-millimeter diameter. So a caliber conversion is the same as a metric to English measurement conversion (inches = mm /25.4).
A few common caliber conversions: .223 .56 mm, .22 5.7 mm, .30 7.62 mm, .357 9.07 mm, .3 9 mm, .40 10 mm, .45 11.43 mm and .50 12.7 mm.
Because the caliber is a machined value and very precise, specific ammunition is required for each gun. That is to say, a gun chambered for 7.62 mm might not have the ability to fire a .30 cal round. However, there are actually some firearms that are designed to fire a range of calibers, but users should remain extremely careful in determining whether this is the case or not prior to attempting to use and fire the weapon.