Many firearms, Glocks by way of example have serialized barrels. The issue is, sticking with the Glock example, that the polymer lower carries the serial plate

Many firearms, Glocks by way of example have serialized barrels. The issue is, sticking with the Glock example, that the polymer lower carries the serial plate and is considered to be "the firearm" since barrels are interchangeable and their sale is not regulated. It is straightforward to get a replacement barrel. Barrels do not have an infinite service life and barrel replacement is not uncommon.