What did the X originally mean on end of a barrel?

The letter X stamped on the end of a barrel signifies that it is an export grade weapon. This practice originated in the United Kingdom in the 18th century, where barrels intended for export were marked with an X to distinguish them from those intended for domestic use. The practice was later adopted by other countries, including the United States. Today, the X marking is still used on some firearms to indicate that they are intended for export, but it is no longer as common as it once was.