Why does your gun have a "barrel?"
The earliest firearms were cannon, used to breach castle walls. These were made from logs having their center cores burned and bored out. They fired round stone balls, hewn at or near the battlefield. (Think casting is drudgery?)
All too often these failed, bursting out the hollowed out log. In order to reinforce these cannon, oak, or similar boards were placed longitudinally around the circumference. and coopers wrought iron bands, or hoops, to hold the reinforcing in place. The result looked much like liquid containers of the day, barrels.