How did the barrels get bored? How were they honed? How did they make the strikers and the plugs that screw into the breachface?

Any well fitting thread in a gun was made with a tap that was made on a lathe. Likewise, the screw that goes in the threaded hole is made on a lathe. Any axles are made on a lathe (anything that holds the work and turns is a form of a lathe)