How do you rifle a barrel?

While there are several techniques used by gun makers, the original method was to start with a smooth bore barrel. A long screw with a single cutting tooth was inserted in the barrel, and pushed down, turning as it went. This would scratch a spiral line on the inside of the barrel. Raise the cutter a notch, repeat. Eventually, the scratch becomes a groove cut into the metal. Rotate the screw, repeat for the next groove. Some barrels had 2 grooves, some 4,5,6, etc.