The tools required to manufacturer rifling in traditional ways are highly cost prohibitive. Electro-Chemical Machining (ECM) is a method where it can be done for very cheap.

ECM is used by employing saltwater, an electric current, and conductive material to rust groves into a blank barrel in a controlled manner. This conductive material is wrapped around a jig (like shown below) which is then placed into the blank to guide the oxidation, and form rifling.

Traditionally barrels are made by cutting, forging, buttoning, or broaching.