How do you make grooves in gun barrell?

Grooves can be on the outside of the barrel- called fluting- and they are cut with a milling machine.

Grooves can also be inside the barrel- that is the rifling. It is cut by a special tool. The earliest rifling tool cut one line at a time, gradually getting deeper and deeper. Now a rifling tool called a broach may be used- made of extremely hard material, it is pulled through the barrel by a machine, and cuts all of the rifling in one pass.