A method for forming a set of gun barrels, comprises: machining an elongate key along a length of a metal billet; mounting the metal billet on a machining jig, clamped along the key; performing CNC milling operations on the mounted metal billet in a 5-axis CNC milling machine to form a set of gun barrels having a pair of bores and a rib, the rib extending lengthwise proximate the elongate key; and, separating the set of gun barrels from the key along the length of the rib section. The present invention provides for significantly faster machine production with no manual handling of the workpiece throughout the milling operations. This makes the milling operations more accurate as the workpiece remains in position throughout, without reloading. Holding the workpiece along a key which runs the length of what will become the rib of the barrels avoids any loading of the workpiece in areas that need to be machined. The use of a 5-axis CNC milling machine allows for variable geometries to adjust the finished shape and configuration of the gun barrels.