The sketch is only a guide for those who would like to build a CNC controlled barrel rifling machine.  As shown, the machine operates as follows:

The barrel is mounted in a pair of supports and clamped securely so it will not turn.  It might be a good idea to include a set of moveable (or even removable) centers as an aid in locating the barrel bore.  

The spiral guide consists of a round bar having single spiral groove machined along its surface.  The spiral guide is driven longitudnally by means of a bracket and lead screw.  

The drive bracket is made so as to allow indexing of the spiral guide.  Indexing to allow 5, 6 or any other number of grooves can be accomplished by using interchangeable indexing bushings.  Make the bushings with flats on the sides to eliminate any possibility of rotation during use.

The opposite end of the spiral guide rides in a bracket which allows the guide to slide as well as rotate at the same time. It would be wise to install a replaceable bushing in the bracket for this purpose.  A fixed pin mounted in the bracket engages the machined groove in the spiral guide.  Longitudnal movement of the spiral guide imparted by the lead screw will cause the guide to rotate.  This rotation is transmitted to the rifling tool, consequently cutting a matching spiral in the barrel.  The right hand end of this bracket should have a vee block or some other method of supporting the spiral bar as it extends thru the bracket.  This will help preven bending of the tool holder bar which might cause deformed grooves near the barrel ends.

The tool holder can simply be a piece of tool steel with a cutter attached on the end.  Sufficient means should be provided to solidly attach this bar to the spiral guide so it CANNOT TURN relative to the guide.

If a pair of full length round bars are used as ways, all of the brackets can be made using like mounting dimensions, thereby insuring accurate centering of all parts.  

The base for this unit should be built very sturdy to resist the forces developed while "egraving" the rifiling.

With a bit of forethought, it should be very easy to install a removeable rotary axis and a horizonal spindle for cutting the spiral groove required in the guide as well as machining the indexing flats. Use the CNC system to make all the spiral guides needed for ANY desired twist rate.