A golf club shaft is provided that has a grip end opposite a head end and defining a length of the shaft extending between those two ends. The shaft has a tubular cross-section over at least one portion of the shaft length which cross-section has a substantially circular outer periphery and a polygon inner periphery formed of a plurality of between 4 and 24 flats, and preferably formed of 8-16 flats. A mandrel having the shape of the inner periphery of the shaft is also provided, as is a method of forming the shaft that uses the mandrel and composite matrix materials.