Draw a parabolic curve on a large sheet of paper, cut it out and wrap it on your blank. Trace the line of the edge of the curve onto the blank. Cut a grove along the line. You can repeat the process if you like to get the additional groves or you can use just one grove and index either the cutting shaft or the barrel.
A friend and I built a rifling device based on a plan he came up with which allows us to cut regular rifling right or left and gain twist right or left. We rifled a 54 Cal. barrel for a gun I'm building with 12 groves, left hand gain twist from 1:70 to 1:18 buttress rifling. We tested the barrel and it shoots great. Can't wait to get the gun finished so I can try it out as a completed rifle.
