This information is straight from the Shilen web page at: http://www.shilen.com/calibers.htm
 

"The information below will help you select the best barrel twist for your specific needs, providing of course there are multiple choices. If you’ll be shooting one bullet weight, choose a twist from the chart just fast enough to stabilize it. Too fast a twist simply overspins the bullet and may result in reduced accuracy. On the other hand, a twist too slow for any bullet will likely produce instability, keyholing and rotten accuracy. So if you’re going to be using several bullet weights, be sure to select a twist that will stabilize the heaviest one, since lighter bullets will also be stabilized.

By way of explanation, the numbers in the "Twist" column indicate how far the bullet must travel through the bore to make one full revolution. This is determined by the rate of rotation of the rifling. For example, a 9"-twist barrel spins the bullet one full turn when it passes 9 inches through the bore. In a 16"-twist barrel, the bullet makes one revolution in 16 inches. So at the same bullet velocity, the 9" twist is faster (spins the bullet faster) than the 16" twist. Consequently, as the chart reflects in the .224 CF section, the 9" twist is necessary for stabilizing heavier bullet weights.

Again looking at the .224 CF section, if you’re planning to shoot 55-grain bullets, the 14" twist is ideal. This twist is also the best choice for 52- and 53-grain match bullets driven at moderate velocities. But if you’re also contemplating shooting a 60-grainer, go with the 12" twist, since it will stabilize all the bullets you’ll be using. As you can see from the 15" and 16" twists, higher velocities also spin the bullet faster and may enable a slower-than-recommended twist to stabilize a marginally overweight bullet. However, this is tricky stuff best left to those who are highly experienced in operating at the ragged edge of stability and sanity.

The basic principles just outlined apply to all the calibers below. But, if you have questions or need a  recommendation, let us know.

                  Caliber Twist
 

                        .172                                  9" For bullets heavier than 30 gr.

                                                               10" For bullets up to 30 gr.
 

                        .204/20                           12" For all bullets.
 

                        .222 RF                          14"*    Twist for pistol barrels

                                                               16" Standard twist for rifle barrels

                                                               17"*    Special twist for rifle barrels
 

                        .224 CF                           8"        For bullets heavier than 70 gr.

                                                                9" For bullets up to 70 gr.

                                                             12" For bullets up to 63 gr.

                                                             14" For bullets up to 55 gr.

                                                             15"*    For bullets up to 55 gr. driven 4,100 fps or more

                                                             16"*    For bullets up to 55 gr. driven 4,300 fps or more
 

                        6mm/.243                       8"        Special for VLD bullets over 100 gr.

                                                             10" For bullets up to 120 gr. and VLD under 100 gr.

                                                             12" For bullets up to 85 gr.

                                                             13"*    For bullets up to 75 gr.

                                                             14"*    For bullets up to 70 gr.

                                                             15"*    Special for bullets up to 70 gr.
 

                        .257                                9" For bullets heavier than 100 gr.

                                                             10" For bullets up to 100 gr.

                                                             12" For bullets up to 90 gr.

                                                             13"*    For bullets up to 80 gr.

                                                             14"*    For bullets up to 70 gr.
 

                        6.5mm/.264                    8"        For bullets heavier than 130 gr.

                                                               9"        For bullets up to 130 gr.
 

                        .270                               10" For all bullets
 

                        7mm/.284                       9"        For bullets heavier than 140 gr.

                                                              11" For bullets up to 140 gr.
 

                        .307                               13"*    Special size and twist
 

                        .308                                 8" For bullets heavier than 220gr.

                                                              10" For bullets up to 220 gr.

                                                              12" For bullets up to 170 gr.

                                                              14"* For bullets up to 168 gr.

                                                              15"* For bullets up to 150 gr.
 

                        7.65mm/.311                 10" For all bullets
 

                        8mm/.323                      10" For all bullets
 

                        .338                                10" For all bullets
 

                        9mm/.355                      14" For low-velocity wadcutters

                                                               16" For all other bullets
 

                        .38/.357                         14" For low-velocity wadcutters

                                                              18" For all other bullets
 

                        .358                               14" For all bullets
 

                        .375                               12" For all bullets
 

                        10mm/.400                    16" For all bullets
 

                        .411                               14" For all bullets
 

                        .416                               14" For all bullets
 

                        .44                                 20" For all bullets

                                                              16" For low velocity wadcutters
 

                        .451                               16" For all bullets
 

                        .458                               14" For all bullets
 
 

                  *Stainless steel only

                                        Black powder barrels (1.250" x 30")
 

                        .32                       14"                  .320" groove

                        .38                       20" .                 379" groove

                        .40                       20"                  .403" groove

                        .457                     20", 14"          .457" groove"