The easiest way to determine the proper diameter bushing, is to measure the neck diameter of several loaded or dummy cartridges with an accurate micrometer.  Subtract .001”–.002” from the average neck diameter and this diameter bushing will generally size case necks to create the proper press fit for the bullet.  Another method is to measure the neck thickness with a ball micrometer or our Case Neck Gauge.   Double the neck wall thickness and add this number to the bullet diameter.  The result is the neck diameter of a loaded cartridge, and bushing size can be determined as above.  After loading several cases, it’s a good idea to test the neck’s grip on the bullet.  The simplest method is to push the bullet in a loaded cartridge against the edge of your reloading bench with moderate hand pressure.  The bullet should not move easily in the case neck.  If the bullet pushes deeper in the case, select the next smaller bushing and start again.  When using your bushing die, Redding has found that lubricating case necks and installing the bushing numbers down may improve results.  Many experienced reloaders like to adjust the die to size 1/2 to 3/4 of the case neck.  This has been shown to improve accuracy in most instances.