Cicognani steel bushings

They are available in .001" size increments throughout the range of .185" thru .368", covering all calibers from .17 to .338 and clearly marked with size I.D. By selecting the correct bushing, you are now able to provide just the right amount of neck tension to properly hold the bullet without excessive resizing.
To determine proper Bushing size, measure the outside neck diameter of a loaded cartridge with a precision micrometer, then subtract .001" to .002", to allow for brass spring back and bullet tension.

— Tech Talk —
Bushing Selection and Use

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 smallest average neck diameter and this diameter bushing will generally size case necks to create the proper press fit for the bullet.
Note: If the neck wall thickness of your cases is on the thin side of the SAAMI tolerance, your fired case necks will measure considerably larger (.006-.010” larger) than your loaded cartridges. Under these circumstances, our tests have shown that a bushing .001“ larger may give the desired results.
Another method is to measure the neck thickness with a ball micrometer. 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, we have found that lubricating case necks and installing the bushing numbers down may improve results. Many 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 some Instances.

CAL. 6 PPC range from .256 to .260