The Redding Type S Bushing Neck Die allows the reloader to precisely control case neck size and tension. Type S Dies use interchangeable sizing bushings that are available in .001" increments. Neck wall thickness varies from manufacturer to manufacturer. Depending on your situation, you will probably need a selection of bushings if you are loading different brands of brass. One way to measure bushing size is to use a ball micrometer and measure several cartridges neck wall thickness and then average the results. To that average you double it and then add the bullet caliber. Another way to find the correct bushing size is to measure several loaded rounds, and then you average the readings. The calculation you arrive at is used to determine your neck bushing size. Common wisdom suggests that you for every application you will neck a certain amount of neck tension.
For single loading in a bolt gun, optimum neck tension would be 0.001”.
For loading from bolt gun from the magazine, optimum neck tension would be 0.002”.
For loading from auto loading gun from the magazine, optimum neck tension would be 0.003”.
For an example in the 308 Winchester the measurement came to 0.015”. So, we double that and add .308” bullet diameter; yielding a reading of 0.338”. So your bushings would be 0.337”/ 0.336” / 0.335”.
