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Adjusting Sizing and Seating Dies

By: Mike Bellm
     With some understanding of measuring techniques you can do a pretty fine job of determining what the throat to breech face measurement is, what the overall cartridge length is, how to seat to this measurement, and how to adjust your size die... all based on simple measurements and the threads per inch on your die parts.

     Take the barrel off the frame first!

     If you drop a sized case into the chamber, its head should be flush or protrude no more than a couple thousandths. If it does not, you best learn now how to set a size die, because you are absolutely correct about these break open guns being different from bolt guns. This same case with a bullet seated way out can be dropped into the chamber, leaving the head sticking out the end a measurable amount. Measure this distance the case protrudes with your caliper or a depth mike if you have one or access to one.

     The difference between protrusions is the amount you need to seat the bullet deeper to get to "0." You will then know exactly what the overall length is. You can seat the bullet down to get "0" protrusion, or simply subtract the amount of protrusion from the round's present overall length to arrive at the distance from throat to breech face for that given bullet. If you measure the actual gap between your barrel and breech face with feeler gauges, you can get it down to the thousandth of an inch.

     You don't need any gauges or gizmos to do the job. Most of these gizmos are like fishing lures designed to catch fishermen and make a buck rather than catch fish.

     While on the subject... you have a micrometer on your seating die and did not know it.

     Follow this. Let's use an RCBS seat die for an example with a 1/4-28 thread.

     100 divided by 28 = .0357. So one full turn of this 28 thread per inch seat stem equals .0357" of movement up or down.

     Half of this is .0179," half of this or 1/4th turn is .009," and half of this or 1/8th turn is .0045." Follow the logic? While you cannot readily get it down to the last thousandth of an inch, you can get mighty close, normally plenty close to the order of magnitude needed to be of practical value. Do the same thing with any one's seat stem, no matter what the thread pitch is. Just divide the number of threads per inch into 1 inch to find out the value of each full turn.

     You can "eyeball" a reference point, or make marks on the die itself. I just note the position of the stem and turn it however many turns or fraction of a turn I need to.

     If you cannot determine exactly how many thread per inch are on the stem, you can buy a little thread pitch gauge from Brownell's or one of the tool supply houses. Or you can hold the stem against a ruler/tape measure and count the threads in one inch as close as you can. So far as I know, all seat dies use standard thread forms, usually National Fine thread. Look at any chart of thread sizes and find the one with the diameter and threads per inch closest to what you counted.

     This same principle is quite handy for adjusting size dies also. You have a 14 pitch thread on the die body. That is .071" per turn. Following the logic outlined above, you can with a little practice get your size die adjustments down into the order of magnitude necessary to stay within the normal .006" maximum headspace allowance.

     For example, you performed "The Experiment Every Contender Shooter Should Perform" on my web site and found that you need to bump fired case shoulders back about .010" to get to "0" headspace. 1/8th turn is .009," rounded off, so that would leave you just .001" headspace... perfecto. No jamming the case head forward or up when you close the barrel.

- Mike Bellm

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This article reprinted with permission of Mike Bellm and
Warning: All technical data mentioned, especially handloading and bullet casting, reflect the limited experience of individuals using specific tools, products, equipment and components under specific conditions and circumstances not necessarily reported in the article or on this web site and over which The Los Angeles Silhouette Club (LASC), this web site or the author has no control. The above has no control over the condition of your firearms or your methods, components, tools, techniques or circumstances and disclaims all and any responsibility for any person using any data mentioned. Always consult recognized reloading manuals.