Most reloaders learn of two basic resizing dies for bottleneck rifle cartridges: the Full Length (FL) and Neck Only sizing dies.
FL dies will resize the entire case including the neck, body, and bump the shoulder. Neck dies will resize only the neck, and will not resize the body or bump the shoulder down. Neck dies help to maximize case life because the body/shoulder portions of the brass are not being constantly sized down and fired out during each iteration of use.
Most reloaders understand that neck only dies should only be used when 1) reloading fired brass for the same gun that the brass was fired out of and 2) only in bolt-action rifles.
That leaves the semi-auto / gas rifle shooters left with using a FL sizing die. Does this mean that people reloading for autoloaders are doomed to short brass life? Not at all.
FL dies are actually fairly misunderstood dies. Reloaders tend to read the die manufacturer's instructions in setting the die and use it as is. Manufacturer's instructions are correct in terms of getting the die set to size the brass to within SAAMI specifications which will result in properly sized cases in terms of proper feeding, reliablity, and use in any firearm chambered for the respective cartridge. But this also means that the brass is most often being sized more than necessary in terms of the shoulder setback process.
Before continuing, it is important that the reader understand what headspace is. Headspace refers to the distance between the breech face and the part of the chamber of the gun that stops the forward movement of the cartridge case. In rimless bottleneck cases such as the .223 Remington, the headspace of a rifle is the distance from the bolt face to the chamber datum line that contacts the shoulder of the .223 case. In terms of the .223 cartridge case itself, it is measured from the bottom of the case (head) to the datum line on the shoulder of the case.
When a bottleneck case is resized in an FL die, the body and neck are sized down, and the die pushes down/back or 'bumps' the shoulder of the case. This shoulder bump is the 'headspace' resizing of the case.
This aspect is what determines the life of a case being reloaded for a semi-auto rifle. If a reloader follows the FL die instructions, the shoulder of the case is often bumped down anywhere to .005" to .015" from it's fired state (and maybe even more, depending on the respective rifle's chamber). Assuming a reloader is constantly bumping the shoulder of the brass .015" from the fired state, there is going to be a constant stretch of the brass in the body, typically near the web. This constant stretching of the case and shoulder bumping from firing and resizing is what leads to the eventual wear of the brass near the web and leads to the case head separation.
So it is apparent that the reloader can minimize case stretch when reloading cases for a semi-auto rifle by minimizing the amount of shoulder bump during the resizing process. How do we accomplish that?
By using a headspace measuring gauge.
There are several tools to measure or compare the headspace of cases. One inexpensive option is the Hornady (formerly Stoney Point) Lock-N-Load Headspace Gauge. The Hornady headspace gauge is a device that attaches to the jaws of any standard caliper. It can be purchased with the body only, or in a kit with several bushings for most of the popular cartridges.
Hornady LNL Headspace Gauge Kit: Body (far left) and bushings.
Gauge attached to caliper.
In order to effectively manage the amount of shoulder bump during the FL resizing process, the reloader should measure several cases fired from the rifle in which the ammunition is being reloading for, using the headspace gauge. The resulting measurement of the fired cases will be the value used as the headspace 'length' of the rifle. Measuring several cases should ensure a correct 'average'; the sample fired cases should measure the same and not vary more than +/-.001", if at all.
Measuring a fired case. Headspace value approximately 1.459".
After that value is recorded, the reloader should work on setting the FL die appropriately. This will be trial and error, with the operator constantly adjusting how high or low the die is threaded in the press until the desired resulting headspace measurement is acheived.
Basically, start the die a bit higher than normal and run the brass through it. Clean lube off of it and measure. If the shoulder is still too high, turn the die down more and resize the case again. Clean the lube and measure. Repeat until the desired amount of shoulder set back is achieved. Test the setting with a few other fired cases.
What is a desired measurement? Bumping the shoulder back .002" from the measurement of the fired brass will be ideal. This ensures that there is actual resizing of the shoulder taking place while minimizing the amount of shoulder bump after each resizing.
Measuring a resized case. Headspace value approximately 1.451". .008" difference from fired measurement of 1.459"; a bit too much shoulder bump.
Measuring a resized case after die adjustment. Headspace value approximately 1.457". .002" difference from fired measurement of 1.459". Good amount of shoulder bump.
There are several other gauges out there used to measure headspace of cases, but the Hornady LNL Headspace Gauge is a readily available product at a respective price (~$45 MSRP for the kit; ~$30 street price).
Whatever headspace gauge is used, semi-auto rifle reloaders can still maximize case life while at the same time maintaining ammunition reliability using a Full Length resizing die. The shooter reloading for a semi-auto rifle is not hindered in any way by the inability to use implement a neck only sizing die.
Note: It is important to understand that the measurement taken of the fired cases is applicable to the firearm the cases were fired from. Brass resized using the above methods should only be used in the one firearm that it was originally fired out of. It is possible to use the same reloads in another rifle of the same chambering, assuming the headspace of the rifles are exactly the same.
Addendum: The above concepts can also apply to bolt action rifles. While many bolt action rifle reloaders use a neck only sizing die, eventually, the shoulder on cases that have been neck only sized continuously will eventually need to have the shoulder 'bumped' back. Some bolt action rifle reloaders full length size everytime for increased reliability. Using the above methods will work to get a FL die set appropriately for a particular rifle's chamber.
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