Savvy sizing: the case, that is

Guns Magazine, Dec, 2008 by Glen Zediker

Since it's a semi-auto, full-length spent case resizing is mandatory. Further, I think full-length resizing is mandatory for anything other than a Benchrest rifle, but I'll leave additional details dormant along with that statement, just believe it.

There are subtle and often touted differences in most of the better-grade sizing dies, but most standard full-length .223 Remington sizing dies do a fully adequate job of getting the case to fit back into the chamber, and that's the focus here. We'll look more closely at die designs in another article. I don't think it should be necessary to run a small-base sizing die (smaller diameter near the case head) for most competition rifles with the usual chambers, but it won't ever hurt accuracy.

The main thing is figuring out how to correctly adjust the sizing die body to give the case all the sizing it needs to have. More is better than less, but too much, as with most things, is excessive.

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As a fired case, which is larger diameter and therefore shorter than it was before firing, gets squeezed to a smaller diameter by a sizing die, it gets longer again. It gets longer until the case shoulder contacts the corresponding portion of the sizing die.

Back to the pre-sizing case dimensions: The case body will be bigger and the case will be shorter, but the shoulder area will be located higher than it was prior to firing. The shoulder, therefore, is taller than before. When a case expands fully inside the chamber, its shoulder moves forward as the case head moves back against the bolt and its body grows in diameter to fit the chamber walls.

Measuring overall case length (base to case mouth) doesn't really matter, not now at least. What matters is knowing the amount of case shoulder expansion, how far it moved forward or "up." Of course (of course) there is a tool or two to show this. Drop-in style case gages are popular, but the best kind is one giving a number corresponding to chamber dimensions.

The Gage

This next can't be done without a gage, so get a gage. My recommendation is part of a photo caption. Measure a new case. Write that down (write everything down). Measure a fired case. Write that down. Do some math. For best use in a semi-auto, the difference between fired case shoulder height and resized case shoulder height should be .003". Most cartridge case shoulders are going to be shorter going in when new than they will coming out after being fired. If they're not, that means the rifle is very tightly headspaced and therefore should never be fired with a round with a headspace dimension longer than the chamber headspace dimension. It also means your gunsmith done a bad thing, unless you requested it has.

As you're adjusting the sizing die body downward you're going to use this case headspace gage to check your progress. Again, you'll see the case getting longer, meaning the shoulder is getting higher, but that's because the case shoulder has yet to make contact with the die. When it does the case will stop getting longer. Keep threading the die body lower and checking the case shoulder height. When it reaches fired case dimension, proceed very carefully but turn the die down a tad amount more. Stop it when the case shoulder is .003" lower than it was on the fired case. That is called "set back." We have set back the shoulder on this case .003". Fix the die in place with its lock-ring, and then check more cases at that setting. Settle when it's showing consistent cartridge headspace gage readings.

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It's easy. It's also important. I don't think it really matters to accuracy, but it sho 'nuff does to function. I know you all know there are those who talk about "fitting" a case to a chamber and how that has to improve its accuracy. Fitting a case into the chamber is more important and that's why the .003" set back. A bolt-action rifle can get away with less, but nothing should ever be run that's not at the very least .001" back from fired dimension. I set my bolt-gun cases back .002" because I want the bolt to close easily.

Case-to-case consistency in this dimension might matter to accuracy, but the dimension itself, as long as it's "enough," really can't. If it did, then new cases, which can blow shoulders ahead a good deal in some chambers, wouldn't group as well as they do. There's no harm in setting back a case shoulder more than .003", but the reason for a minimum figure is some improvement in reuse. There's less material movement on subsequent shots, and the brass flowing forward does so from the case head. I run that little extra in my AR-15s compared to a bolt rifle to provide a space cushion to the shoulder during chambering. Depending on bolt gap, and whether or not it was addressed by a builder, there's often a little (to more than a little) additional and unintentional case shoulder set back after a round has been chambered. Plus, residue buildup in a chamber can reduce net headspace a might.

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