Case Sizing

If you are a handloader, pay attention. Since it’s a semi-auto, full-length spent case resizing for AR-type rifles is mandatory.

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 an entirely adequate job of getting the case to fit back into the chamber, and that’s the focus here. 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 with the press to give the case all the sizing it needs. More is better than less, but too much, as with many things, is excessive.

As a fired case, which is larger diameter and therefore also 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 that is reamed to fit it. 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 that will show this. Drop-in-style case gages are popular, but the best kind is one that gives a number that corresponds to chamber dimensions.
This next can’t be done without a gage, so get a gage. An option is shown. Use it to measure a new case. Then measure a fired case. 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 that has a headspace dimension longer than the chamber headspace dimension. It also means your barrel man done a bad thing, unless you requested it.

AR 1

Here is what we’re working with, and for. The case datum line on a .223
Remington is .330″. From the point of that diameter (dotted line) back to
the rifle bolt is headspace in the chamber, so from that line to the base of the
cartridge case is our concern in sizing. Get it .003″ under the chamber height
and we’re good to go.

Gage The Distance

As you’re adjusting the sizing die body downward (threading it into the press) 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 case shoulder “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. Call it good to go when it’s showing consistent cartridge headspace gage readings, and (one more time) they are .003″.

It’s easy. It’s also important. I don’t think it really matters to accuracy, but it sho’ does to function. Everyone who has read much about “precision” loading knows there are those who talk about “fitting” a case to a chamber and how that just has to improve its accuracy. Fitting a case into the chamber is more important and that’s why the .003″ set back.

AR 2

This new gage from Forster works extremely well. Their Datum Dial
accommodates virtually all bottleneck rifle cases, as well as having
additional rings that let it be used as a bullet comparator. This gage
measures off the case datum line, which I say is the correct means. Slick.

Bolt Actions

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 of the future spent cases. There’s less material movement on subsequent shots, and the brass that flows 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 when a round is chambered. Plus, residue buildup in a chamber can reduce net headspace a might (keep the chamber clean).

I’ve seen sizing dies with problems, and enough times to say, “What’s next?” Generally, the problems are from inadequate capacity to set back the case shoulder. If a die is threaded all the way down to the shellholder and still won’t push back the case shoulder enough to get the fired/sized difference recommended, then the die either has to have its bottom ground down or be replaced by one that will. Threading the die down farther does absolutely nothing but stress the press. The case will only go in so far. Any and all grinding should be done by a machinist, but it won’t cost much and usually is preferable to trying to get satisfaction from the die manufacturer. There are accessory shellholder sets that have different thicknesses but these also won’t solve such a problem. They are handy, though, when loading for different rifles that need different amounts of sizing on their cases.

Information in this article was adapted from The Competitive AR15: The Ultimate Technical Guide, published by Zediker Publishing. For more information, including many downloads, please check or call (662) 473-6107.
By Glen Zediker

Forster Products Inc.
310 E. Lanark Ave.
Lanark, IL 61046
(815) 493-6360

5875 W. Van Horn Tavern Rd.
Columbia, MO 65203
(800) 243-3220

Sinclair International Inc.
200 S. Front St.
Montezuma, Iowa 50171
(800) 717-8211

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One thought on “Case Sizing

  1. Garry Zelfer

    What’s the name of the device you have on the caliber? I can’t find it at any of the shooting supply company’s. Thank you Garry


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