Case sizing: the inside

Guns Magazine, Feb, 2009 by Glen Zediker

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The "nicer" sizing dies are those featuring changeable bushings to specify the amount of case neck sizing. These bushings, of course, are dimensioned to the outside of the case necks but their ultimate effect, also of course, is on case neck inside diameter. Most dies incorporate an expanding appliance, usually called a sizing button (some--me too--call it an "expander"), which comes back through the case neck when the case is withdrawn from the die. These are affixed to the decapping rod. When used (most are removable), the button diameter determines the inside neck diameter.

Contrary to many competition-focus handloaders, I do not recommend removing the sizing button from a sizing die, unless other steps are taken to effectively (and more effectively) replace its effect. Not for what we're needing from case sizing. It is possible to select a neck bushing that will result in some inside neck diameter change, but unless we're using perfected cases, the inside sizing is responsible for the consistency and correctness of case necks.

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Now. Since the inside diameter is important, the question is why? I have an answer. One part is the inside diameter determines case neck constriction, which some call case neck "tension," and that matters a whole lot. It matters to safety and accuracy, and matters more to accuracy than some might assume.

Runout

In effect, there are two centers on a case neck, one outside and one inside. The inside matters most because that's where the bullet is, and is what it gets seated into. To see how literal this is, size a case with no expanding appliance, so only the outside wall is touched, then run it on a concentricity fixture and it will almost always show zero to very little runout. Take the same case and size it using the expander so the inside neck wall is the last thing touched, and another check with the fixture will likely then show runout. Expanders get blamed but the bigger thing is it's only pointed out neck wall thickness inconsistency.

If case neck walls aren't uniform in thickness, then whatever amount of inconsistency there is displaces either center. If we want the inside wall center to be in the center, then an inside expanding appliance should be the last sizing tool used on the case neck. The only case necks that respond better to no inside sizing are those on uniformed brass, and specifically that means they've had their necks outside-turned. When we're using cases exhibiting neck wall thickness differences, or not, however slight, having something to final-size the inside of the neck actually produces a more concentric case neck, from the bullet's point of view (literally). Go with that.

More Important

Inside sizing does something else of much importance for us. I don't strongly believe most of the neck-bushing-style sizers are of great help to the most of us with our rifles for one really easy reason, and that is they usually won't size the whole of the case neck. Not sizing the full length of the neck is a contributor to the influence of the case neck doughnut shown in the October 2008 issue.

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In case you missed that, this narrow elevated ring of brass increases constriction by reducing the inside neck diameter. It's just like rolling an o-ring down inside the case neck. It's located at the case neck/case shoulder juncture and, although other things contribute to its severity, is the result of the large differences in wall thicknesses between those areas. It even exists in many new cases.

As guessed, it's also for this reason I do not recommend sizing a case without a sizing button in place. Squishing down the outside diameter of the neck without opening its inside back up will, I promise, form a doughnut. The sizing button reopens the neck inside and helps alleviate the effects of this condition. Another option, discussed shortly, negates the majority of this last paragraph, and that's just the nature of handloading tool discussions.

I am a proponent of making life easy on case necks. Many dies have a sizing button you can chuck into a hand drill (chuck up the decapping stem). It's way wise to polish the fool out of the piece even before its first use. I use 320 emery. The difference in use, and its effect on the case neck, is astounding. If needed, the button can be run on the emery until it's the right size (smaller) to get the grab we want from the case neck. I've encountered a number of .223 Remington dies needing a smaller diameter button to net the recommended .003" constriction for magazine-led rounds. And lube the inside of the neck. Right. Some seem adamant about not doing this, but case necks ought not to squeak. I use plain old case tube.

The point of the bushing dies is being able to control the amount of "down" sizing a case neck gets. That's good, just not so truly necessary. The amount of "up" sizing ultimately matters more. I like bushing dies for bolt rifles, and l use them for gas guns too, with precautions. One reason I don't get as adamant about recommending them for the control they give us and all the other good points I could make is case life isn't going to be all that long with most .223 competition loads. Oversizing the brass, which bushing dies can reduce, is a concern only over the life of a case, not in its next use, so its value is directly related to how many uses one case will get. If its next use will be its last, and it's one or two uses since new, then there's really no point to it, as long as the die is correctly dimensioning the case for those uses.