Let’s Look Inside

Case Sizing Part II

Last time, we talked about resizing a spent cartridge case for best reuse in an AR-type firearm. The focus was on the outside of the case. This time we’ll talk about the inside, the case neck area. Options matter more than you might imagine. Before talking options, the first step is understanding the essence.

There are two diameters on a case neck, both matter. Inside diameter and outside diameter. On firing, the case neck will expand to fit the limits of that area in the rifle chamber. For safety’s sake, most factory chamber neck areas are fairly generous in size (compared to what we might get away with in a custom chamber and consistent brass). That is done to accommodate a wider variety of cartridge case specifications. Not all case neck walls are the same thickness, so not all neck diameters will be the same. There has to be room for the case neck to expand to release the bullet. If there’s not, then pressures can escalate.

In resizing, the case neck gets its outside squeezed down and then its inside gets opened back up.

Most conventional sizing dies are going to take the fired neck outside diameter down a significant amount. These dies incorporate an expanding appliance, usually called a sizing button (some, me too, call it an “expander”) that comes back through the case neck when the case is withdrawn from the die. This is affixed to the decapping rod. The button diameter determines the inside case neck diameter, and also then the outside neck diameter.

It’s inside diameter that’s important. Inside diameter determines case neck constriction, which some call case neck “tension,” and that matters a whole lot. It matters to safety and accuracy. It’s the difference in diameters of the bullet and the inside case neck. For a semi-auto it should be at least .003″. Less than that and retaining the bullet against movement prior to firing can be a question. This movement can be induced through inertia or impact. Much more than that (more than .005″ difference) and then the bullet may get its jacket damaged on seating, as well as having the bullet become an unwanted contribution to the sizing operation. If there is excessive seating resistance, the case shoulder may get additional setback.

Some, me included, are concerned with the amount of down and up in the sizing operation. No doubt, more sizing “works” brass and shortens its life. There are sizing dies that feature changeable bushings to specify the amount of outside case neck sizing. It’s possible, certainly, to use this to reduce the amount the expander opens up the neck.

I really don’t recommend bushing dies for semi-autos. One of the main points against them is that they don’t size the full height of the case neck. Not sizing the full neck tube is a contributor to the influence of the case neck “doughnut” I talked about in the October 2008 issue. (In case you missed that, this is narrow elevated ring of brass that increases constriction by reducing the inside neck diameter. It’s like rolling an O-ring down into the case neck, stopping it right at the case neck, case shoulder juncture.) As I hope makes sense, it’s also for this reason I don’t recommend sizing a case without a sizing button in place. This is easily possible with a bushing die. Squeezing 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. It can also influence accuracy, in a bad way, contrary to the intentions of following this procedure.

AR 1

Exaggerated, of course, but because wall thicknesses vary there literally
can be two different centers in a case neck with non-uniformed walls.
Which influences depends on whether the case neck was last sized
inside or outside. If you don’t neck-turn cases, it should be inside.

Neck Centers

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 that’s what it gets seated into. To see how literal this is, sizing a case with no expanding appliance, such that only the outside wall is touched, and then running it on a concentricity fixture will almost always show zero to very little runout. Take the same case and size it using the expander such that 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 that 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 so their wall diameters are consistent. When we’re using cases that exhibit neck wall thickness differences, 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.

Make life easier on case necks. Many dies have a sizing button that can be chucked into a hand drill (chuck up the decapping stem). It’s way wise to run it against emery paper to polish the fool out of the piece before even its first use. I use 320 grit. 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 bullet grip we want from the case neck. I’ve encountered a number of .223 Remington dies that needed a smaller diameter button to net the recommended .003″ constriction. 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 lube.

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

AR 2

This WW-brand case has .012″-thick neck walls. Double that to get .024″. Add that to the
bullet diameter, .224″, to get the outside case neck diameter of a loaded round. Of course
you can always just measure a loaded neck, but this progression of attaining numbers shows
more. For example, take that figure, .248″, and reduce it until you get the .003″ recommended
constriction amount. That means we need a ready-to-load outside diameter of .245″, which
would be an inside diameter of .221″. If using a neck-bushing die, it would be a .244″, or a
sizing button diameter of .222″, both account for the .001″ spring-back. (Always either add
or subtract .001″ from any sizing appliance to arrive at an anticipated net result. Brass isn’t
completely pliable, or plastic. It will rebound roughly this amount after any sizing operation.)

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