January 04, 2011
By Glen D. Zediker
Proper case sizing is crucial to loading safe ammo that will function properly in your rifle.
By Glen D. Zediker
Determining proper sizing die adjustment is best accomplished with a gauge such as the Hornady
LNL. By comparing new and fired cases, you can determine the right amount of set-back and adjust the die accordingly.
Certainly one of the most basic operations in reloading, cartridge-case sizing, deserves more attention than it often gets. This should help.
When a round is fired, a few things happen to the case. All factor, in some way, into the reuse of the spent case. Brass is both elastic and plastic. It's elastic in that it will expand and contract; it's plastic in that it will also exhibit permanent dimensional change and won't return fully to its original state.
Under heat and pressure upon firing, brass flows. The case swells to the limits of the chamber, in all directions. The case head is slammed back against the bolt face as the case shoulder is blown forward, filling and conforming to that area in the chamber. The case neck likewise expands to fit against the chamber walls, as does the case body.
Momentarily (all this is momentary) the case is essentially the same size as the chamber, assuming we're using a full-power load--by which I don't necessarily mean a maximum load but one that's delivering factory-level velocity.
Now we need tools. Get a full-length sizing die. I don't care what you're reloading for. A neck-only sizing die, in my experience, is an appliance and a method that should be restricted to single-shot, high-precision rifles only.
I know all the ideas touting the superiority of neck sizing, believe me. One often given is that by not sizing the case anywhere but the neck, and often sizing only a tiny portion of the neck, then we're preserving the "fit" between case and chamber that will result in superior accuracy. Nope.
Any rifle with an ejector on its bolt face that's pushing against one side of the case base will not eject a "straight" case. Also, even the best-made repeating rifles aren't perfect. While they do in fact meet their tolerances, there has to be some "gap" to make the rifle run. That's especially true in a semiautomatic, of course, but it applies equally to virtually all repeating rifles. Similarly, there has to be a little gap in the ammunition.
The only folks who see a benefit from neck-only sizing (keep in mind what we're saying is that the case neck is the only area sized) are benchrest and perhaps long-range, single-shot competitors. These rifles are constructed with such small tolerances that it's common for a benchrest shooter, for instance, to get 100 uses from a case--and see the case get shorter rather than longer during its life.
But that's not "us," the shooters who reload for accurate hunting or general recreational ammo. I promise you that not only will neck-only sizing not make your rifle shoot better groups, it will make it more difficult to operate.
A full-length sizing die contacts the entire case body and nearly all the case neck, and it can also be adjusted to contact the case shoulder area. A "small-base" die is a variation on the full-length and constricts the case body even more. Some semiautomatic rifles need this, but most don't. I've not yet encountered a .223 Remington, for instance, that needed a small-base die, but I have loaded for some match-conditioned M1As that benefited from additional reduction.
Cartridge case headspace is a critical, crucial thing to understand--and, of course, control. For virtually any sporting rifle chamber, any new case will be undersize. That means it's smaller in diameter and shorter. If we fire a brand-new case in a rifle chamber, what emerges is a fair representation of that chamber. Brass contracts about 0.001 inches after expansion. That varies on the brass brand, and some exhibit more or less change after firing. Likewise, not all brass brands or even lots of the same brand will have exactly the same pre-fired dimensions.
Bushing dies allow you to control the amount of case neck reduction. Even though these can be run without an expander, the author doesn't recommend it.
Chamber headspace and cartridge headspace aren't exactly the same thing, but they're measured in the same way. In a chamber it's the distance from the bolt face to the datum line on the case shoulder (for a bottleneck cartridge). Whoever cuts the rifle chamber determines chamber headspace.
The datum line is a point on the case shoulder that coincides with a certain diameter used for a given cartridge. For a .308 Winchester, for example, the datum is 0.420 inch. If there's insufficient chamber headspace (too short), the round may not chamber or not without deforming the case shoulder. If the headspace is excessive (too long), then there's too much room for the case to stretch; structural failures can result.
I strongly recommend purchasing a cartridge headspace gauge. Some gauges are like go/no-go chamber headspace gauges because they provide a means to see that a case is within standards. These are the "drop-in" gauges that have a step machined into their bases that defines a minimum and maximum. To use, drop the case into the gauge and sight along the surface of the gauge to see that the base of the case is within the steps.
This type of gauge usually will keep your ammunition safe, but you get a lot more control if you use a method that provides a numerical reading. I prefer gauges such as the Hornady LNL that produce a reading which correlates to what a gunsmith would read when chambering a barrel.
Use such a gauge to check a new, unfired case. Write down the measurement. Check the case again after firing. Write that down (write everything down.) Now use the gauge to set the sizing die to produce a case that's been adequately sized but not sized too much.
Here's how. Thread the sizing die down toward the shell holder on the press ram until it touches the shell holder. Back it off a couple of turns. Lube a case and run it into the sizing die. Check with the gauge. You'll see that the case is getting longer rather than shorter; that's because the case body is being squeezed inward, making it taller, sort of like rolling a piece of clay on a tabletop.
Turn the die down toward the shell holder a little at a time and keep checking (making sure the case is continually lubed).
When the case ceases to get longer, that's usually because the case shoulder has finally made flush contact with its corresponding area inside the die.
Turn it in a little more and soon you'll get a reading that's the same you had from the fired case. Any more sizing now comes from the case shoulder being pushed on or "set-back."
My suggestion for a case sized for reuse in a semiautomatic is 0.003 inches of shoulder set-back. For a bolt action, 0.002 is good; 0.001 is the minimum. Flush is too long. There's a huge difference in how easily reloaded ammunition chambers between something with 0.000 set-back and 0.002 set-back. There will be no difference in group sizes.
Now, what if the die is turned all the way down flush with the shell holder surface and you're still showing no or inadequate set-back? This happens. Once it's flush, there's no more case going into the die. At least a few times a year I get this question from someone. While you can complain to the die maker, the faster solution is to take it to a local machine shop and get a little shaved off the die bottom.
Lubing is an essential part of sizing cases, and the author thinks Imperial Sizing Die Wax, which is applied with the fingers and cleans off easily, is the best he's tried.
An alternative to traditional expanders is to use an expanding mandrel. It's a stand-alone operation, separate from case body and case outside neck sizing.
Now that we've sized the case body, it's time to consider the neck. The case neck has to be sized to accept a bullet--and of course it also has to retain the bullet. That means there's some amount of tension or constriction the bullet must overcome when it's seated.
Since a case is a tube, there's an inside and an outside diameter on a case neck. The case outside diameter has to have adequate clearance between it and the rifle chamber walls, and the inside has to be smaller than bullet diameter to retain the bullet.
Different brass brands and lots will exhibit differences in outside diameters, whereas their inside diameters will be the same when the case is prepped for reloading using a standard sizing die. This is due to case neck wall thickness differences.
To someone using a standard sizing die this may not matter much. In these, the case neck area dimension is fixed (sizing dies are cut with a reamer similar to what's used for cutting a rifle chamber). For anyone wanting to use a sizing die with interchangeable neck sizing bushings, then it matters--a lot.
It's just math: Measure case-neck wall thickness, double it, add the bullet diameter, then subtract the desired amount of constriction. For instance, 0.012-inch wall thickness on a 0.224 caliber will result in a loaded neck diameter of 0.248 (0.224 + 0.024).
If it's a magazine-fed semiauto and we want 0.003 constriction, that means a sized case-neck outside diameter of 0.245 will do it. However, since brass is elastic and shows that to the tune of about 0.001 inch "spring back," choose a 0.244 diameter bushing. Easy.
For a bolt rifle, you want 0.002 constriction. Another 0.001 won't hurt. More than that will stress the case neck and shoulder; less constriction increases the risk of a bullet getting pushed into the case during feeding. Lighter constriction does not automatically equal better accuracy.
As with cartridge headspace, measuring the outside diameter of a fired case neck gives a pretty accurate representation of this corresponding dimension in the rifle chamber. Most sporting rifles (or let's say anything that's not been custom chambered) will provide more than adequate space for case neck expansion so the bullet can release.
Full-length sizing dies are going to have an expanding button--some call it a sizing button. It's there to set the case neck inside diameter. It works by opening up the neck after the neck outside has been brought down by the die itself.
This little thing has a bad reputation with many, and that's because it can cause a displacement of the case neck. One reason is that many dies--from experience, I'd say most--bring down the neck outside diameter an excessive amount. This places a good deal of stress on this area when the case is retracted over the expander.
Bushing dies reduce this a good deal, as can a machinist by opening up the neck area a little bit in a sizing die. There doesn't have to be more than 0.005 (difference between actual and desired) outside sizing for the expander to do its job.
One of the ideas in using a die with bushings--which means we can set the amount of sizing the outside diameter gets--is it makes it possible to run the die without its expanding button installed. However, unless you are willing to very closely monitor the ultimate inside diameter of case necks each loading, I don't recommend running a die without an expander.
With a drill, the right abrasives and some care, you can polish the expander for better operation.
And don't be fooled by checking concentricity (via runout) of a case neck sized with and without an expander. Just because the one sized without the expander will almost always show less runout doesn't mean all you may think it does. If there's any thickness difference in the case neck walls, a die with no expander pushes them toward the inside of the case neck and the expander pushes them to the outside. Make sense? Check runout after seating bullets to know if the expander is hurting anything (assuming we rule out the bullet seating die as a source of any problems).
My advice is to polish the heck out of the expander. Chuck up the expander stem (with the expander installed on it, of course), in a drill and run it against a fine whetstone to round any sharp edges. Then polish it using 600 grit emery or abrasive compound.
Go carefully. If you encounter a combination that won't adequately downsize a case neck to hold a bullet, be a little more aggressive with this routine and simply reduce the expander diameter.
Once you done that, if you're conscientious about lubing case neck insides, sizing with the expander will give you safe and consistent ammo as the brass ages and the neck walls stiffen.
The other option is to use an expanding mandrel instead of a traditional expander. It functions in the same way to establish correct inside diameter but does so with less potential harm. It's a stand-alone operation, separate from case body and case outside ne
Glen Zediker is the author of Handloading for Competition.