Functional ammunition through correct sizing


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Tapered cases that are fired at high pressure in chambers that are too deep may need more frequent trimming.

The principal function of the brass cartridge case is to contain all the necessary components of the load, but it is also crucial to the correct and safe functioning of the ammunition, whether it be factory or handload. Besides being a receptacle for the powder and bullet, it expands to seal the breech against gas leakage and takes away some of the heat of the gun.

All cases commercially manufactured anywhere in the world for a given calibre must conform to precise specifications as to external dimensions. This is assured by a number of controls by standardizing bodies such as SAAMI (U.S.A) and CIP (Europe) who assign very cartridge a maximum set of pressure values which are then used as control points by ammunition manufacturers to ensure that the maximum average pressures generated by factory ammunition will, in all rifles, be the maximum allowable. This is a remarkable feat considering the extremely small tolerances to which ammunition must be fabricated, but it is true, and shooters and handloaders benefit greatly from this system of standardization.

However, although external brass dimensions are standard throughoutthe industry, internal dimensions and certain other charactaristics such as hardness of case heads are left to the engineers of each individual manufacturer and may vary considerably between brands or even lots from the same brand-name maker. Obviously if case walls are made thicker and external dimensions are standard, the volume of the cases (powder capacity) will be reduced, and this will have an effect upon powders, charges and pressures. Similarly, up to a point, if case heads are made harder and thus able to withstand higher pressures, charges may be safely increased in a given case brand beyond a point which might be safe in another brand of equal quality.

This means that the handloader should sort his fired cases by headstamp, and preferably by lot as well, and keep his various batches separate. If he is careless, and assembles ammunition using three or four different makes of case indiscriminately, his reloads will surely produce widely varying pressures from shot to shot (which is detrimental to accuracy) and he may, in extreme cases, even encounter dangerous pressures.

Research has shown that there’s no fundamental difference in the ultimate strengths of the modern rimless and belted cases. Due to differences in hardness and web thickness, it’s possible to find a lot of rimless stuff that will withstand more pressure than a given lot of belted brass, or vice versa. But overall, the two types of cases are equal in ultimate strength. Perhaps it’s academic from the handloader’s point of view, but the strongest cases in manufacture today are probably the Winchester Short Magnums and Remington SAUMs. Brass is the starting point for all reloading operations. The reloader can effect a real saving because he saves and re-uses the cartridge case which is the most expensive part of the whole round. Once he’s purchased a die set, all it costs him is some labour to get the case ready to use again. We start assembling a new round by reprocessing the fired case – expelling the old, fired primer, and resizing the body and neck of the case. But while resizing is a necessary part of the reloading process, the amount of resizing you have to do depends upon a number of factors. Knowing exactly how much resizing to do is the key to making your reloads function better. It is important that reloaded rounds chamber easily and that the case neck holds the bullet firmly.

The single most critical factor in determining the amount of resizing necessary is whether you are going to use the cases in the same gun. Problems sometimes arise if a shooter who has only one gun attempts to reload brass obtained from another source. Maybe he picked them up off the ground at the range? Or, maybe someone gave him a quantity of cases fired in another gun? Either way, they may need full-length resizing before they will fit the chamber of his gun. If, however, the cases chamber easily in his rifle, they may not need full-length resizing, and he can get away with neck resizing only.

The factors that control this are the relative dimensions of the chamber in your gun and the original gun. Chambers may have a range of dimensions that are within tolerances and thus acceptable. But if your chamber is a little bit larger than that of the original gun, the cases won’t need full length resizing.

Most reloading is done using 7/8×14 reloading dies that are designed to squeeze the entire case down to dimensions slightly smaller than those specified for the maximum size case. This size is nearly always smaller than the smallest “standard” chamber, although in a few cartridge designs the diameter of the smallest standard chamber and largest maximum standard cartridge are line- to-line at the same size. Headspace in this condition might even show a bit of interference.

If both the die and shellholder are made to the “correct” size, when you push the case into the die until the shellholder touches the bottom of the die and the press can’t close any farther, the case will be resized to duplicate the dimensions of a new factory case. Reloading die manufacturers get as close to this ideal situation as they possibly can, but they try to stay near the maximum to keep from working the brass too much. That’s ideal isn’t it? Not really, because resizing cases back to factory tolerance isn’t what the handloader wants. His goal is to make the case fit closely the chamber of his gun. He doesn’t need to, nor does he want to bring the case back to factory new diameter each time he resizes.

For rifle calibres, it’s easy to control how much you resize the case body by simply resetting your die. Screw the die into the press until it touches the shellholder,then screw the die back out about 2mm from the shellholder when the press is fully closed. Nearly all rifle cases are tapered to some degree, although some use straight-sided cases. Backing the die off a bit leaves the entire body of the case just a little bit larger than before. This results in partial resizing. You can experiment with moving the die in or out to get no more than the amount of resizing you want. The idea is to resize the case as little as possible.

What happens to the neck when you set back the die? The neck isn’t tapered and backing off the die just moves the point where the shoulder blends into the neck without changing the neck diameter in the least. You’d think that this process would let the case’s shoulder move forward, but actually, it doesn’t. The position of the shoulder will have been determined by the chamber dimensions of the gun and stopping the neck sizing a bit short won’t change the headspace dimensions as long as the case is fired in the same gun. On rare occasions, I’ve seen cases that were fired in chambers that were so deep that they wouldn’t chamber in another gun even after full-length resizing to set the shoulder back to its correct location. But this is not a common happening.

There are some cartridges, especially those with tapered cases like the .300 and .375 H&H Magnums that are fired at high pressures in chambers that might be a tad deep, causing them to grow in length with every shot. If you have one of these, you may have to full-length resize every time, trim to length frequently and junk your cases after very few firings. You might also do well to back off a little on your powder charges. Most die manufacturers make dies designed to neck-size only. These dies don’t make contact with the case walls, and size only the neck. For some situations, such as varmint shooting, neck-sizing is often better than full-length resizing. It usually results in more accurate ammo because the cases are a closer fit in the chamber. Some say that cases last longer too, but I’d argue that point. Often neck resizing is all that’s needed, but if you are going on an overseas or distant hunting trip I’d recommend you either use new brass or full-length resizing your fired cases to ensure troublefree functioning.

One of the fundamental requirements of assembling accurate reloads is that the bullet must be absolutely aligned with the bore when the round is chambered. If the bullet is cocked, even at a slight angle, it will end up leaving the barrel with a little yaw and the group will open up.

For hunting rifle cartridges, the bullet is aligned to the cartridge by the case neck, and the cartridge is aligned to the barrel by the chamber including the neck area of the chamber. But if it so happens that the case neck isn’t concentric with the long axis of the case, you’ve got a crooked bullet.

The rule of thumb is that each .001”of crookedness in the bullet as seated will result in a displacement on the target of about .125”. Most factory ammo reveals a misalignment of from .002” to .006”. Each reloader must decide for himself how important to his purposes this element of accuracy is, but a fair assessment might be: for benchrest shooting, maximum acceptable bullet tilt of .0005”; for serious long-range varminting,, not more than .001” run-out; for general big game hunting, .004”, preferably less.

Guns with a shrouded bolt face that supports the case head on the centreline of the chamber produce more concentric cases than guns with flat bolt faces that allow the cases to lie on the bottom of a large chamber. When this happens, the case doesn’t expand symmetrically, it bulges more on one side than the other. After you neck-size only, you are likely to rotate the case into some new position that leaves the neck crooked in the chamber. But you don’t really have a lot of choice. A gun with a very large chamber isn’t likely to win any benchrest matches, but it can perform perfectly well for hunting.

As long as your reloaded cases chamber easily in your gun, you don’t have to be afraid to minimize the amount of resizing you do. With rimless, bottleneck cases that control headspace with the shoulder, it’s sometimes possible to increase headspace by setting back the shoulder. But this is rare, since good dies won’t allow this to happen because they bottom the shellholder before they start moving the shoulder back.

What it all boils down to is: the final test of case resizing is whether your handloads fit the gun. If your reloads chamber without having to force them in, you’ve resized them enough. Actually, the ideal situation is when the bolt closes on a chambered round with just a barely perceptible amount of “feel.”

This article was first published in Sporting Shooter, December 2010.


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Nick Harvey

Nick Harvey is one of the world's most experienced and knowledgeable gun writers, a true legend of the business. He has been writing about firearms and hunting for more than 65 years, has published many books and uncounted articles, and has travelled the world to hunt and shoot. His reloading manuals are highly sought after, and his knowledge of the subject is unmatched. He has been Sporting Shooter's Gun Editor for longer than anyone can remember. Nick lives in rural NSW, Australia.

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