Reloading basics: Tolerances in Reloading
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- Last updated: 21/08/2019
One of the biggest advantages of making your own ammunition, is that you have control over exactly what goes into each and every finished round, but just how accurate should you, or can you, be? The tolerances within which you work when making your own ammunition fall into two categories; those that are within your control and those which are not.
When you are making ammunition, the level of quality control you need to exercise can vary, depending on what you are going to use the ammo for. If you are shooting for fun, maybe with a pistol calibre carbine, then using a progressive press, you can happily feed cases and bullets into the machine straight from the box, with powder fed through an automated measure and be satisfied with the performance of the ammunition you make. If, however, you are making match-grade rifle ammunition for competition, then you are better using a single stage reloading process and checking each component of every round carefully (this can most easily be done on a turret type press with the indexing rod removed, so that you can move the turret manually for each step). In all cases, it is important to keep the amount of pressure you apply to the press when cycling rounds through it constant, this will affect all the steps in the reloading process.
The other element of quality control is buying the best quality components to suit your needs. This does not mean buying top quality match bullets for ‘plinking’, it simply means buying components that will perform sufficiently for your particular type of shooting. Often, home cast bullets are more than good enough for target shooting at relatively short distances and are, by far, the cheapest.
Weighing out each powder charge does take longer, but it is much more accurate and consistent than using a powder measure. For both .308 Winchester and 45-70 Government calibre, I weigh out each charge on an electronic scale but for .357 Magnum, which are shot in far higher volume and just for fun, a Lee Pro Auto Disc on a Lee Auto Breech Lock Pro does the job.
The quickest way to measure out individual powder charges is to use an accurate powder measure, the Lee Deluxe Perfect Powder Measure being my preference, to discharge a slightly lighter charge than needed into the pan on the electronic scale.
This is then topped up to the required weight using a powder trickler. The reason for this, is that the readout on this type of scale will have a limited degree of accuracy; for example, the figure of 3.1-grains will appear on screen for any weight between, 3.051- and 3.149-grains, so your powder charge will vary within that range. This degree of tolerance is normal for this type of scale. By slowly trickling powder onto the scale, the display will switch to the desired weight as soon as it enters this range at the bottom end, giving you far greater consistency. In the case of the above example, the powder weights are all going to be close to the 3.051 weight. This really does work and improves the consistency of the ammunition produced using this method.
It can also be worthwhile weighing your bullets, if time permits, depending on brand and quality. Brands like PPU and Sierra show very consistent weights within batches of bullets and so tend not to need checking. With some other makes, it is worth weighing them, to see if any fall outside an acceptable weight range. In .308 calibre, a batch of 180-grain bullets contained a handful that were around two grains lighter than the rest, which may not seem a lot, but it could well effect where they hit the target. They were still used and shot fine but were fired at a separate target from the others to avoid spoiling an otherwise tight grouping.
Even fussier, you can weigh each case to separate out any that weigh significantly more or less than the average. Case weight has a direct relationship to case volume. which in turn significantly effects pressure and velocity; this is really fussy reloading and tends to only be practiced by top end bench rest shooters.
The Cartridge Overall Length (C.O.L.) is something that is easy to check. Tolerances in bullet and case manufacturing may cause this to vary slightly but otherwise it should remain pretty constant. It is well worth keeping a dummy round on hand in case you need to reset your dies to the correct C.O.L. if they get out of adjustment. Measuring a completed round at the start, middle and end of a reloading session will ensure that the C.O.L. remains constant. If it does vary by more than a millimetre or two, check that the bullet seating die is not clogged with bullet lube in the seating plug.
Components like bullets are all made within acceptable tolerances and many of these are far too tight for us to measure without very expensive and accurate equipment. We pretty much have to accept that a .308 diameter bullet has a diameter of .308-inches.
With powders, the way they perform will vary slightly from batch to batch, so when one tub empties and a new one is opened, it is possible that your ammunition may perform a little different. Apart from avoiding starting a new tub of powder half-way through making a batch of ammunition, especially if it will be used in competition, there is not much you can do about this. If finances allow, you can buy more than one tub with the same batch number to keep things consistent for that bit longer, but other than that you might just have to tweak ammunition made with a new pot of powder. The point of impact is unlikely to change very much when you switch to a new tub of powder, but if it does move by an inch or two, then adjusting your rifle sight will be necessary.
Cases from different manufacturers and, to a lesser degree different batches of cases with the same headstamp, in any given calibre will vary and so it is best to stick to one maker and not mix batches of cases together. There is nothing you can do about the tolerances in case manufacture, other than stick to one brand. Two different brands of 45-70 Government cases I have differ in weight by around 26-grains each and all of this difference will be reflected in the internal volume of the cases and will affect pressure. Cases of the same make vary by up to three-grains and this is far less significant and unlikely to cause noticeable effects on the performance of your ammunition.
The level of quality control that you apply in your reloading is totally up to you, but the more care you take, the better your ammunition will perform. Using quality components is the easy part, but carefully weighing out powder charges, checking your components and separating them into consistent batches is time consuming and far harder. With the cost of reloading supplies constantly spiralling upwards, it is well worth investing time and money into getting the very best out of them.
Henry Krank & Co Ltd. henrykrank.com
Norman Clark Gunsmiths. normanclarkgunsmith.co.uk
JMS Arms. jmsarms.com
1967 Spud Reloading Supplies. 1967spud.com
Hannam’s Reloading Ltd. hannamsreloading.com
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