Reloading bullets: Working with tolerances

Reloading equipment

All reloading equipment, component and methods have tolerances that we have to work with - Credit: Mark Underwood

This month Mark Underwood looks at the tolerances we have to work within when reloading, and how to control what you can!

Quality control levels
The more effort you put into your ammunition, the better the results will be. But you should think about how much work it is actually worthwhile putting into ammunition for different applications. If you are just plinking at tin cans on the 25m range you won’t be looking for the same level of pinpoint accuracy that you need for shooting target competitions at 300yd, so do you really need to devote the same amount of time to your plinking rounds?

For pistol-calibre carbines, often used for fun shooting at shorter ranges, you can happily run a large quantity of rounds through a progressive press, with cases, primers, powder and heads all fed automatically, and produce perfectly satisfactory ammunition for the intended purpose.

However, if you are going to be making match-grade ammunition for competition, this level of automation is less desirable and you need to check each and every component as each round is put together. The more effort you put into quality control, the fewer rounds you are going to be able to make in a given time, so it really is a case of deciding what is the best balance between time and output for you.

I use a Lee Precision Breech Lock Pro press to make .357 Magnum ammunition, and with everything set up and components fed automatically, I can knock out 50 rounds in a matter of minutes. For .308 Winchester ammunition I operate a Lee Classic Turret press with the auto-index deactivated, and carry out each stage of each round by hand. When I work in this fashion, 50 rounds of this calibre can take a couple of hours.

Buying Quality
Quality comes at a price and, although reloading components are getting more expensive all the time, it is certainly worth buying the best equipment and supplies you can afford. Bullets and cases are probably the most quality sensitive of the components and with both the more you pay, within reason, the more consistent weights and critical dimensions will be.

You also have to balance cost and quality with affordable alternatives. Currently factory-made lead bullets in 45-70 Government calibre cost around £0.25 each, but you can cast your own at home at a fraction of this cost and they work very well at relatively short ranges.

Trickling up powder charges for bullet reloading

Trickling up powder charges will increase accuracy and minimise weight variances - Credit: Mark Underwood

Trickling up
The powder charge is absolutely critical to ammunition quality. To control the weight of your powder charges it is important to understand how your scale works, including its tolerances, and use it to its very best capabilities.

A digital scale will typically have an accuracy of 0.1gr, which means that it will display a single figure for a range of weights within that range. Such a scale will display 40 grains for any weight that falls between 39.951 and 40.049 so the powder charges weighed on this scale can vary by up to 0.1 grains (within the range between this maximum and minimum figure).

To manage this degree of accuracy you can use a process called ‘trickling up’. To do this you first put a powder charge that is slightly lighter than needed into the pan on the electronic scale. You can discharge this light load from a powder measure or by using a powder scoop.

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The next step is to add powder very slowly until the scale display just changes to the desired weight, say 40.0 grains. You can add the powder using a powder trickler or a scoop. By slowly trickling additional powder onto the scale in this way, you know that as soon as the weight reaches the lower end of the tolerance range you get a reading change. This produces far greater consistency and reduces charge weight variation. All your powder charges will weigh in at or just over 39.951 grains. This really does work, and improves the consistency of the ammunition produced.

This process is very slow, but with practice you do develop the ability to trickle powder in at just the right rate to get the scale display to change without going over.

Sorting bullets by weight

Sorting through bullets and separating them by weight is time-consuming - Credit: Mark Underwood

Weighing up
It can also be beneficial to weigh your bullets and cases. The purpose of this is to weed out components that weigh significantly more or less than the rest. It is unusual to find any bullets that vary in weight significantly, and when I recently checked a box of 180gr .308 calibre bullets, just a couple of them proved to be two grains lighter than the rest. A variation of 1% is of little importance, but precision shooters would probably weed out these lighter bullets. These bullets can of course still be shot, but are probably best reserved for a separate practice session. Higher-quality brand bullets such as PPU and Sierra do not tend to need checking.

You can also check each case to see if any weigh significantly more or less than the average. The external dimensions of cases are fixed to tight tolerances, so if a case is much heavier than the rest (indicating that it contains more brass), the only place this can affect is the space inside the case. Case volume directly affects chamber pressure and muzzle velocity. This process is time consuming and tends not to be done by your average reloader.

The finished product
Cartridge overall length (COL) has been discussed in a previous article, but is worth another mention. Checking it on the first round you make, one part way through your reloading and one at the end, will ensure that nothing has slipped out of adjustment. Digital dial calipers for reloading tend to be accurate to 0.001", which is pretty good, but this doesn’t mean you have to get hung up on getting every single round to measure precisely the same.

Sierra, with their millions of pounds worth of equipment, manufacture bullets to a tolerance of 0.001" and it would be hard for you to match that at home. The combined tolerances on your reloading press, dies, components and even your measuring equipment mean that there will be slight variations in your finished rounds.

It is also a good idea to weigh completed rounds. If you weigh a primed case and you know the weight of the bullet and the powder charge, you can add them together to give you the expected weight of a finished round. By weighing one completed round and then zeroing the scale, you can quickly weigh the others and see how much they deviate. If you find one is maybe overweight by the equivalent of half the powder charge, it should be pulled apart and checked out.

Calipers measuring a bullet

Accurate calipers will measure to 1/1000 of an inch but do not expect all your rounds to be the EXACT same length - Credit: Mark Underwood

Outside your control
All components are manufactured within tight and acceptable industry tolerances, which would be very hard for you to check without extremely expensive equipment. There is little we can do with these tolerances, and they will be one of the reasons that every bullet does not go through the same hole every time.

Powders will usually perform very consistently from batch to batch. But when you change to a new tub of the same powder you might get a slight shift in the point of impact. This is normal and you just need to tweak your scope to get back on target.

Brass cases are mass produced to very tight tolerances, but different manufacturers will use different sources of brass and treat them differently, so you need to stick to just one brand. The difference in weight between cases of the same calibre but from different manufacturers can be significant and, as mentioned above, this means the case capacity will be different.

Conclusion
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. This is why for some people reloading becomes a separate hobby in its own right, rather than just a way of making cheaper ammunition.

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. If there is one of these elements in particular that has proved to be the most beneficial for me, it is definitely trickling up to the desired powder charge. 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.