Step back: Byron Pace takes a careful look at pressure build up

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I want to discuss some important aspects of safety and the production of better loads. For those of you who already reload, you may have noted a number of gaps in the presented knowledge base and equipment to this point. We are yet to discuss the use of chronographs or the understanding of cartridge pressures. Not every hunter is going to rush out and buy a chronograph, and in truth it probably isn’t needed for general hunting applications; hence the reason I have made no mention of it. We will be looking at this fairly soon, but for now we will focus on cartridge pressures and signs of excessive pressure.

Beyond the understanding of how pressure builds inside a cartridge on ignition, knowing the symptoms of excessive pressure is very important when it comes to loading cartridges, especially if pushing the boundaries in terms of hot loads.

Different calibres will have different pressure profiles, and importantly different max allowable pressures

Different calibres will have different pressure profiles, and importantly different max allowable pressures

Gunpowder by itself, in an open space, is of little danger. If ignited, it would burn fairly slowly and be far less dramatic than most people expect, producing only an audible whoosh and a few sparks as it ignites. In a confined, sealed space, the build-up of pressure with gas expansion helps to accelerate the rate at which the powder burns, leading to a rapid increase in pressure. The bullet, which remember is only fitted via friction into the neck of the brass case, is the stopper inside the pressured vessel. This is the weak point, and will be propelled out and into the barrel. If for whatever reason this is not the weakest point, then trouble can arise and a case blow out could occur. This would tend to be as a result of a fault in the case, and is unlikely to occur under normal loading conditions. It is, however, quite possible to load cartridges in excess of the pressure they are designed for. In this case there are normally a number of signs long before we get to a point where it could be dangerous.

As I have said before, loading has to be done with care, storing powders separately and never mixing them. It is not wise to test random powders for a calibre for which you have no load data. The most dangerous scenarios occur when a fast burning powder is used where a slow burning powder should have been loaded. Store powders in a dark, dry place at a steady, moderate temperature, and only use powders recommended for the bullet weight and calibre in question and you should be fine.

It’s the bolt face and locking lugs that resist the pressure of the cartridge. This also forms a component of head-spacing

It’s the bolt face and locking lugs that resist the pressure of the cartridge. This also forms a component of head-spacing

When looking at the pressure profile of cartridge ignition, we can see a steep increase in pressure to a peak

Of course there are other measures in place to prevent serious harm occurring from an over-pressurised cartridge. For a start, modern rifles are rated to far greater pressures than we will be subjecting them to. They are also designed with gas checks in the receiver to vent excess pressure away from your face in the case of an incident. This is, however, a last resort.

When looking at the pressure profile of cartridge ignition, we can see a steep increase in pressure to a peak, before it declines as the bullet is pushed further down the barrel. It is this peak in pressure that we are primarily concerned with, and governs suitable, safe loads for the calibre in question. This peak pressure usually occurs fairly soon after the bullet leaves the case, with neck tension and crimping increasing the force required and hence the highest pressure achieved. A bullet seated deep into the lands of the rifling will have a similar effect. This is an important point to note. As we will be loading rounds intended for hunting, our bullets will not be touching the lands, although as we saw in our tests on optimal seating depth, they may sit very close to the lands.

QuickLoad is an excellent piece of software for this, and if you want to push your loading beyond the most basic, it is definitely an essential addition to the armoury. By tweaking the additions of powder to the cartridge spec in terms of OAL (set by our seating test), it is possible to compare peak pressure to max load pressure, and in turn how this relates to case fill. As I touched on previously, this can really help when it comes to deciding on a powder for a particular bullet weight.

With excessive pressure you may be able to see the extractor slot imprinted on the case head

With excessive pressure you may be able to see the extractor slot imprinted on the case head

Take my latest load development as an example, shooting a 168gr A-Max through my .308 Win Mauser M03. On sampling a few of the powders I have in store, my initial testing is going to be completed with IMR 4895. This represents a good compromise across all the important factors. With the pressure curve peaking around 55,700psi, and a max pressure available of 60,191, it sits comfortably below on a 100 per cent fill, offering a fairly quick muzzle velocity of 2,721fps. I would likely be loading the bullet slightly further out as well, pushing the capacity marginally. Interestingly, it also offers a ballistic efficiency greater than 30 per cent (32.4 per cent). I say interestingly, because unless all your inputs are spot-on this can give some misleading results. Basically it gives you a measure of energy output compared with the powder’s potential energy. Anything 30 per cent or above is good. Another important aspect is the percentage of powder burnt, and this number should always be as close to 100 per cent as possible. You will note that increasing the length of the barrel, or reducing the charge, will alter this. On one hand there is no point having a case stuffed with powder if a good chunk of it remains unburnt, but on the other, I like to settle at a point just below 100 per cent, as this means at no point during the bullet’s flight will no powder be burning behind it.

Case inspection for pressure signs is an essential part of reloading

Case inspection for pressure signs is an essential part of reloading

Digging a little bit deeper into the analysis available also shows that at the peak pressure, the bullet will have travelled 1.42in down the barrel. With the gases expanding rapidly, the bullet starts to move, engaging the rifle, adding resistance and helping to build the pressure further. At the same time the space for the gases to expand in is getting bigger. At some point this exceeds to gas expansion rate and the pressure starts to drop. Looking at the graph output from QuickLoad it is easy to see that the velocity of the bullet is still increasing, and this is due to the continued burning of the powder down the barrel and further gas expansion accelerating the bullet. As soon as our bullet leaves the barrel it will only be getting slower.

Although QuickLoad is an exceptional guide, it should be used in conjunction with a reloading manual like the Hornady bible provided in the kit we have been using. Even still, you should be aware of the signs if a case is over-pressured. The first thing you may notice with a hot load is difficultly in opening the bolt due to excessive expansion, or even flowing of the brass. If this occurs you then need to examine the cases.

Factory load showing signs of a flattened primer, but this is not an over-pressured load

Factory load showing signs of a flattened primer, but this is not an over-pressured load

The first port of call is to look at the primers. Comparing a fired and unfired case side by side it should be fairly evident if the fired case has a flattened primer. There should still be a definitive bevel around the edge of the primer with a noticeable gap between the primer edge and the brass. After shooting the primer will vary in shape marginally, and a good way to gauge a hand load is to compare the fired case to a spent factory load from the same rifle. If the primer has been flattened, this could be a sign of excessive pressure, although this is not always reliable. It may also be apparent that the indent imposed by the firing pin is noticeably shallower, and this is a result of the excess pressure pushing the thin metal of the primer back out around the protruding pin. Do recall however that both of these can also be a sign of excessive head spacing. It should also be noted that some primer brands are softer than others, so a side-by-side comparison can give misleading results. An extension of this is primer cratering, which is where the metal from the primer flows back along the firing pin into the space between the pin and the hole in the bolt face. However, this can also be caused by excessive head space. It is also possible to have the firing pin pierce the primer with excessive pressure, but I haven’t personally seen this. A further sign can be a sooty discharge around the primer, but this can also occur with an overworked primer pocket that has become loose after a number of reloads. You should get a fair idea when priming the cases using the Hornady priming tool how uniform and tight the pockets are. After a bit of practice it is quite easy to feel the variations.

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QuickLoad is an invaluable resource and shows pressure profile limits for powder loads

Away from primers, the head of the case can also produce signs, with an extrusion of the brass around the head stamp flowing into the bolt face recess used for the ejector. Those rifles with a spring plunger ejector will show a distinct round ring as the excessive pressure pushes the plunger back and beyond the face.

 

It is also possible to measure for excessive pressures, but this requires callipers accurate to 0.0001in, and I have never had a need for such an item. Within normal operating pressures brass expansion should be fairly predictable for the particular calibre and make. With a benchmark set, you can then see if the brass expansion sits above the base line.

Next month we will look at the measurement of head-spacing before offering an alternative to the ladder test; a test which is actually my preferred method for powder development. n

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