Reloading: Under Pressure

Pressure – the dictionary tells us that it is the ratio of force to the area over which it is applied. Let’s take a simple illustration. You’ll need an apple and a typical table knife. Put the back of the blade against the apple and push down… it just dents the apple. Turn the blade around, apply the same force and the apple should be cut in two (or you need new knives!). The same force was applied but because the contact area of the cutting edge was ‘X’ times smaller than the back of the blade, the applied pressure was ‘X’ times greater. Second fact, according to the rules of physics, pressure within a closed vessel is equal in all directions. Just blow up a balloon… assuming it has a consistent wall thickness then there are no low pressure dents or high pressure bulges.

THE ONLY WAY IS MUZZLE!
It’s this pressure effect that moves the projectile down the barrel, whether by gas (firearms) or CO2 and air in the case of airguns. In a metallic cartridge the brass case acts as the balloon, expanding through the effect of pressure until constrained by the bolt and chamber. The pressure relief valve is the projectile – if it fails to move then we have a potential bomb. When the bullet does move, the pressure energy of the gas is progressively imparted to it in the form of acceleration.

FIZZ TO BANG
We know that a given powder charge in a particular calibre with a specific bullet weight will give a muzzle velocity that falls within a predictable range. It is a fact that this charge has a finite calorific value. However, the conditions of burning will determine the rate at which this energy is released. If it is placed in a heap on the bench and ignited it will simply fizz brightly until consumed. At the other extreme, if constrained in a sealed space and ignited, the process will avalanche – the increasing gas pressure accelerating and so on until the container explodes or the charge is consumed.

We had the same amount of energy (calorific value), but released in a different way. If the bullet weight increases, the crimp and neck tension increase and the bore diameter reduces (increasing forcement) so the pressure within the case and barrel will increase, further accelerating the rate of deflagration and resulting pressure. Increasing the charge weight will inevitably add to this rate of increase. From this it is easy to understand that in a cartridge the percentage relationship between an increased charge and the resulting performance is not linear.

INDUSTRY STANDARDS
It’s partly because of this mix of conflicting variables that the industry has working data for almost every recognised production cartridge. As we have already discussed, both CIP and SAAMI publish not only dimensional information for the chamber and cartridge but a maximum working pressure. To establish the pressures generated by each loading in each cartridge the various bodies and ammunition manufacturers use either Copper Crusher or Piezo/Quartz systems. The former is a purely mechanical system in which a copper cylinder of defined dimensions is placed against a drilled cartridge case through the wall of a dummy chamber. When the round is fired, the copper element collapses to a disc or slug. The thickness of the slug is measured and compared against a standard conversion chart in order to read off the peak pressure that was generated.

LIGHTING THE GAS STOVE
The piezo/quartz electronic system arrived about 50-years ago… using the same basic principle that makes a B-B-Q igniter work. Basically, pressure deformation of a sliver of quartz generates an electrical signal, the amplitude being a function of the deformation. A piezo sensor fitted with a thermal protector is inserted into the side of the chamber/cartridge in place of the copper cylinder. This pressure deformation produces an electrical signal that can be computed to a value for pressure. With the use of an oscilloscope the shape of the pressure ‘wave’ can be observed, or the output digitised and viewed on a computer. There is a slight disparity between the results achieved from each of the two main measurement systems, especially in very small calibres such as .22 LR. Based upon these ‘standard’ figures a proof pressure is calculated which tests a firearm to a set percentage pressure above that specified as the safe working figure. All of which brings us to the point I’m trying to make.

SETTING THE LIMITS
Each of our shootable firearms has been proof tested, that is, it has had a ‘proof’ cartridge fired in each of its chambers – and survived without blowing apart or changing any of its critical dimensions. The resulting proof mark is a piece of insurance, telling us that the firearm is safe to use with factory ammunition of the calibre for which it is chambered. That leaves us with our handloads.

All of the loading manuals quote either a max pressure or a max velocity – or both. For a given calibre/bullet combination they will give a maximum charge for each of the propellants suggested as suitable for use with the cartridge. Where the manual quotes velocity we will see a wide variation in the maximum resulting figure for each of the suggested propellants. This is no mistake and should not be ignored. It is likely that the maximum velocity will be greatest with the slowest propellant that can achieve the maximum specified pressure.

As we move up the range of burning speeds of the specified propellants, so we will see an irregular but steady decline in maximum velocities. However, it is likely that they all generate a pressure that is close to the safe maximum. In a few instances the velocity will be uncharacteristically low – usually because even a compressed charge of the propellant fails to make the maximum pressure. It is because of these factors that we must never interpolate from one propellant to another without reference to both the safe pressure and maximum velocity. Get the sums wrong and you’re potentially in a world of hurt!