In the first article discussing the mechanisms by which a firearm locks its breech, we mentioned briefly two quantities that are important for small arms design: Pressure, and bolt thrust. The latter we’ll discuss in a later installment, but today we’ll discuss pressure, and why it is important for both the downrange performance of a bullet, and the design of a firearm’s mechanism.
In physics terms, pressure is simply the average force applied perpendicular to a surface, per unit of area of that surface. The best way to convey this meaning is to think about a balloon. Taking a balloon out of the package, it’s limp and unstressed. However, fill it with air and the pressure inside increases. Fill it too much – and it pops!
When a gun fires, a similar effect is at work. Just like inside a balloon, or in a champagne bottle, pressure is exerted in all directions after the ignition of a cartridge. Some of this pressure acts upon the base of the bullet, pushing it forward. Likewise, the pressure also pushes backward, against the case head and the breech face. In both cases, the pressure acts upon an area, and therefore the force against either the case head or the bullet is proportional to the area of the maximum internal diameter of the case and the bullet base, respectively. These subjects will come up again when we cover bolt thrust and swept volume, later.
Over the course of the firing cycle, pressure in a firearm will increase rapidly and then decrease more gradually, and the graph of this change in pressure for a specific load of ammunition is called its pressure curve. Over the course of the pressure curve, several notable highlights occur: These include the maximum pressure, which is the highest pressure level on the curve, the uncorking pressure, which is the pressure as the bullet is leaving contact with the rifling, and – in the case of weapons with gas systems – the gas port pressure, which is the pressure as gas enters the gas port.
The pressure of firearms ammunition can be measured in several units: Most commonly used in the United States is pounds per square inch (PSI), but the older copper units of pressure (CUP) is still in use as well. In Europe, Megapascals (MPa) is common, as well as occasionally bars or atmospheres. Importantly, the way that pressure standards are created is very important. It’s common for a cartridge to be held to a Maximum Average Pressure (MAP) limit, which is the average of the maximum pressures measured for a set of test cartridges. Note, the way these tests are conducted and how they are measured can be quite different, so it’s a good idea to only compare data from one source with data from the same source, even if the data uses the same units.
For the shooting hobbyist, pressure is an important piece of information of which to take note. Firearms produce tremendous internal pressures. As a comparison, an inflated balloon has a pressure of approximately 12-15 PSI. An unopened champagne bottle has an internal pressure of about 80-90 PSI. A high performance RD-180 rocket engine – one of the highest pressure rocket engines in existence – used on the first stage of an Atlas V rocket to lift payloads to orbit has an operating pressure of almost 3,900 PSI.
In contrast to these, even the lowly .22 Long Rifle is specified to run about 25,000 PSI, and it’s normal for a modern rifle cartridge to reach pressures of 60,000 PSI or more!