In a previous article (Barrels Bulged and Burst, August 2017) I briefly touched on the potential danger of under-filling cart- ridges. Although a rare occurrence, the consequence can sometimes be devastating. This matter deserves closer scrutiny because it is one of the most perplexing problems many shooters and ballisticians are struggling with.
Very light loads are sometimes used to fire-form cartridge cases or when using a modern smokeless powder in a large capacity cartridge case such as those originally designed for black powder.
The cause of this violent occurrence remains elusive because of inadequate results from experiments that have tried to repeat the condition under controlled conditions, and because of the many contradictory opinions expressed. For example, reputable authors warn of the danger of using light loads of slow-burning powders in large capacity cases, while other equally reputable authors warn of using fast powders in light loads.
The phenomenon is sometimes called the Secondary Explosion Effect but both the term and the meaning are confusing and lack definition. However, I will venture to suggest some plausible hypotheses as to the cause. With this in mind, it may be interesting to take the first step in an engineering analysis by applying some simple physics, the rules of which will set concrete boundaries to exclude all the incorrect conjectures. I will try to explain this without a resort to mathematics.
Firstly, the terms power and specific energy must be explained. As with any fuel, a charge of gunpowder will produce a certain amount of energy in the form of hot gas to propel the bullet. This is the specific energy. The amount of this energy will be in proportion to the mass of the fuel, in this case, the weight of the charge. A reduced charge will release less gas to propel the bullet. The power on the other hand is determined by the rate at which this energy is released. In a conventional load, a fast powder will generate a larger peak power for a given weight because its specific energy will be spent in a shorter time. The sharpness of this spike in power is called the crest factor. This number indicates how extreme the peaks are in the waveform. In a pressure vessel like a gun chamber, the power generated is equated to pressure. This brings us to the first fact. There is no doubt that it is an exceptionally large crest factor that causes the failures.
Read the full article in the October 2017 issue of Magnum.