Shotgun Patterning from First Principles: Part I

The inspiration for this article (in several parts) comes from several sources. Firstly, it comes from the recent analysis the SmallBoreShotguns team have done for the Lyalvale 2″ .410 cartridge containing 9g / #6 shot where we said we’d discuss some of the problems inherent in analyzing the performance of cartridges containing fewer than what we’d call an acceptable minimum number of pellets. Second, it comes from a recent discussion on a major UK shooting forum where the question of equivalence-of-performance between 30″ and 20″ pattern circles was raised and the general difficulties of comparing differently-sized patterns was further discussed.

Despite appearances, the science and method behind patterning is not an easy topic, but we’ll do our best to discuss the difficulties involved in both comparing patterns (with a view to determining which represents “best” performance) and common approaches to shotgun patterning in general which, although better-understood, are not without their difficulties. We hope that this article may give some insight into the commentary we provide elsewhere on this site regarding the comparative performance of various small bore cartridges and the conclusions we have drawn from testing them.

An Imperfect Science

Patterning is an imperfect science. Many people think that, because it is not scientifically-perfect, it has no value at all – a view reinforced by the fact that many of the significant investigations of shotgun performance were performed at the beginning of the smokeless powder era, 80-120 years ago. They therefore see patterning as an anachronism and naturally assume that 100 years of progress in cartridge design has either solved or ossified all of the difficulties which that early work revealed, leaving nothing new to be discovered.

Contrary to this view however, it is the case that cartridge designs have advanced since the early 20th century and perhaps more so in the last 20 years than in all the years preceding. Modern cartridges are generally, very, very good and even cartridges theoretically unsuitable for a given purpose will – within reason – work most or some of the time. The most significant and recent advances have been in wad design and the formulation of powders. These components affect the performance of gun and cartridge just as much as bore size and choke constriction – if not more so – which serves to make continued investigation worthwhile.

Of course, it is also possible to put too much weight on the study of shotgun performance through patterning. There are certainly a minority who treat patterning as gospel to be followed, pored over and analyzed and who use often quite arbitrary distinctions to allow or disallow the use of cartridges which would, under normal circumstances, all be broadly adequate. The SmallBoreShotguns team disagrees with this approach as much as it rejects the view of patterning as an anachronism: patterning is an imperfect science but can be a useful guide, provided that one does not lean too heavily on it or allow theory to override tried and tested experience.

Destructive Testing

It is stating the obvious to say that, since it is impossible to break the same clay, or kill (or fail to kill) the same bird in the same presentation with two different cartridges, it is physically impossible to achieve any direct comparisons between the performance of two different cartridges. Even were such an experiment achievable – perhaps with the use of a time machine!? – it would require controlling many other variables; so many, in fact, that it’s difficult not to feel some sympathy with those who argue that it’s all too much trouble when the only data that can be obtained is indicative rather than conclusive.

However, like many of the more significant and well-recognized branches of science – astronomy, quantum physics, pharmacology, to name three examples – one does not have to have direct measurements or directly comparable data to draw useful conclusions.

Much is known in astronomy about the location, composition and behavior of stars because of the light that they produce, for example. We do not have to possess a sample of a star’s material to determine that it has a composition of mostly Hydrogen, with some Helium and a “signature” of heavier elements but rather detect that by the way it interacts with our telescopes and spectrometers. Likewise, we can make judgments as to the probable behavior of a shotgun cartridge via its interactions with our particular kind of sensor – in this case, a large, square piece of cardboard, paper or painted steel.

To emphasize: our data may be derived by indirect means, but that does not make it necessarily invalid.

We rely on repetitious experience: it is still possible, of course, that when someone finally manages to take a sample of a star and discover of what it is made, that our long-standing conclusions about the relationship of a star’s spectrograph to its composition will prove to have been a misunderstanding. There is no formula which absolutely defines the spectrum of light a star will produce if it has a particular elemental composition; the measurement of stars’ spectra and composition remains solidly based in empiricism.

Until then, however, we may say as spectra are to stars, so patterns are to shotgun cartridges: circumstantial but consistent and useful evidence from which performance in the field can broadly be predicted, supported by the millions of measurements made over hundreds of years which have most often reinforced rather than contradicted the principles generally held to be correct.

Controlling the Variables

Provided we accept that shotgun patterns can be indicative of performance and that continuous empirical testing over the last 120 years or more has elucidated the general principles by which the factors affecting performance are linked, then we can state with reasonable confidence that the way in which a shotgun pattern is created is primarily dependent upon:

  • the cartridge creating the pattern
  • the gun and choke through which the cartridge is fired
  • the distance at which it is shot
  • the ambient temperature, air pressure and other environmental conditions.

If the variable of interest is to be the cartridge itself, then to be truly scientific, one ought to control (or otherwise account for) all of the other variables in the list above (and, as we implied, a few more besides). Some of the ways the SmallBoreShotguns team address these issues are:

  • The pattern testing performed for other sections of this site is always done with the same test gun for a given gauge, using the same set of choke tubes.
  • The distances at which the patterns are shot are carefully measured with the same tape every time.
  • The paper used to “catch” the patterns is always the same.

Left unmentioned in the examples above are the environmental conditions which, without God-like powers, we cannot, obviously, control. This is perhaps the most obvious of the confounding factors which can affect the behavior of a shotgun cartridge, the pattern it throws and with it, our ability to determine in scientifically-rigorous terms, its performance. It would be marvelous to have the opportunity to test all of our cartridges in 15°C undisturbed air of fixed pressure, but no such facility exists. Muzzle velocities will therefore be higher or lower dependent upon the time of year and the prevailing weather, and patterns will change as a result.

Lest anyone think that we seek here to undermine much of the purpose of this site by stating that our data is suspect, let us say this: to a degree, it is, as is all pattern data. In admitting this however, we do not suggest that our data is worthless, but only that it requires a degree of interpretation (which we seek to provide) and that it remains – as we have said repeatedly – indicative, rather than deterministic.

Our interpretations of pattern data seek to take into account the variables we cannot control and afford them appropriate significance in reaching our conclusions. Often, this is more art than science, but it is backed up by broad experience, a strong theoretical understanding of internal and external ballistics and indeed, the better- and lesser-known conclusions of over 100 years of study into modern shotgunning.

To return to our example: we have observed the differences in cartridge behavior between sweltering summer days and the cold of winter in the data we have collected. (It is for this reason that we provide the relevant dates with the pattern data we have published.) We have therefore made the relevant observations in the analyses, but at the same time, recognized that the ambient temperature, though affecting the behavior of a cartridge, has a far lesser effect on pattern and performance than, for example, choice of powder. A change in muzzle velocity of 20-30fps because of temperature change between summer and winter is insignificant compared to a change of powder which might cause the muzzle velocities of two otherwise identical cartridges to differ by ten times that amount.

The question of controlling variables other than choice of cartridge therefore requires controlling the significant factors, whilst accounting for (but not necessarily controlling) the marginal factors whose effects will not exceed, for example, ordinary shot-to-shot variation in a given cartridge’s performance.

Conclusion To Part I

The justification for patterning shotguns is more easily found in empiricism than clearly defined scientific theory. To most, it is of no relevance; to some, a needless distraction; to a few, a means of supporting with evidence their tried and tested experience which brings confidence. To a handful of us, it is of great interest as a scientific pursuit, albeit one which the newcomer must recognize is necessarily vague, incomplete in places and heavily reliant upon interpretation.

Provided, however, that one is aware both of the limitations of what pattern testing can demonstrate and, most of all, that it is a study of the probable rather than the determinable, the application of the scientific method and statistical analysis to the data generated by it is not only appropriate but often enlightening. We will discuss the common approaches and their origins in the second part of this article.