General Principles
The general principles of hunting game with a shotgun, with respect to ballistic performance, are well established.
- Shot must be big (energetic) enough to kill at the range it is used.
- The shot cloud (pattern) must be sufficiently dense that our quarry cannot escape through the gaps between pellets.
As most of us are well aware:
“Shotguns work best with big loads of large shot.”
Unless one is in possession of a gun and cartridge combination which throws a sufficient number of pellets of sufficient energy, one will not reliably kill one’s quarry.
The primary factors influencing the density of the pattern thrown by a gun and cartridge combination will always be a) the quantity of shot, b) the size of the shot and c) the geometry and constriction of the choke through which it passes. A change in load weight, shot size or choke can account for a change in average pattern density of tens of percent and should be the first port of call if the behaviour of one’s cartridge is found to be unsuitable for the quarry hunted.
A list of secondary factors which can influence the density of a pattern is given below. None of these factors should be considered unless the cartridge employed is reasonably “balanced” and the gun has been configured to be broadly appropriate to the quarry hunted. Without pattern sufficiency (or being very close to it) for the quarry and shooting range in question, adjusting for factors that might influence pattern density by only a few percent is likely to be a waste of time.
The previous point deserves some emphasis. The fundamental factor in the effectiveness of a shotgun is the number of pellets in the cartridge. Imagine that we are intending to hunt small to medium game birds. If we have 300 pellets in the cartridge, (e.g. a 12-gauge hunting load), we only need approximately 40% of them in the standard circle at any given range to have a usable pattern: losing or gaining a few percent of average pattern density is of no consequence. After we’ve chosen the appropriate choke for the range we’re shooting, there is very little else worth adjusting.
Conversely, if we start with fewer than 200 pellets in the cartridge (e.g. a 28-gauge hunting load), a loss or gain of 5% average pattern density can equate to a noticeable change in the effective range of gun and cartridge. Since it is rarely the case that one wishes to lower pattern density, particularly in the smaller gauges, we can focus on the problem of trying to achieve better performance from the gun and cartridges we are able to acquire.
If we find that performance is not sufficient for our needs in a fixed choke gun, for instance, we do not have the option of screwing in a tighter choke tube to affect the pattern. If we cannot use a different gun for the task at hand, this usually means trying to adjust the cartridge to be better suited to both our gun and the task at hand.
Secondary Factors Affecting Shotgun Pattern Density
Most of the factors in the list below are related to the deformation (or not) of pellets as they travel from the cartridge / chamber, down the barrel of the gun, through the choke and into the environment. Pellet deformation is the major cause of “fliers” – pellets which exit the muzzle and fly on a curved path, away from the axis of the barrel, and do not contribute to the usable pattern which the gun throws.
N.b.: Un-deformed pellets will also fly away from the axis of the barrel, but will tend to do so along a straighter path and may still end up in the usable pattern, dependent on their initial direction of travel relative to the barrel axis and the range at which the pattern is measured.
Shot Size
Various physical factors mean that smaller shot is likely to deform to a proportionally greater extent than large shot. Furthermore, assuming all other factors – including proportional deformation – remain constant, the path of a larger pellet, having greater mass / momentum, will be straighter than a smaller pellet. The use of larger shot is therefore likely to increase the percentage of the original shot charge which ends up in the useful pattern at any given range. This does not, of course, guarantee that an absolutely larger number of pellets will end up in the pattern.
Shot Column Height & Bore Size
Substantial friction forces are generated between the shot column and the barrel wall when the shot charge is propelled by the combustion gases towards the muzzle of the shotgun. These forces can deform the pellets on the outside of the shot charge to a significant extent and cause them to become “fliers”. This is known as “scrubbing”.
Where the bore is small and the shot column tall (e.g. the 3″ .410 case) a large proportion of the pellets will be in contact with the barrel wall as the shot column travels down it. Naturally, greater scrubbing of pellets increases the number of that deform and fly rapidly away from the barrel axis and out of the usable pattern.
Many years’ experience has shown particular loads to be well-suited to each of the available gauges. Traditionally, this has been ¾oz for the 24 and 28 gauges, ½oz for the 32-gauge and 7/16oz for the .410. These “historical averages” upon which millions of hunters over many generations have settled can be taken as representing the best balance between implied velocity (lighter loads will tend to have higher muzzle velocities) and implied column height (lighter loads will have shorter shot columns) for a given bore size.
Plastic / Fibre Wad & Case-Chamber Length Synergy
This scrubbing effect described immediately above effect is particularly pronounced in cartridges loaded with a fibre wad, where there is no protective layer of plastic between the pellets and barrel wall. Furthermore, the use of a fibre wad can introduce detrimental effects related to wad damage which tend to be less significant with the use of skirted plastic wads.
There are two factors which, when they exist together, make fibre wad damage more likely: firstly, where the cartridge case is substantially shorter than the length of the chamber and second, where the forcing cones at the end of the chamber are short or non-existent.
If this situation occurs, it is possible for various detrimental effects to accompany it. Upon firing, the wad may be sheared on the edge of the end of the chamber if, as is likely, it does not fly exactly along the axis of the barrel. Combustion gases may also pass round the wad as it “jumps” from cartridge to barrel or because it was damaged upon entry to the barrel and does not properly contain the pressure behind the wad. Escaped combustion gases under high temperature and pressure may interfere with the pellets, which may become welded together or deformed through melting. Whatever the cause, this interference is likely to cause pellet deformation and reduce pattern density.
These effects are likely to be reduced or mitigated entirely if the gun has progressive forcing cones and / or the cartridge case length matches the chamber, since the opportunities for combustion gases to get in front of the wad will be significantly reduced.
Choke
Generally, we think of choke as contributing to increased pattern density, with more choke making for tighter patterns, but this is usually only true up to a point. Suddenly forcing a quantity of shot through an opening smaller than the space in which it has previously existed exerts substantial forces on the shot – some through direct impact with the angled wall of the choke, some through collision with other pellets so deflected. All of these collisions carry the possibility that the pellets will deform as a result, reducing their ballistic efficiency and increasing the likelihood that they will become “fliers”.
Whilst this effect can occur in shotguns of any gauge, it is more significant in the smaller gauges where there is literally less wiggle room inside the bore for the pellets to collide – to use a term borrowed from physicists – elastically. Rather, like an immeasurably more complex version of Newton’s cradle, the impact of one pellet into another, into another and so on will be much more likely to end with a collision into the essentially immovable barrel wall, ultimately making the collision inelastic with the energy transferred into the pellets as a deformation effect.
A greater degree of choke, achieved by using a larger angle of incidence rather than a longer choke tube, will increase the collision force of the outer pellets upon the tube, leading to larger collision forces affecting the “inner” pellets by transmission. This will increase the likelihood of and degree of pellet deformation observed and reduce the density of the resulting pattern, perhaps, in the worst cases, rendering it “blown”.
A “blown pattern”, in guns which are said to be “over-choked”, often results in a noticeable donut-shaped ring around the centre of patterns. The “hot centre” of the pattern remains intact, but the vast majority of the remaining pellets follow curved trajectories and impact the pattern plate close to or outside of the standard circle. This effect is more pronounced at long range.
Muzzle Velocity
In spite of the current trend in the UK towards the development of cartridges with higher and higher muzzle velocity, “speed” is actually the enemy of shotgun performance. It is telling that, outside of the UK market and perhaps the water-fowling market in the US, muzzle velocities of 1200-1350fps are considered quite adequate for hunting and there is very little interest in increasing them. Perhaps the majority of hunters worldwide feel that the prospect of reducing their shotgun’s performance at range, at the expense of significantly increased recoil is a trade-off not worth making? We may yet come to refer to the delusion that high muzzle velocity is desirable as “the English disease”.
The reason that velocity is the enemy of good patterns (and therefore, shotgun performance in general) is simple. Every one of the other factors described in this list relates to the deformation of pellets under mechanical force. In every case, increasing the velocity of the shot column will increase the forces exerted on the pellets, in turn increasing the likelihood and degree of deformation, degrading pattern quality.
If you doubt this fact, ask yourself why turkey hunters in the US often speak most highly of cartridges which have muzzle velocities of 1100fps or less, even though modern powders can drive their huge magnum loads much faster? The answer is that for them, pattern is everything and pushing the velocity to even 1200fps reduces the number of pellets that end up in the head and neck of the bird.
Shot Hardness
The ability of a pellet to resist deformation and remain spherical under stress is a property of the material of which it is made. Lead, a soft metal, is much more easily deformed than steel, Bismuth or Tungsten, though this property can be mitigated to some degree by the addition of small quantities of Antimony, and by the method of cooling employed when the shot is manufactured. Using harder shot can, to some degree, increase the quality of patterns.
Shot Lubrication
Related to the hardness of the shot and the degree of choke employed, the use of copper plating or powder-based lubricating agents on the shot (e.g. molybdenum) may reduce the degree to which pellets deform each other upon collision by allowing them to slide past each other and through the choke more easily. However it should be noted that, to date, the author has seen no convincing evidence for the improvements in performance which are claimed for these techniques.
Chamber Pressure, Wad Shock Absorption & Crimp
Chamber pressure is outside of the ability of all but the most dedicated reloaders to manipulate, as is the presence / absence of a shock absorbing stage in the wad of the cartridge. Very few brands of cartridges offer variation between loadings which extends as far as being able to choose a roll or folded crimp. However, the reason that attention may be given to all three factors is the same.
As the cartridge is fired, the burning powder expands rapidly and forces the shot column forwards. Prior to the column’s entry into the barrel, the resistance-to-opening of the crimp must be overcome, as the burning combustion gas continues to expand. This causes a rapid, short-term increase in pressure inside the cartridge and compression of the shot column until the crimp gives way and the column starts to move forwards.
Whilst this compression and rise in pressure is necessary in many cartridges for the powder to burn efficiently, it is damaging to the pellets and can cause deformation via crushing and – in some cases – “cold-welding” of the pellets, such that two or more of them become fixed together for the duration of their journey out of the gun. (This is often seen on paper / cardboard pattern plates as conjoined pellet holes.)
Lower chamber pressures, the use of roll rather than fold crimps and the presence of a shock absorbing section in plastic wads can all reduce the degree to which this crushing effect occurs, with a commensurate improvement in pattern density.
Analysis
The secondary factors described above are not all easy to manipulate. Some of them exist in opposition with each other and some are in opposition to the primary factors influencing pattern and performance listed at the top of this document. None of them can be fully mitigated – they are the inevitable by-products of the fact of igniting a charge of powder to propel a shot column out of a shotgun barrel. All of the effects they produce are particular to a given combination of gun and cartridge combination and their influence on such a combination can only be measured or spoken of in the most general terms, but not predicted.
What then, is the purpose of listing or analyzing them?
Taken together, they can provide a clue as to the steps which can be taken to improve shotgun performance in guns which seem unable to pattern beyond a certain percentage or give an even pattern at a particular range. There will always be limits: sometimes nothing will make a gun put more than a certain percentage of its shot in the circle at 40 yards, but understanding the factors preventing that may at least give an indication of adjustments worth attempting before settling on that conclusion. Indeed, sometimes the adjustments which lead to an improvement in performance will initially appear counter-intuitive.
Take, for example, the Eley “Extralong” cartridge for .410, loaded with #7 shot. Perhaps our imaginary shooter has tried the cartridge with every one of his .410s and found that none of them will put 120 pellets in the standard circle beyond 30 yards, in spite of the fact that they’re all tightly choked. (This is roughly equivalent to a standard 40% or “cylinder” performance.) What might he do to improve the situation?
Our shooter cannot use a bigger load – 18g is just about all the shot you can put in a .410 cartridge – and he cannot use a tighter choke because all of his guns have fixed chokes. It therefore falls to the manipulation of the secondary effects, through the use of an alternative cartridge, to improve the situation.
Our shooter might choose, counter-intuitively, to use a cartridge with a lighter load – perhaps one loaded in a 2½” case. Yes, performance might be damaged by the introduction of a big jump between the end of the cartridge and the end of the chamber and will certainly suffer from the lower pellet count, but the reduced influence of shot column compression might be enough to make a positive improvement in pattern density overall.
Or, our shooter might look for a box or two of the classic Eley 3″ subsonic load, containing 18g of #6 shot. True, because of the larger shot size, the number of pellets in the case will be 20% lower, but it’s not uncommon to find that subsonic loads pattern in the 80-90% range, such is the damaging effect of acceleration to supersonic velocities on pellets. Our shooter might find that in absolute terms, the number of pellets falling within the standard circle is doubled – perhaps achieving a usable number.
Perhaps, if – in spite of our earlier suggestion – we allow our imaginary shooter the use of a multi-choked .410, he might try loosening the chokes substantially. It could be that his pattern plate shows the tell-tale donut-shaped gap in his patterns and that the chokes of his other guns are simply too tight for good performance to obtain. Reducing the “crush” of pellets at the choke may tighten the patterns through reduced pellet deformation.
Of course, without patterning alternative cartridges with the gun intended for their use, this can be nothing more than a hypothetical discussion of possibilities. The reader should remember too that, if by some chance we were able to control every one of these secondary factors and manipulate them in the direction of theoretically better performance, we may still not see that occur in practice. An apparently perfect combination of adjustments might even damage performance, such are the vagaries of internal ballistics. Beyond a certain point, the success or failure of any given combination of gun and cartridge may simply elude all explanation, after which it is a matter for the gods.
Thanks for reading!