Shotgun Patterning
Importance of Patterning
Now that we've talked about the different types of ammunition available for use with your shotgun, it's time to start the real work. Once you have decided on the type of payload best suited for delivery on your target, careful consideration on how it gets there is required. This consideration happens in a study we call patterning which you conduct with all potential load combinations you've selected for a potential application. It is probably one of the most important things you will ever do with your shotgun.
While this section of our website is focused primarily on shotguns in a tactical application, patterning is so essential to extracting maximum performance from any shotgun we are going to talk about it from a slightly wider perspective.
Why is patterning so important? In order to illustrate the significance of patterning, let's imagine for a moment in slow motion what is happening in-between pulling the trigger on your gun and the projectiles striking their target. While it all happens so quickly, there are a lot of different things that happen and they cumulatively can have a great impact on the performance and ultimate effectiveness of your ammunition.
Inside the Hull and Chamber
Our starting point in this mental exercise is the shotgun's firing pin falling on the shell's primer, causing it to detonate. Ultra hot gases from the primer ignite the quick burning propellant, and pressure rapidly begins to build with violence. These gases push on the wad which, acting like a cushion absorbs some of this violence, and then the wad starts to push on the shot column. Now, the projectiles have considerable combined mass, and their inertia resists the movement of the wad, in essence crushing the shot against itself as it is forced to move and accelerate. In the case of lead shot, the lead is a relatively soft material and those nice little round pellets get dented and deformed significantly as they bash against themselves while resisting acceleration. While steel shot is somewhat harder and deforms less, it has what engineers call elastic memory and absorbs energy that we'll talk about in a minute. The hot gasses continue to expand, and the wad with its payload move out of the hull and into the barrel of the shotgun.
The Forcing Cone
There are many different types of shotgun barrels: some of them have 2 3/4 inch chambers, some 3 inch and some even 3.5 inch. Some shotgun chambers have a distinct lip which demarcates the transition from the chamber to the barrel, and others have what is called a forcing cone which is a smooth transition from the inside diameter of the chamber to the inside diameter of the barrel. The length of this transition will vary from barrel to barrel. Remove the barrel from your gun some day while cleaning and have a peer down the barrel from both the chamber and muzzle end to see how your smoothbore is set up. Regardless, the wad and shot in our imagination exercise now have to pass through this point before they can continue to travel down the barrel and further deformation and energy absorbsion by the shot occurs here.
Additionally, the wad likely does not fit tightly against the chamber wall, nor will it seal effectively against the wider part of the forcing cone. Gas from the burning propellant can leak around the wad and sometimes does so unevenly, possibly damaging the base of the wad which will likely further destabilize the shot column. This phenomenon is known as blow by.
Inside the Barrel
As the wad and shot column clear the forcing cone they accelerate rapidly as they are pushed down the barrel. Depending on the wad design and condition of the barrel, some of the ultra hot propellant gasses may slip continue to slip around the wad in as blow-by. Before they exit the barrel, they pass through the constriction of the shotgun's choke. The choke is a reduction in the inside diameter of the muzzle, and it varies in severity from no reduction in the case of "cylinder bore" (effectively no restriction) to significant, extreme reduction in the case of ultra tight chokes popular for turkey hunting these days. The entire payload is squished through the shotgun's choke, and many shotgunners liken its function as similar to the nozzle on a garden hose sprayer.
At the Muzzle
The shot/wad combination now leaves the barrel as essentially one big mass. At this point, any blow-by gas will expand immediately when the shot column leaves the barrel and will rapidly and greatly effect the dispersion of the shot. The wad begins to peel back, and the shot begins to tumble as it is subject to the effects of air resistance. In the case of steel shot, all that mechanical energy absorbed by the violent acceleration of the wad/shot column is released by the elastic memory of the steel. Shot pellets actually spring away from each other. In the case of lead shot, the no-longer-round shot pellets start to create all kinds of unpredictable aerodynamic forces as they begin to spin.
While the wad bleeds off speed and quickly falls to the ground once it has left the barrel, keep in mind that it is the last thing to leave the barrel and has been accelerated over a slightly longer period of time than the shot column. As such, it is sometimes traveling faster than the shot column and some high speed photographs show it traveling through the middle of the forming shot cloud. Some people believe that this is where the primary benefit to the choke is in that it retards the forward movement of the wad, giving the shot enough of a head start that the wad does not blow through the middle of the expanding shot cloud. This phenomenon is often debated, as other photographs show the wad falling back without passing through the shot column.
Traveling Downrange
Now, the shot column has completely left the influence of the shotgun and begins to bridge the gap between the gunner and the target. As it travels, it begins to disperse in three dimensions. It is important to re-iterate the notion of a three dimensional shot cloud, for the patterning exercise we are about to explore will only illustrate two of these three dimensions. While they are the most important ones, it is valuable to remember that not only does the shot cloud expand (or grow wider) as it travels to the target, but it also elongates such that all the shot does not arrive at the same time. External forces now come to play on the shot column as it travels downrange. Shot at the front end is subject to higher air resistance and decelerates faster than the shot at the rear of the column, and both wind and gravity exert considerable influence. By the time the shot arrives on target, many many many variables have been brought into play.
If anything, this little exercise in imagination should illustrate to you that there are a lot of different variables governing the performance characteristics of projectiles fired from a shotgun. While it won't give you a better understanding of these variables as they pertain to your particular gun/barrel/load combination, patterning your shotgun should allow you the opportunity to observe the cumulative effects of these variables on the dispersion of a particular loads' shot column. What you will learn from these observations will allow you to develop a partial understanding (partial because it does not tell you anything about the terminal ballistics of the shot pellets) of the maximum potential effective range of any given load, and to reiterate, it is probably one of the most important things you will ever do.
The Patterning Exercise
So when should you conduct the following patterning exercises? Any time that you consider changing loads, choke tubes, barrels, or the sights you have on your shotgun, some form of patterning is required if you are to have any understanding how they will perform at any given range.
Patterning consists of firing a given load through a given shogun/barrel/choke combination at a large piece of paper suspended on a frame. One round is fired, the target is examined and every pellet within the load is accounted for and circled with a felt pen. The target gets marked with the particulars of the load/shotgun data and distance between the shotgun and the target. The paper target is then replaced and the process is repeated with the same load/gun combination, only this time the range is increased. This procedure is repeated again and again, out to the maximum anticipated engagement range.
This process is extremely time consuming, as it should be repeated for every different load you anticipate deploying with your shotgun. Even loads of identical charge weight and shot size, but from different manufacturers need to be proved up on the patterning board, as your own empirical testing will undoubtedly demonstrate that they all pattern differently. The variables are numerous enough that even the same load run through two identical shotguns with consecutive serial numbers might pattern significantly different; so much so that the load could be an excellent fit for one gun, but an extremely poor choice in the other. Bottom line: the only way to see how that ultra-tight-printing wonder buckshot load that the gun rags are all raving about will perform in your gun is to get out and do the homework yourself.
While this book is targeted primarily at the application of a shotgun in a defensive role, we're going to talk about patterning in a much more general sense for several reasons. The first is a bit of a twist on Jeff Cooper's first rule of gun fighting (have a gun); any ammunition is better than no ammunition and as such it is useful to know how those light trap loads you have right on hand are going to pattern. The second is that, despite what we've talked about in the load selection part of the book, there are many people who will evaluate the whole equation (for example, those living in an apartment and who are significantly worried about target over penetration or building material pass-through) and still choose a birdshot load for use in a defensive application.
Required Equipment
The first thing you'll need to do is either find or build a patterning frame. Many trap or sporting clay ranges have fixtures (and some even have paper!) set up for patterning and are often very convenient to use. If you are not a member of such a facility and don't have any friends who have access to something similar, don't despair as one can be built in a few minutes on about $10.00 worth of lumber. You want the frame to be large so that it is less likely to be damaged by shot at extended ranges. On our pattern board, we have small, folding clips with string attached to them and a small weight attached to the end of the string after it passes through an eye attached to the corner of the frame. The clip is attached to corner of the paper target, and with all four corners attached, the weights hold the paper taught.
The next step is to find some extra large paper. To avoid downtime in their printing operations, newspaper printing companies often change their paper rolls before they run out. More often than not, they are happy to give these (what they call "end rolls" in our part of the world) away to someone willing to come pick them up. Meat packing paper also makes excellent patterning target material, as does Kraft style parcel wrapping paper. Often, it is a good idea to source your paper before you build your patterning frame such that you can build the frame around the dimensions of your paper.
After acquiring your paper, cut it off in segments of approximately 4 feet and mark a point of aim in the middle of each one. This is important, as when you shoot for it you will quickly discover and be able to quantify any deviance between your point of aim and your point of impact. You might be surprised at how different they could be. We'd suggest you mark with a black felt marker a large, solid dot approximately an inch in diameter. You next need to consider what the intended use for the load you are wishing to test. This consideration dictates the type of constraints you will use to establish the effectiveness of your pattern, and suggests the shape and size of the shape you should draw around your aiming point. The shape you choose should be considered in the context of your application, and for home defense, a life sized hominid silhouette (with an 8 inch circle drawn to represent the heart and lungs and another to represent the pelvis) is probably pretty applicable. Bird hunters will typically draw a 30 inch diameter circle, and those testing slugs for bear defense, hunting, or longer range type defense work will likely fall back to the silhouette model and draw an 8 to 10 inch diameter circle in the appropriate place to represent the vitals.
Conducting the Test
Now that you've built your patterning board and marked up your targets appropriately, assuming you've found a safe range environment to conduct your testing, you're ready to start! One of the most important considerations in the upcoming testing is range. The whole purpose of the patterning exercise is to develop a good understanding of what your projectiles are doing as they travel downrange. As such, you'll want to ensure that you have enough range increments to clearly illustrate what is going on. A good starting point is around 3 yards. At this range, almost invariably the shot cloud travels through the target as a single mass. With some loads though (especially higher energy loads), dispersion is already evident. Things which are useful to note on your target include where the wad is in relation to the shot cloud, where the shot cloud is in relation to your point of aim, and if there are any errant pellets. You'll then mark some of the particulars on the target such as the load type, which shotgun/barrel combination was used, what range the round was fired at, and anything else that may have been of interest.
After replacing your target with a fresh one, it's time for round number 2. This is usually around 5 yards. With shot, here you'll typically start to see some dispersion of the shot cloud. It is normally still pretty tight, however you may be able to differentiate the wad from the shot.and if there were any errant pellets on your first round, this phenomenon would be magnified and even more evident at this range. This target is marked up the same as the first, and then the process repeated on new targets at 5 yard increments all the way out to what your maximum intended range is.
Pattern Evaluation
So how do you establish the maximum effective range of your load? This will depend greatly on what your intended use for the load is. It becomes obvious through this patterning exercise that as the distance between the gun and the target increases, so will the dispersion of your shot cloud. How much shot is required on your target to get the job done? What are the risks associated with shot that has dispersed enough that it will bypass the target? Across many different applications, you will come up with different answers. Let's look at some of the more common ones.
If you are patterning buckshot for use in a big game hunting or defensive application, then you'll likely have marked the 8 to 12 inch circle as a target possibly inside a silhouette. The circle represents the approximate size of the vital organs you'll need to concentrate sufficient shot within to incapacitate your target. Evaluating the effectiveness of the load/pattern requires that you consider how much of the load is required on target in the circle and what happens to projectiles that fall outside of the circle. Do you have enough lead on target with enough kinetic energy to create the type of trauma you need for effective target neutralization? As an second consideration, it is also important to have a target paper large enough that at your maximum expected range, you can still account for all the pellets in the load as in this type of application, the pellets are usually significantly larger and carry with them more energy such that you are likely concerned with what happens to pellets that might bypass the target.
Slugs, while they fall into a category somewhat different than shot and will be discussed in their own chapter, require a similar type of testing as well. While you are not looking for patterns of shot, you are looking to develop a sense of both how accurate your gun/barrel/slug combination is at any given range and what the trajectory of the projectile is as it travels downrange. This is typically done by shooting 3 round groups at your target, working your way out in 20 yard increments to your maximum anticipated range. You'll likely be surprised the type of varied performance you get from one slug load to another. Finding one that groups well in your particular gun/barrel combination might take some work.
If you are patterning for a bird or clay load in a bird hunting application, you've most likely marked a 30 inch circle as your target with your point of aim directly in the middle. There is typically little risk associated with bypassing targets in this application, and as such what you are primarily interested in is whether or not there are any "holes" in the pattern where at less than three pellets are covering an area the size of the bird or clay. There are two things that you should look at here: the first is to count all the pellet holes and establish what percentage of them falls within your thirty inch circle. Usually 80% is where the pass or fail mark is set. If the 80% threshold is met, then the next thing you are looking for is how even the pattern is, or conversely how splotchy it might be. A nice, even pattern with 80% of the shot contained within the circle will usually have at least 3 or 4 pellets within the area of a clay or bird. Once you get to a range where you see "holes" in the pattern, the load/gun combinations' effectiveness drops off significantly.
Documentation
Conducting pattern testing with potential loads and your shotgun is so important that it is often worth the effort to keep notes on your experiences with a given gun/barrel/load combination. Given the versatility of the shotgun with respect to the myriad of payload choices you have, it can be a daunting task to remember which loads perform well and documentation is a great memory jogger.
Conclusion
Aside from safe handling, patterning is the single most important aspect of understanding and applying your shotgun effectively in a defensive application. You will build empirical evidence that suggests via first-hand experience the ammunition you might employ in any given situation in addition to developing a very vivid sense of both the advantages and limitations associated with this potent firearm system. A lack of this familiarization is a significant liability. Neglecting patterning familiarization in your training deprives you of this invaluable exploration; a significant risk and something that should be seriously avoided.