If you have ever wondered why, this post will attempt to explain the science of shotgun shells. But first, lets become familiar with the components that make up a shotgun shell. In comparison to most centerfire cartridges with shotgun shells the only component visible is the hull or shell casing, whereas a metallic cartridge you will see the bullet sticking out from the case. While this may seem confusing, there is a huge benefit, as many shotgun hulls are made of transparent plastic allowing the internal components to be viewed without the shell being cross sectioned. As is shown:
So an explanation of the components. From left to right, the brass colored section at the rear is called the base, which includes the extraction rim. The next visible component is the powder, the dark stuff viewable just in front of the base through the plastic body. Next is a concave cup that provides a gas seal, in front of that is the wadding, followed by the shot, and then the hull is crimped at the front to close it and keep the shot in. With this in mind, a discussion of each feature in descending order of importance.
Crimp
Shotshell crimps come in several varieties, the 6 and 8 point crimps are by far the most common these days, the difference in each is mostly determined by the preference of the manufacturer, cheap disposable single use shells most commonly have the 6 point crimp, and more durable reloadable hulls utilize the 8-point type. Two other types that have been common over the years are the over-shot card, which is most commonly used on buckshot loads. An example is shown here:
The other crimp, which is performed in a similar method to the over-shot card is that used on shotgun slugs. In this case the overshot card is simply omitted and the shell casing is roll crimped around the slug.
Roll crimping is performed using a roll crimping machine, or tool which rotates against the leading edge of the shell, forcing the rim to be rolled over, and will continue rolling the front down until it encounters the card or a slug. There are several sources for roll crimping tools. Ballistic Products manufactures a series of roll crimping tools for every common gauge of shotshell. The alternative is an antique roll crimping tool made by Ideal (now lyman), I couldn't find a convenient picture.
Crimp is one of the two most important features of any shotgun shell, because the type, length and geometry of the crimp determine the internal capacity of a closed shotgun shell. Folded multipoint crimps are the most restrictive in terms of internal volume and do not allow for much tolerance in terms of internal volume, a large shot, powder charge, and long wad selection will result in a recipe that will simply not stuff into the internal volume of the shotgun shell. This typically will result in the crimp bursting and all the shot coming out during handling. The inverse is also true, a reduction in shot, powder, or wad length will allow the crimp petals to cave inwards, again allowing the shot to spill out. In a certain sense, this is a good thing as it makes detecting rounds with an inadequate powder charge easy to pick out from properly loaded shells.
Roll type crimps (either for slugs, or for over-shot cards) are the most tolerant, but are only appropriate on shells that have been roll-crimped before, or new shells. Generally, the roll crimping process is not kind to plastic shells, and they are unlikely to survive more than one or two goes, paper shells tend to be more durable in this respect. That said, the advantage to a roll crimp is the internal capacity can vary more as the crimping process is not as rigid because the shell only needs to be able to fold back into itself to retain the components, it does not have to fold exactly closed.
Wad Selection
It is designed for a nominal load of 1 to 1-5/8oz of lead shot. For different loadings, the amount of powder or wad can be changed to make a good match for the internal volume of the shotgun shell you are loading it into. In Black Powder loadings, the WAA12R is a common wad, the buffer section (the part in the middle) is significantly shorter. In the WAA12SL, it is much longer.
The modern 1-piece wad has two functions that makes it so important, first, it compresses the gunpowder against the battery cup, and most importantly, it provides some elasticity to the internal volume of the shell. When the hull is being loaded, the wad will be compressed by some percentage, this has two functions, to tamp down the powder, and to make sure the shell is loaded to the correct over-all length. So finding a good match between your hull, the type of crimp you're using, your wad, shot charge, and powder charge is essential to making a good shell.
Powder Selection
Shotguns are very low pressure guns, where modern rifles may perform constantly at 60,000 PSI the shotgun would burst at those pressures, SAAMI lists a max pressure of 13,000PSI for 12ga shotguns. However, to complicate manners, the powder used in shotshells if not properly compressed with have difficulty reaching full ignition, resulting in a "blooper" as it's often called, based on the noise it makes.
The reason for the ignition problems is because shotgun bores are so large by comparison to their metallic cousins. The diameter of a 12ga barrel is .729" or thereabouts which is .5" greater in diameter than the venerable .30-06. This means that the volume of the 12ga will increase five times faster as the projectile travels down the barrel. So quick burning powders are a must. To keep pressures low, most shotguns use only 3-5 times the amount of powder used by pistols, where rifles may be 10 times (on a weight-weight basis) more powder.
Powders such as Hodgdon Clays, W231, bullseye, IMR700X are choice powders for loading shotgun shells. All of these powders tend to be very bulky, large flake powders, making them easy to ignite, and able to take up the necessary space at the base of the shotgun shell in addition to having relatively fast burn rates.
Hull Selection
The hull selection is quite critical in terms of internal volume, as the length of the hull will change where the crimp goes, however hulls are standard sizes (or very close to it) being at 2-1/2", 2-3/4", 3" and 3-1/2" each of these numbers represent the "open" length of the hull, or the length before crimping. While a 3" loaded round may fit into a 2-3/4" chamber, firing it would be very dangerous as the crimp would open up into the forcing cone of the chamber (think of this like the throat on a rifle) causing pressures to spike.
Hulls come in a wide variety of shapes, sizes, constructions, and internal volumes. So while hull selection is a critical factor for maintaining consistent and safe performance for a recipe, hull variation in a single brand/model is almost non-existent.
Primer Selection
I may be a bit cavalier, but in my years of loading shotshell, I have yet to notice many significant differences between shotgun primer types. They all seem to be about the same power, and same dimensions. While I tend to run minimum loads, I have never had problems switching between primer brands. But as a cautionary note, this is considered among the cardinal sins to adherents of the shotshell recipe cult. But it is worth noting that the #209 primer, no matter manufacturer should be exactly the same, as #209 is a primer standard that specifies both the size of the primer, the battery cup but also the amount of priming material and any other ignition characteristics of the primer.
Shot Selection
When it comes to recipe creation, the size of shot, the hole size in the bar that meters the shot, and the type of shot can greatly affect how the load will perform. The primary reason for this, is the smaller (the higher the number) the more densely the individual pellets can stack together in the shell. This is very easy to visualize when you try to stack a few pieces of buckshot into a shell, vs a few spoonfulls of #12.
As a consequence of these density variations, a load of #12 may be the volumetric equivalent of 1oz of #7.5, but may be 50% heavier, which could be a dangerous combination. The reverse is also true, 1oz of #12, could be equivalent to 3/4oz of #7.5, but the lack of inertia may lead to a blooper.
In this way, knowing your components, is a very critical factor when creating a shotgun recipe and tends to be one few people will mess with with any regularity.
As long as I'm on the topic of shot, there is more to load than simple wads full of shot, at least two manufacturers offer molds for casting your own slugs, lee and lyman are the largest, I am sure there are others out there. Lee however takes the most interesting approach. The Lee slug, is equal in length, to the length of a similar weight of #7.5 shot (approximately). Therefore if you are set up to load a 1oz #7.5 load, removing the shot bottle, and instead inserting a slug after the wad has been inserted and precompressed, and before crimping you can load your own shotgun slugs for pennies compared to what they cost at the gun shop.
Shotgun cartridges are much more complex than their metallic counterparts, they have more components and the way they are assembled seems to be a floating quantity. For most people, sticking to the recipe is the safest, that said, at least you now know why.
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