A Dummy's Guide to Shotgun Cartridges

When it comes to cartridges we generally know what we like and what works for us, but we don’t really know why.

It is understandable. Cartridges are a blizzard of numbers - shot size, weight, chamber length, muzzle velocity, pressure - as well as some quite complex chemistry and engineering.

But, knowing what all the numbers mean, what’s actually inside the cartridge case, and how all the components interplay with each other and your gun not only makes that purchasing decision easier, but makes it more likely that you’ll make the right choice for your particular style of shooting.

Anatomy of a Shotgun Cartridge

The components of a cartridge are pretty universal. Where things get complicated is when we look at the myriad variations within those components.

Starting at the back, we have the head, the metal base of the cartridge. This is sometimes called the ‘brass’, but is often made from nickel-plated brass. The length of the head can also vary, with some extending nearly half the length of the case, others measuring just a few millimetres. Brass length doesn’t not impact performance all that much, though it can have an impact on how (or if) the cartridge ejects from the gun once fired.

The head contains the primer, a small charge of shock-sensitive chemical. When you pull the trigger on your gun, some very clever engineering takes place inside the gun’s action, causing the gun’s firing pin to strike the percussion cap. The primer generates heat, which ignites the propellant (gunpowder).

The Propellant

The propellant sits atop the primer, and is typically precisely measured and blended nitroglycerin powders (unless its a ‘black powder’ cartridge). When ignited the powder burns rapidly, creating expanding gasses which generates enormous pressure, or - in layman’s terms, goes bang. This pressure is what propels the cartridge’s payload down the barrel of the gun. The exact choice and quantity of powder affects the amount of pressure generated, and thus the amount of recoil and the speed at which the shot leaves the barrel. 

The propellant is one of the most important ingredients in a shotgun cartridge. “We take great pains to make sure that we use the right powders for the right jobs,” says Paul James, Director at Gamebore. “In our high performance game range, small bore loads and premium lead-free loads we use Alliant Powder. The cutting edge double base powder gives a performance that single-based propellants can’t replicate. On the other hand, in our traditional game range we use Vectan powders, which still give great performance but with smooth recoil in lightweight guns.”

The Wad

Over the propellant lies the wad, the principal function of which is to form a gas seal between the propellant and the payload. Wads can be made of fibre, plastic, bio-degradable or hydro-soluble polymers, or even paper and cork.

Some wads also incorporate a shot cup - these help to protect the barrel from the shot as it travels down the bore, both preventing damage to the barrel walls when using steel shot, and minimising deformation of the pellets, giving better ballistics and patterns.

In recent years research and development into non-plastic materials for use in shotcup wads for steel loads has resulted in products like Gamebore’s Quad Seal wad, which is used in their premium, high performance range. This employs the same biodegradable material as the company’s steel cartridges to create a wad system that offers previously unachievable performance. A series of four ridges and furrows in the wad’s outer wall give them a greater sealing area even than conventional plastic wads, ensuring that the maximum pressure is preserved, generating greater striking energy, better consistency and reduced recoil.

The Shot

The shot, either inside or outside a shotcup, is the final ingredient. Shotgun cartridges are traditionally loaded with lead, though there are many other materials used, including lead coated with copper or nickel, which manufacturers say result in reduced pellet deformation, and a harder strike. The rapidly expanding gasses behind the wad generate a huge amount of pressure, leaving the wad and shot only one way to go - down the barrel, which it does with great speed.

From April 1st 2029 the use and sale of lead shot for live quarry and target shooting will be banned in the UK. There are a number of non-lead alternatives on the market, including steel, bismuth, bismuth, tungsten (Tungsten Super Shot), metal alloys and wildflower seed, even (deceased) people’s ashes, though those last two are unlikely to give a partridge much cause for concern.

“The quality of the shot is paramount,” notes Paul. “Whether steel, lead or bismuth, the more perfectly spherical the pellet, the better the ballistic performance. As the only UK cartridge company with a working shot tower, our lead shot is the highest quality in the world - our Diamond Shot is graded 5 times to make sure that it it is! We also make our own Bismuth shot, and source our steel shot from one of the world’s leading manufacturers.”

All of the above is contained in the case. Shotgun cases can be made from paper, plastic or biodegradable polymers, and come in a variety of lengths.

Understanding the numbers

A box of cartridges will display a variety of different numbers, and knowing what these mean (and don’t) is not only critical for making the right choice for your quarry, but for safety too.

Most important is the bore/guage. This refers to the diameter of the chambers. Using the wrong gauge cartridge is incredibly dangerous. While it is impossible to put a larger-gauge cartridge into a smaller bore gun, putting, say, a 20-gauge cartridge in a 12-bore will result in the cartridge getting stuck - firing a 12 bore cartridge in above the stuck 20-bore usually results in the barrel failing, and can result in serious injury.

Next, is the weight, which refers to the amount of shot in the cartridge, not the amount of powder/propellant. A 28g load contains more shot than a 26g load, and less than a 30g load (assuming the shot size is the same.) 

The next number is the shot size. The lower the number, the larger the pellet, the higher the number, the smaller the pellet.

The Case Length

Finally there is the case length. 12 bore cartridges are typically available in 65mm. 67mm, 70mm and 76mm. It is important to match the length of your cartridge to the length of the gun’s chambers - using a cartridge that is too long for the gun is dangerous. The length that is printed on the box is fired length, ie, once the crimp at the end of the cartridge has opened. If the case is too long for the chamber the opened crimp can extend into the forcing cone, constricting the mouth of the case and causing an obstruction to the shot and wad as they try to pass. This can result in higher pressures than intended, damage to the gun or even a dangerous failure.

The cartridge box will also indicate what kind of wad the cartridge contains. Most, if not all, game shoots and some clay shoots prohibit the use of plastic wads, while others have restrictions on the type of non-plastic wads that are permitted.

Finally, non-lead (especially steel) cartridges will indicate whether they are ‘standard’ or High Performance (sometimes called ‘Superior’). This is a subject best suited to a more detailed article, but the general rule is unless your gun is stamped with a fleur de lys symbol or ‘STEEL PROOFED’ you cannot use High Performance steel cartridges.

Choosing the Right Cartridge

Working out what cartridge is right for your particular needs is a game of experimentation, trade-off and personal preference, so there are no definitive answers. There are, however, some rules of thumb.

As a general rule, the larger the quarry, or the greater the distance at which you are shooting, the larger shot size you will need, so if you are shooting geese you will need a larger shot than if you are shooting ducks, and if you are shooting 50 yard birds you will need a larger shot size than if you are shooting 30 yard birds.

This basically all boils down to momentum. Not to get too technical, but the larger (or more accurately, the heavier) the pellet, the more momentum it carries (momentum = mass x velocity). Pellets with greater momentum maintain speed further down-range and penetrate better. A useful analogy is to consider throwing a ping pong ball and a golf ball at a window with equal force - one will bounce off, the other will result in a glaziers bill. The golf-ball’s greater mass means it carries more energy.

Bigger Does't Always Mean Better

So, you might think that it is as simple as ‘bigger shot size = better’. Certainly that seems to be the view held by some, but there are, as stated above, trade-offs. For example, a 28g load of no.5 shot will contain fewer pellets than a 28g load of no.6. This results in a reduction in the likelihood of any one pellet connecting with the target, just by virtue of there being fewer of them. What’s more, larger pellets - or a larger quantity of the same size pellets - also require a larger charge to achieve the same muzzle velocity, so simply sizing up the shot doesn’t necessarily result in greater momentum. BUT, increasing the powder charge results in greater recoil.

The size of the quarry plays a part. It doesn't really matter how fast you push a load of no.7 shot, the pellets are unlikely to carry enough momentum to bring down something big like a goose. Conversely, shooting a small bird like a partridge with a no.4 more suited to long-range pheasants will result in a bird that is not fit for the table - unless you like a lot of feathers and bone-fragments in your game terrine.

You also need to factor in the weight of your gun - heavier guns better absorb the recoil of more powerful loads than lighter guns, while some older guns may have their insides scrambled if you use too heavy a load.

Ultimately, choosing a cartridge is less about finding the “best” cartridge than finding the right compromise for your gun, your quarry, and your shooting. Try different cartridges in your own gun. Pattern them if you can. Pay attention not just to what kills cleanly, but to what you shoot well with. Confidence counts for a great deal, and the cartridge you trust is often the one you will shoot best with. 

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