What's inside a shotgun cartridge?
How a shotgun cartridge works
There are five parts of a shotgun cartridge. :
- A plastic (sometimes paper) case
Primer in a shotgun cartridge plays one of the biggest parts of putting a load of pellets in action.
Different primers burn with a different “hotness” and the skill of every loader is to marry the right primer to the right powder. If the primer is wrong the cartridge could give a strong ‘kick’ or just fade and falter.
In light, high performance clayshooting a hot primer with fast ignition will generally be used.
In game and wildfowling a slower burning powder and primer is favoured.
A skilful loader will use the right combination of primers and powders, enabling superb control of the way in which different cartridges come up to speed within safe pressure limits.
Crimps and rolled turnovers
A crimp closure or a rolled turnover holds the shot, wad and powder in place inside the shotgun cartridge.
The rolled turnover (RTO) is an old-fashioned way of doing this but still in use for shorter chambered guns such as the 2in and in the smaller bores, so that a heavier load can be fitted in the cartridge.
Crimp closure is popular nowadays as it tends to give better patterns than a RTO. It also manages the pressure and velocity of the cartridge.
Usually today you will see a six star crimp closure. You might see a rarer eight star crimp sometimes, which is still used for higher performance competition shells.
The propellant used in a cartridge dictates its performance.
Different powders are purposely designed to generate speeds and pressures specific to the quarry being pursued, and for use in guns proofed to fire them safely.
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Black powder used to be the only propellant available. In those days guns had longer barrels because the extra length gave the powder time to burn completely and develop sufficient pressure to fire the pellets.
Today we have modern nitro powders which burn faster and more efficiently. So it is easy to create a cartridge for different ballistic requirements and a shooter’s specific needs.
A fast burning propellant might reach full speed and performance inside the barrel within 12in of the standing breech, whereas a slower powder may take up to 18in of barrel length to do the same. However both shot charges will leave the barrel at the same speed.
High performance, fast burning powders for light loads tend to be what’s called ‘single based’. Nitrocellulose is the single main constituent making them more susceptible than ‘double base’ propellants to absorbing damp. To counteract this, single-based powders contain stabilisers such as diphenylamine.
Slower burning double base powders tend to be used in heavier loads and have nitrocellulose and nitro glycerine as their two main constituents. They also contain oxidisers to sustain combustion.
If you cut a single base powder cartridge open you will see that it is flaked with a larger surface area to ensure the burn is faster.
Double base powders are more likely to be granular, with a smaller surface area.
A shotgun cartridge wad is made from plastic or fibre. However more and more cartridges are now being made with fibre wads so that they biodegrade. In addition, many shooting grounds now ban the use of plastic wads.
Cartridge wad variants include dispersant wads, for maximising the spread of the pattern and Bior wads, which are partial cups designed to deliver the best pattern distribution for a chosen discipline or quarry.
Plastic wads tend to have “petals”. These expand to create a seal within the bore of the barrel whereas fibre wads have to be compressed when they are placed in the case. Fibre wad cartridges often produce a little more felt recoil because they are loaded at a slightly higher pressure.
The most important element in shot is antimony. This hardens the shot and most high performance clay shells contain up to 5% to ensure clean breaks.
Game cartridges on the other hand carry softer shot that deforms more readily, offloading more energy on the quarry to bring about humane kills. Antimony in game shot is usually around 2 or 3%.
Occasionally cartridges are loaded with pellets coated in nickel or copper. Nickel creates even harder shot. However in both cases the coatings prevent pellets sticking together (balling) on firing.
As a result they give better and more even patterns. However the coatings do make the cartridges more expensive.
You will find the shot load weight printed on the cartridge in grammes. The heavier the load, the more pellets the cartridge carries. This does not however mean that you have a greater chance of hitting the target.
Heavier loads do not offer more power either. In fact, heavy game loads tend to use slower burning double base powders which generate a lower muzzle and down range velocity.
This is now a disadvantage because if a 36gm (1.1/4oz) fibre wad high bird shell had the same velocity as a 28gm plastic wad competition cartridge, it would soon suffer from exceptionally heavy recoil.
The ‘power’ of a cartridge relates to velocity but for gameshooting it derives from the kinetic energy of the pellet. So a larger pellet size will therefore have more striking energy for a given velocity.