What Makes up the Anatomy of a Shotgun Cartridge
The Anatomy of a Shotgun Cartridge is really very simple but how it actually works and performs is not quite so easily understood.
Cartridges comprise of few components – namely a plastic (sometimes paper) case, primer, propellant, wad and shot – but it’s how these ingredients balance and work together that determines the shell’s effectiveness.
Anatomy of a Shotgun Cartridge – Primer
For instance, take a look at the primer – it might well be the smallest component part involved but it actually plays one of the biggest parts in putting a payload of pellets into orbit.
Different primers burn with a different “hotness” and the skill of every loader is to marry the right primer to the right powder.
Get it wrong the cartridge might go off like a damp squib or, conversely, kick like the proverbial mule.
In general terms a hot primer with fast ignition will be used in light, high performance clay shooting loads whereas a slower burning powder and primer will be favoured for heavier game and wildfowling loads.
By using the right combination of primers and powders our loader is able to control the way in which different cartridges come up to speed within safe pressure limits.
Anatomy of a Shotgun Cartridge – Crimp Control
Shot, wad and powder are held in place by either a rolled turnover (RTO) or a crimp closure.
The RTO is an old fashioned method but still used for shorter chambered guns such as the 2in and in the smaller bores to get a heavier load in the cartridge.
Crimp closure is virtually ubiquitous these days and will usually be a six star crimp, although the somewhat rarer eight star crimp is still used for higher performance competition shells.
Crimp closure tends to give better patterns than a RTO and is also intrinsic on managing the pressure and velocity of the cartridge.
Anatomy of a Shotgun Cartridge – Powder Power
The propellant used determines cartridge performance and is the hardest part of a cartridge’s make up to understand.
That’s because 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.
In days of yore when black powder was the only propellant around, guns generally had longer barrels than they do today because the extra length gave the powder time to burn completely and develop sufficient pressure to send the pellets speeding on their way.
Modern nitro powders behave very differently in that they burn more quickly and efficiently therefore making it easier to tailor a cartridge to different ballistic requirements, and a shooter’s specific needs.
To illustrate the point 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, yet both shot charges will leave the barrel at the same speed.
And that’s only the start of it.
High performance, fast burning powders for light loads tend to be what’s called ‘single based’ with nitrocellulose as the single main constituent making them more hygroscopic and, hence, more susceptible than ‘double base’ propellants to damp and cold conditions.
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. How can you tell the difference?
Well, cut one open and you will generally find that single base powders are flaked and have a larger surface area to ensure the burn is faster.
Double base powders, though, are more likely to be granular, with a smaller surface area.
Anatomy of a Shotgun Cartridge -Wadding
Next up is the wad, made from either plastic or fibre. Plastic wads are many and varied with the full cup version being most common.
However, different shot such as steel or tungsten, require thicker wads made from more robust material to protect a gun’s barrels.
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.
Fibre wads, sometimes referred to as felt, are made from biodegradeable materials and now predominate in game loads.
Plastic wads tend to have “petals” which 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.
This compression ensures that when the wad enters the barrel it expands to make the same seal.
Fibre wad cartridges often produce a little more felt recoil because they are loaded at a slightly higher pressure.
Anatomy of a Shotgun Cartridge – Shot
Finally, we come to the ‘real deal’ in every cartridge – its shot load. Usually manufactured from lead, shot contains a good many other elements of which the most important is antimony.
Antimony hardens the shot and most high performance clay shells contain up to 5% of the stuff to ensure clean breaks.
Game cartridges on the other hand carry softer shot that deforms more readily, thereby offloading energy on the quarry to bring about humane kills.
Antimony in game shot is usually in the order of 2 or 3%.
Occasionally cartridges are loaded with pellets coated in nickel or copper – the former creating even harder shot – but in both cases the coatings prevent pellets sticking together (balling) on firing.
Upshot of this is that they give better, more even patterns. The downside is that these coatings add significantly to the cost.
However, Gamebore’s patented Diamond Shot has the same effect of preventing balling under pressure and costs no more than standard lead shot.
The weight of a shot load will always be printed on the cartridge in grammes so the heavier the load, the more pellets it carries.
It could be argued that by using a heavy load we have a greater chance of hitting the mark; if only this were the case!
A faster burning powder is likely to be used for this light 24gm load of 7.1/2 shot (above) whereas something much slower will sit underneath the 36gm 4s (right).
A popular misconception is that heavier loads give us more “power”, but the reverse is true: heavy game loads tend to use slower burning double base powders which generate a lower muzzle and down range velocity.
The important thing is not to view this as a disadvantage – if a 36gm (1.1/4oz) fibre wad high bird shell had the same velocity as a 28gm plastic wad competition cartridge, we would soon suffer from exceptionally heavy recoil.
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