Shotgun cartridges - inner workings

The purpose of a shotgun cartridge is to propel a ‘load’ of pellets (shot) towards a target, whether clay or game, at a fast enough speed to be able to ‘kill’ the quarry cleanly.

View to a kill

A kill is made using the energy of the pellet which is calculated as mass x velocity2. In basic terms it takes 11/2 lbs/ft of energy to break a clay and about 41/2 lbs/ft to kill a pheasant humanely. Other factors need to be considered as well, such as the size of the pellet, weather conditions and so on.

Construction

So what does it take to propel the shot at the target?  A cartridge is built using the following elements (from back to front):

Exterior

  • Case - this is a tube made of either plastic or paper.
  • Base wad – this forms the bottom end of the case and is made of high density polyethylene.
  • Brass head - this is crimped together with the case and base wad to hold them together.

Explosives

  • Primer - this protrudes into the base wad and is a very important part of the cartridge.  It contains a cup, a cap which encloses the primer itself, and the anvil which detonates the primer using pressure.  Priming compound is highly explosive.  NEVER TRY TO DISMANTLE A PRIMER AS THIS COULD CAUSE SERIOUS OR EVEN FATAL INJURY.
  • Propellant – placed in front of the base wad, propellant is a chemical compound which is not explosive but burns very rapidly.  Contrary to popular belief, it is not gunpowder (which is used in fireworks and called black powder in the trade).  Propellants are specially developed by each cartridge manufacturer, the grade of compound depending on shot weight and purpose.

The wad

This separates the propellant from the shot and forms a gas-tight seal. When the propellant converts from solid to gas when ignited, it forces the wad and shot charge forward. There are three types of wads:

  • Plastic – the most common type of wads which is a single piece comprising a cup to hold the shot, an obturating disk to form the gas seal and a shock absorbing section holding the other two elements apart.  Most plastic wads aren’t biodegradable or decomposing, although photodegradable plastic (PDP) is now available which breaks down over time and is more environmentally acceptable.
  • Fibre – vegetable fibre bonded with adhesive and lubricated with paraffin wax.
  • Felt - woven material of different types but usually referred to as felt.  Sealed with a card which is fully biodegradable.

Shot

  • Material - shot is normally made from lead, but steel is also used.  Lead shot is hardened with a substance called antimony and this is normally 2%, 3% or 5% depending on purpose.  5% is never used for game to avoid the shot penetrating the bird and causing an inhumane kill.
  • Size - shot is measured by size.  The diameter of the individual pellets range from 2mm – 8.4mm.
  • Weight - the load weight is printed on the packaging the cartridges are sold in. This shows the total weight (the load) of all the shot in the cartridge and ranges from 21gm-50gm for a 12 gauge cartridge.

The closure

This is the method used to seal the end of the cartridge. This is done in two ways:

  • Crimped closure - folding the end, normally using a six star crimp although some manufacturers use eight. This is the most common method of closure.
  • Rolled Turn Over (RTO) – A Perspex or card disc is placed on the top of the charge of shot and the tube rolled over the disc to close the end of the cartridge.

Choice

Choosing the right cartridge depends on the type of gun, purpose and personal preference. During the shooting season, R. Ward Gunmakers stocks the finest range of cartridges for all gauges and purposes.



Thanks to Mike Haw at
Hull Cartridge for his advice in producing this article.

Author: Peter Brill

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