You can spend hundreds – even thousands – of pounds on a shotgun, but without a simple little cartridge costing just a few pennies it would be redundant
For all the good it could do you might as well hammer the barrels into the ground and use it as a fence post or sapling support.
Yet cartridges are amazing things. they contain a minimum number of simple components but deliver a performance based on some quite complex processes. Most shooters, however, don’t give the cartridge a second thought simply because they are cheap, expendable and the user can’t see what happens to it once he closes the gun. Cartridges carried loose in a pocket or stored at home in bulk won’t explode in the event of an accident.
We certainly don’t recommend that you try it, but if you were to use a hammer and blunt nail to sharply tap the primer the cartridge wouldn’t go off with a big bang, as you might expect. Instead you’d hear a sharp crack from the primer followed by a fizzing sound as the powder starts to burn. The hot gasses might have just enough power to burn the plastic case, open the crimp and let some pellets fall out – and that’s about it.
A violent reaction
Put a cartridge into the gun, however, and a whole different story unfolds. Now that it’s held captive in the chamber a violent reaction is in the offing. As soon as the hammer strikes the primer a hot jet of flame ignites the powder creating an explosion inside the case. Some of the energy
tries to come backwards (which explains the recoil you feel on your shoulder) but it’s stopped by the gun’s standing breech. The only way this energy can dissipate is to shoot forward and, as it does so, it propels the wad and shot past the crimp and into the bore of the barrel at an incredible pace. Within milli-seconds the expanding gasses reach pressures of around three tons per square inch, and by the time you hear the bang and feel the bump of the gun in your shoulder the shot charge will have already left the barrel.
It’s as soon as the shot charge starts moving that the cartridge wad comes into play. Its function is to protect the column of lead pellets from the hot gasses propelling it through the bore of the gun and past the muzzle ends. To do this the wad needs to expand slightly and so create a seal, which prevents the gas getting past. If it does, the heat can fuse pellets into clumps that affect the patterning quality of the shot load.
The most popular type of wad is the plastic shot cup. It has a skirted base that flares out to create the seal we’ve just mentioned, but it has added benefits over other wadding systems. It has collapsible “legs” which cushion lead pellets from the initial power surge and the cup walls prevent abrasion by the barrel bores and choke cone. The more pellets that emerge from the barrel in perfect shape, the straighter they will fly and the better they will retain their energy.
And there’s more…
The primer in a cartridge contains a small charge of explosive, which ignites when struck by a firing pin. Primers used to be made of soft copper but today they are made from a cheaper alloy of zinc and steel and zinc, which can easily be dented when hit by the firing pin. The tiny amount of explosive held in the primer is made up of a lead styphnate that doesn’t corrode or cause pitting to the bore of the barrel if not cleaned away.