How the Shotgun Shoots
Here are the steps showing what happens when you shoot a shotgun.
A shotshell is inserted into the chamber. Closing the action pushes the firing pin back and holds it under spring tension.
Pulling the trigger releases the firing pin. The firing pin strikes the primer producing sparks.
Heat and sparks from the primer ignite the gunpowder. Gas converted from the burning powder expands in the shell. The expanding gas forces the wad and shot out of the plastic body of the shell.
The escaping gases produce a "bang" as the wad and shot leave the barrel. The wad quickly opens and falls away. The shot cluster spreads. This spread is called the shot string.