Primers are spark plugs! When hit they explode and give out a sheet of flame which sets fire to the powder in the cylinder. When unfired these primers have a domed appearance like a convex mirror. The ones around the outside are unfired. |
(These shotshell primers are approx 8mm in diameter.)
When the primer is fired it has a dent in the middle caused by the firing pin. It is this impact which ignites the primer. During the "explosion" of the powder high pressures are generated. These high pressures force the bullet out of the chamber but also force the primer back against the metal of the gun. This can flatten the primer, and there is some slight flattening most visible on the one in the bottom left of the four fired primers. (6.0 grains Herco, round ball, ball seated at chamber mouth. This is a normal load.) Very flat primers are a sign of excessive pressure in the cylinder and should be taken as a warning to BACK OFF the powder. |
The primer fired without powder or bullet has only a small depression. There is only low pressure in the cylinder. In the top fired primer the pressure had to build up in the chamber to push the bullet out, and so the primer was pushed back onto the firing pin making a deeper dentand there is a small flatening of the primer. |