Primers are held in the primer pocket by friction alone in commercial ammunition. Military ammunition has primers which are crimped and then glued/sealed into place. Because of the friction-only hold in commercial ammunition, primers can sometimes come out of the primer pocket during firing. This is due to the pressure on the primer while the powder is burning. The primer cup can deform when there’s too much pressure in the cartridge because the material needs to be soft enough that it can deform when struck by a firing pin. A tell-tale sign of too much pressure in a cartridge can be seen by
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