What is "primer wipe"?
Oh, boy. Got some time?
To explain primer wipe, first let me explain what the "firing cycle" is in a modern firearm.
The "firing cycle" is the series of operations involved in a modern weapon to fire a round. Generally speaking it goes something like this:
1) Feeding. This is the step in which a complete cartridge is introduced into the chamber of the firearm. It is followed by:
2) Locking. In this step the bolt or breech locks to the barrel, so as to contain the pressure generated by the next step, which is:
3) Firing. Sometimes this is called 'ignition', it is when the primer is ignited by the firing pin or striker, and the round is fired. Next is:
4) Unlocking. Once the pressure in the chamber of the firearm has dropped to a safe level, the bolt or breech unlocks from the barrel, which leads to:
5) Extraction. The empty case is removed from the chamber; and finally:
6) Ejection. The empty case is removed from the firearm.
And the cycle continues back with "Feeding".
Primer wipe is a condition in which the firing pin or striker impacts the primer during the "Firing" portion of the cycle, and remains embedded therein through "Unlocking", "Extraction", and "Ejection". As the empty case is kicked out of the firearm, it is dragged across that un-retracted firing pin, causing a characteristic oval or tear-drop gouge, and -- in extreme cases -- smearing the metal of the primer out of the primer pocket and onto the case itself.
There are many different causes of primer wipe, but they all boil down to one of two reasons.
The first of which is that the firing pin or striker is unable to retract after firing.
While, in and of itself, not a dangerous condition, the typical firing pin/striker assembly is a light-weight metal alloy designed to be driven hard enough by a relatively dinky mainspring to ignite a primer.
It is not designed for lateral stress. Sooner or later being yanked sideways every time the weapon fires is going to break, bend, spindle, mutilate or otherwise damage the needle-shaped firing pin or striker assembly to the point that it can no longer function.
Which, since the firing pin or striker is kind of necessary for the whole 'bang' part of your bang-stick, pretty much guarantees said bang-stick is going to be Paws-Up until fixed.
This can range from being Rather Annoying if it happens at the mid-point of the Palma match or as the trophy of a lifetime disappears over a ridge; all the way up to A Bloody Nuisance if it happens during the third shot at five critters trying to get up under your hat with you.
The second reason for primer wipe is when the firing pin simply doesn't have the time to retract.
In other words, the firing pin itself and the firing pin channel are clean and to spec, but the firearm unlocks, extracts and ejects before the firing pin spring has time to push the pin back to its' resting position.
Folks, this can be a Bad Thing.
There's a reason for the second and fourth segments of the firing cycle. When that pin hits the primer for a brief part of a second there is all sorts of nastiness going on inside that chamber. The locking and unlocking is to make sure all that nastiness stays in the chamber and does its' job: driving the bullet out the muzzle-end at velocities up to, and including, trans-sonic.
If the firearm unlocks too soon -- well, that afore-mentioned nastiness is all of sudden up under your nose, introducing itself to you.
I'll let the Gentle Reader ruminate on that for a moment.
Anyhoo, "primer wipe". Hope it was informative.