Wednesday, January 26, 2011

Dear LawDog,

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.

LawDog

21 comments:

Old NFO said...

It was, and it STILL scares the hell outta me :-)

AND you explained it much better and in more detail than I've seen in a while.

Keads said...

Thanks for that write up! One of the best I have read on the subject!

Anonymous said...

Way better than anything I found on the subject. Fantastic. Thanks!

Sarah said...

Thanks, LawDog. I'd never heard of primer wipe before and now I've learned something new and important.

Vine said...

Bravo, Sir. We went over this very thing today. All I intend to say about that rifle was that it was interesting in the Chinese sense.

fuzzys dad said...

Thank-You Dog for educating the masses

Chris Byrne said...

A note on primer wipe.

One of the side effects of bending, folding, spindling and mutilating your firing pin in such a way can be a firing pin fixed in the forward position.

In many firearms, this will result in unintentional full auto fire, emptying the magazine in one long burst known as a "runaway".

That is a significantly ungood thing.

Larry said...

It was, thank you.

Anonymous said...

1. Agree with all above.
2. Thanks for the education.
V/R JWest

Tam said...

"If the firearm unlocks too soon -- well, that afore-mentioned nastiness is all of sudden up under your nose..."

Or under your ear, in the case of a bullpup...

Aggie said...

Thanks LawDog. That's a new term to this ol' shooter. Appreciate the info!

God Bless Ya'll!

Aggie, Class of '70
An Anglican Firearms "Enthusiast" (ie: Gun Nut)
www.ananglicangn.blogspot.com/

Jake (formerly Riposte3) said...

"In many firearms, this will result in unintentional full auto fire, emptying the magazine in one long burst known as a "runaway".

That is a significantly ungood thing."


Considering the ATF's history of claiming that "malfunction = machine gun", I'd go so far as to say that it's a double-plus-ungood thing. It would truly suck great big donkey balls to have to pay for a federal court case just to prove that it really was a malfunction, it was the first time it happened, and therefore you were not knowingly in possession of an unregistered and untaxed NFA item, just because some twit panicked and called the cops on "sumdood with a machine gun" at the range because of one single burst-fire.

Don said...

Thanks, I learned something new today.

Chris said...

Ahh.. Nothing like having the propellant contents of a .45 cartridge spew up in front of your nose. Even at arms length on a range it was worth a check of the undies..
Don't EVEN want to think about that in a serious social encounter.

Marty said...

Was just sizing and de-priming some once-fired 9mm brass pick-ups last night, and noted this on a number of the primers.

Thankfully, none of the brass had been fired from MY gun.

wolfwalker said...

(possibly stupid) Question: Your description sounds you're talking about semiautomatic handguns. Does 'primer wipe' also afflict revolvers? What about other non-auto-ejecting firearms, such as break-open shotguns or bolt-action rifles?

Loren said...

Wolfwalker, I'd imagine it'd not be an issues on break-opens, but bolts and revolvers are probably just as prone. I'd imagine it would jam a revolver though, as a firing pin stuck out will inhibit the cylinder turning.

GreyLocke said...

"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."

Not trying to be picky sir, but you forgot "fold"

This condition can lead to firing out of battery which is major bad ju-ju's especially if you are firing magnum ammunition. But the possibility of firing out og battery or failure to fire, or a score of other things makes this particular weapon system something I'd avoid until the A1 version comes out.

I found your description of primer wipe to be very informative, and may I have your permission to you is when explaining it to others sir?

LawDog said...

Of course you have my permission. As long as credit is given to The LawDog Files, or to me, I'm more than willing to have people use what they find useful here.

Anonymous said...

Mr. Dawg,
With that style of trigger control you need to get an ITHICA model #37, if you don't already own one. They were in use when I started as a pup LEO in 1977. Just keep it pointed in the correct direction (toward something needing to be shot/shot at), hold the trigger back and pump. Every forward pump yields a very satisfying "BOOM" with the accompanying application of corrective pellets or slug.

Anonymous said...

Darn.....mispelled ITHACA in my first post. Neat shotguns though. I just bought one recently for my left handed son because the Model 37 is bottom eject. As the kids say....SWEET!