"Backup" .380 Kurz Problem

[john.paul.harris]

I have gotten a Kurz .380 "Backup" from a customer complaining of misfiring and odd firing pin placement. After test-firing and inspecting i found that the firing pin isn't long enough to protrude the bolt and strike the primer. I ordered a new firing pin and found that the original was worn short but not that bad, the new pin doesn't protrude either. I'm assuming that it relies on the pin bouncing out of the bolt to strike??? Any thoughts or knowledge would be great!!
Harris Gunsmithing
St. George, Ga

 

[PotatoJudge]

Can it protrude through the breechface with the firing pin spring removed?

Does the gun have an inertial firing pin?

Firearms that use long firing pins, such as pistols, will often use a firing pin that is too short to project when depressed flush by the hammer. This type of firing pin, called an inertial firing pin, must be struck by a full fall of the hammer to provide the momentum to move forward and strike the primer. If the hammer is down, resting on the firing pin, it is very unlikely that a blow to the rear will provide enough energy to the firing pin to detonate the primer. Most variants of the M1911 pistol use this type of firing pin.

 

[hexidismal]

Might also help to know what gun we're talking about here. By "backup", do you mean it's his backup gun ? Or is it an AMT Backup in .380 ?

 

[PotatoJudge]

The AMT Backup has 9mm Kurz (uncommon nomenclature in the states) etched on the slide, kind of a tip-off.

 

[rcmodel]

Sounds like you have what is known as an Inertia Firing Pin.

This is quite commonly used, and dates back to the 1911 Colt, if not further.

In use, the firing pin is shorter then the hole through the slide it rides in, and hides inside the slide at rest.
Only when the hammer hits it with full force is it given enough momentum & energy to compress the retractor spring and fly forward far enough to strike the primer and set it off.

Were it longer, it would protrude through the breech face with the hammer down, and it would be unsafe to carry it loaded with the FP resting on the primer.


rcmodel

 

[john.paul.harris]

Ok thank you all for the help. It does sound like that it has an Inertial FP. And to also assume that this is prob a hammer problem.

 

[rcmodel]

I would clean the firing pin channel throughly with a small bore brush & solvent.

Also clean the gun & chamber throughly, re-lube, and make absolutely sure it is going all the way into battery each time.
If it isn't, it will cause light strikes, and misfires.

Beyond that, I might suspect a weak hammer spring.

Is the problem constant every shot, or at random from time to time?
Constant = Weak hammer spring, or bad ammo.
Random = Not going into battery every time, or bad ammo.


rcmodel

 

[john.paul.harris]

Well its not constant and it tends to leave a weird indent on the primer but sticking out not in. I have also tried a dummy round with putty in the primer hole to check FP depth and get nothing, and that indent so to say as well.

 

[rcmodel]

Say what?
The firing pin dent is an Outtie, not an Innie?

Fraid I'm plum run out of ideas!


rcmodel

 

[john.paul.harris]

Well actually its not the firing pin dent, its a small area around it. If it decides to detonate there is a normal firing pin indent and this raised area around it about the same size as the firing pin chamber.?!.?!.?

 

[Marlin 45 carbine]

I've read that some SxS shotties have that problem after many, many shots, some smiths actually 'reline' the firing pin chamber.

 

[rcmodel]

Primer flow into the firing pin channel is caused by hole or FP wear, a too small fining pin dia. for the hole, improper radius of the tip, or in some cases, a weak hammer spring.

It is nothing to get excited about in a blow-back operated .380.
It can become a problem only if you are getting primer leakage.

In lock-breach pistols, it is a problem, because when the barrel unlocks, it often shears a tiny brass ring off of the protruding primer, and enough of them in the firing pin channel will tie up the gun.

BTW: I noticed Harris Gunsmithing, St. George, Ga. in your first post.
Just wondering, are you in business as a gunsmith?


rcmodel

 

[JaxJim]

Is it a single action or a DA? I have owned both (still own the DA sorry to admit).

I'd put dollars to donuts it's the hammer spring. Many table-top gunsmiths try to improve the trigger pull on the DA units by shaving a couple of coils off the spring. Only thing is, the spring needs all of the length it has (and could use a couple more coils over time) and you will get light strikes. They can be random due to how "fast" you pull the trigger. A slow and careful trigger pull will cause a lighter strike than a quick, rapid (and inaccurate) one.

There is a slight possibility the FP is dragging. Check for smooth movement or dirt as mentioned earlier.

If it's a single action, the FP probably is dragging and/or the main spring is old and weak.

 

[john.paul.harris]

OK thanks again. I'm going to clean the FP hole the best i can.