This post got longer than I intended.
The first part of the post is about guns discharging.
The second part is about investigating an unintentional discharge.
The last part includes practical recommendations for how to avoid unintentional discharges.
Guns discharging: (What we do know about what happened.)
It is extremely rare for a gun to truly just "go off by itself".
For a gun to truly "go off by itself" the gun would have to have enough energy stored in the striker/hammer spring to fire the gun, and that energy would have to be released so that the firing pin/striker hits the primer with enough force to ignite the primer. The release would require, at a minimum for a critical part (e.g. sear or sear equivalent) to break spontaneously.
Why is this extremely rare?
1. Some designs don't even have enough energy stored in the striker/hammer spring to ignite the primer.
True DAO designs are like this. The trigger must be depressed either partially or fully to compress the hammer/striker spring and store enough energy to fire the gun. Single action guns that are not cocked are like this. Until the hammer/striker is cocked, there's not enough energy to fire the gun. But, of course, many designs do have enough energy stored in the hammer/striker spring to ignite the primer.
2. It's extremely unusual for parts to spontaneously break. Sears can break during the firing cycle, but for one to just snap out of a clear blue sky is really unusual. Even under stress (as from being stored in a cocked/partially cocked position), it's very rare for a steel part to just snap on its own.
3. Virtually all guns have some sort of safety system designed to prevent the gun from discharging unintentionally. Things like rebounding hammers, firing pin blocks, transfer bars, trigger safeties, drop safeties are built into designs so that even of something goes wrong (say a sear really does snap out of the clear blue) the gun won't fire.
Well, if that's true, how is it that we hear about guns firing unintentionally? The key is that while it is not so uncommon for a gun to be fired unintentionally, it virtually always requires some input from the user to fire.
What kind of inputs? The simplest way to say it is that virtually always, some kind of energy needs to be put into a gun before it can fire.
Examples are:
1. Pulling the trigger. This is the most common way that guns are fired unintentionally. It may be that the user pulls the trigger intentionally thinking that the gun is unloaded or won't fire for some other reason, they may pull the trigger unintentionally (less common than the previous). Something else may pull the trigger--a foreign object in the triggerguard during holstering, for example--an item in a pocket or a purse.
2. Dropping the gun. Guns are designed to be drop safe, but impacts can apply a tremendous amount of force to parts of the gun and in some cases it is sufficient to cause breakage, or to bypass design features and fire the gun. We worry a lot about drop safety, but in the real world, it's kind of rare for guns to fire from being dropped--but it can happen. In addition to the possibility of defects from the manufacturer, parts in the safeties designed to prevent discharges could be damaged/worn over time, or could be compromised by user modification or by replacement parts that don't meet the manufacturer specifications.
3. Bumping, jostling or shaking a gun that is either defective, damaged, badly worn, or improperly modified could conceivably be sufficient to fire a gun.
I know, that's a lot of words, but the point is important.
When someone says a gun "went off by itself" that should raise red flags. Guns virtually never "go off by themselves". They DO go off unintentionally, but that almost always involves some kind of input or action on the part of the user.
Investigating discharges: (Why we don't know what really happened yet.)
So the first step in an investigation into an unintentional discharge needs to be dertermining what sort of input or action on the part of the user actually started the process that resulted in a discharge.
In this case, the user states:
“I picked up my bag, my keys were on top,” she said. “As I walked around my desk, my purse swings out and it shoots out the bottom of my bag.”
As we should have suspected, the gun wasn't just lying somewhere and suddenly fired without warning. The user was moving the purse containing the gun around. So right away, we know that the gun didn't just "go off by itself".
The next step is to examine the evidence to try to determine a likely cause. The gun, holster, purse, and purse contents should be examined. Obviously we're not going to be able to do that here--we'll have to wait for others to do that examination and provide results.
It's also worth keeping in mind that users often have a significant stake in the outcome of these investigations. Jobs can be at stake. Huge sums of money could change hands if a manufacturer is determined to be liable. Hospital bills can be very expensive. Unfortunately, this means there's often incentive to alter evidence before it can be examined.
Even when the evidence isn't intentionally modified, understandable actions at the time of the discharge may complicate the investigation. In one video I saw, the person grabbed the holstered gun out of his pants after the discharge and threw it away from him. A gun/holster combination that's been thrown across the room could have been damaged or modified in the process. Even just the recoil from the discharge will move things around a bit. I can guarantee you that when the gun fired in the user's purse, it definitely shifted in the purse and moved other objects in the purse in the process.
Anyway, the investigators will look at the gun. Was it modified? Was it in good condition? Can it be made to fire by dropping it, jostling it, etc.?
They will look at the holster. Could something have gotten into the triggerguard with the gun holstered properly? Was the holster in good condition?
If they can, they will try to look at the purse and its contents to determine if there were objects in the purse that might have worked their way into the holster.
What to do: (Whatever happened, how can I keep it from happening to me.)
1.
Understand your gun. If the carry condition of your gun is such that there's enough energy stored to fire the gun, you need to take steps to insure that energy is kept stored until you want it released. This may mean using the provided manual safety. It may mean carefully choosing a holster that protects the trigger and/or safety. It could even mean choosing a different carry gun--not all guns are really designed for carry. Pay attention to what the manufacturer tells you in the manual. People like to talk about lawyerese and scoff about CYA and manufacturer worries about lawsuits, but the bottom line is that if the manufacturer is worried enough about something to try to limit their liability with a warning in the manual, it's worth your attention too. Before you disregard a recommendation or warning, you need to understand exactly what risk you run by doing so.
2.
Make sure your gun is in good condition. Periodically the gun should be inspected for excessive wear. Areas in that particular design known to experience failures should be inspected for cracks or damage. Pay attention to recalls.
3.
Make sure your other equipment is safe and is in good condition. Does the holster protect the trigger? Is the holster starting to sag and lose its integrity? I've seen pictures of a worn leather holster that folded over into the triggerguard and eventually put enough pressure on the trigger to fire the gun. Are you carrying a gun in a case with other items which could contact the trigger?
3.
Make sure your gun meets the manufacturers specifications. Drop in parts are relatively inexpensive, can be easy to install and can offer improvements in trigger feel or other benefits. But it's your responsibility to insure that the safety mechanisms in the gun are still functional. On another forum, a person installed a well-known brand of drop-in trigger into a common pistol and found that it was no longer drop safe with the new part. Some people like to modify their firearms. Be sure you know what you're doing and also know how to test the gun when the modification is complete. There's one common model of pistol that can be easily modified in such a way that all of the passive safeties in the gun are defeated leaving the gun prone to discharges from being dropped or even bumped hard.
4.
Follow the safety rules. People worry a lot about dropped firearms, trigger snags and design defects, but reality is that it's not the firearms that are the problem most of the time. It's the human. Practice the safety rules every chance you get. Keep them in mind when you handle firearms.
5.
Minimize your risk. Since a gun almost always needs some kind of input from the user to fire, think about minimizing those inputs. If you don't really need to load and unload your carry gun daily, then don't. Why manipulate a loaded gun if there's really no need to do so? Don't mess with loaded guns if you don't have to. I've heard too many unintentional discharge stories that started with a show and tell session of a loaded carry gun.
6.
Don't get complacent. Your firearms don't get safer just because you're used to them and have owned them a long time. In fact, just the reverse can be true.