When a new cartridge is being loaded into the firing chamber of the firearm, sometimes the bolt slams the cartridge into the chamber with such force that the weapon fires without the user having pulled the trigger. This is known as a "slamfire".
A slamfire is particularly dangerous because it happens when the user is usually not expecting it to happen. In some firearms, it can happen even with the user's finger off the trigger and the safety enabled. In the case of semi-automatic and fully-automatic weapons, it may cause them to keep shooting until the magazine is empty. Since a slamfire comes as a surprise to the user, it may cause the user to lose control of the firearm and point it in an unsafe direction.
Slamfires are more common in weapons which have a free-floating firing pin, than in weapons which have a spring-loaded firing pin. When a semi-automatic or an automatic firearm is fired, the hammer strikes the first cartridge loaded and causes it to explode and push the bullet of the barrel. Some of the recoil force is harnessed to reload the firearm automatically. The recoil force pushes the bolt backwards and as it moves back, an extractor claw pulls out the cartridge that was just fired. The bolt continues to move backwards and compresses a return spring. Meanwhile the extractor claw pulls the empty cartridge case backwards until it can be pushed out via the ejection port. Meanwhile the bolt moves to its backward most point and is then pushed forward again by the compressed return spring. On the way forward, it picks up a new cartridge from the magazine and pushes it into the chamber. As the bolt reaches the chamber, the firing pin continues to move forward due to inertia, until it is stopped by the cartridge's primer. Now if the firing pin is spring loaded, the spring slows down the firing pin so that it doesn't slam against the cartridge's primer so hard. However, if the firing pin is free-floating, then there is nothing to slow it down as it slams into the cartridge's primer. In either case (but especially for free-floating pins), there is a chance that the firing pin slams into the cartridge primer a little harder than expected and could cause it to detonate prematurely. This causes the firearm to discharge without the user pulling the trigger and then the cycle repeats.
Free-floating firing pins are more common with military firearms, so there is a greater chance of slamfires occurring with them. Of course, this is not to say that firearms with spring-loaded firing pins are immune to slamfires, because it can happen with them as well, but not as often as those with free-floating pins.
As you can see on the above video, the Tavor accidentally slamfires at 0:24 after the user switches magazines.
Slamfires can also happen on some bolt action firearms, when the user is manually cycling the bolt and pushes it forward a little too hard.
In the above video, a slam fire happens as the user chambers the 3rd cartridge on a Stevens bolt-action rifle.
The main causes for slamfires to happen are:
- Dirt and corrosion: If enough dirt gets in the bolt's firing pin channel, then it could cause the firing pin to stick to the bolt wall and protrude out of the bolt, which could cause the slamfire to happen. The same thing could happen if corrosion causes the firing pin to protrude out of the bolt.
- Ammunition: Some ammunition is manufactured with more sensitive primers than others and require less force to detonate. This could cause slamfires.
The solutions to fix this are fairly simple:
- The first solution is to keep the firearm clean to prevent dirt from jamming the firing pin.
- The second solution is to use cartridges with less sensitive primers so that more force is needed to detonate them. This is why military cartridges usually have harder primer caps so that they need an actual hammer strike on the firing pin, rather than inertia, to detonate them.
- The third solution is to design the firearm to minimize slamfires. For instance, the firing pin could have a spring around it, so that it slows down the firing pin as the bolt slams forward. The spring provides enough resistance that the pin will not hit the primer hard enough due to inertia, but will detonate the cartridge when the back of the firing pin is struck by the hammer.
- A fourth solution is to make the firing pin as light as possible, so that it doesn't transfer enough force to detonate the primer due to inertia.
The above video shows a SKS slam firing, the reason why it slamfires and a replacement firing pin that fixes this problem, demonstrated by Ben Murray, the inventor of the replacement firing pin himself. Note that due to slamfire, the SKS fires multiple rounds one after another continuously like a fully-automatic rifle, even though it is actually designed as a semi-automatic rifle and it manages to do this despite the facts that the user's finger is off the trigger AND the safety is enabled! This shows how dangerous slamfires can be, especially when the user is not expecting it.
The Tavor TAR-21 reportedly had a slamfire problem when using some brands of commercial ammunition available in the US, until Israel Arms replaced the free-floating firing pins with spring loaded pins free of charge. The reason this happened was that the TAR-21 was originally designed with military ammunition in mind, which have harder primer caps. The Fabrique Nationale FS2000 semi-automatic carbine also had this same problem with certain brands of commercial ammunition available in the US, causing FN Herstal to recall this model in 2006. The FS2000 came with a firing pin that was originally designed for the military FN F2000 carbine and was therefore heavier, so that it could work with the heavier primer caps of military ammunition. With certain brands of commercial US ammunition with more sensitive primers, the firing pin was heavy enough to detonate them via inertia alone.
There are some pump-action shotguns that are deliberately designed so that they can slamfire if the trigger is held down and the pump is operated. Examples of this include the Winchester model 1897 and Ithaca model 37 shotguns.
This feature is useful for a soldier to rapidly fire into an area (such as a trench or a tunnel), which is why SEAL units in the Vietnam era carried the Ithaca 37 into action.