AR15 slamfire condition

[Ginocide]

1. If the firing pin in an AR15 was to become stuck in the firing position is there any action of the trigger that would stop it from continuously firing all the rounds in the magazine?

2. If you don't depress the trigger, the firearm will continue to fire all cartridges in the magazine, correct?

3. During a runaway slamfire condition like above, if you were to hold the trigger back in the firing postion, would the hammer pop up at any point and keep the bcg from returning to the chamber?

1. No, the trigger only interacts with the fire-control-group, which is no longer involved in the case of a slamfire
2. Correct
3. No. If you were to hold the trigger continuously, the hammer would simply stay held in place by the trigger disconnecter after catching it at the end of the first cycle. Releasing the trigger during the firing would drop the trigger to its primary sear (its normal pre-firing position) but the bolt carrier would simply continue to ride back and forth on top of it.

As you indicated, the slamfire would be caused by the firing pin being jammed in the forward firing position. At this point, it's acting like a fixed-firing pin machine gun, but without a fire-control system designed to control that. Because the AR FCG isn't designed to catch the bolt at the rear of (like an open bolt machine-gun) you can think of this type of malfunction as effectively "short circuiting" the gun and running without the FCG being involved. I'm not sure the AR-15 system would keep sufficient inertia through the camming action of the rotating bolt to reliably detonate a primer, but I could imagine it happening.

Another thing to keep in mind is that if the above is all true and the system does have enough inertia, this firing is unrelated to the trigger, it would begin firing as soon as you drop the bolt on a loaded mag.

As others have said, the best mechanical ways to stop this (if you can) would be to drop the magazine or depress the "catch" nubbin on the bolt release, drop the magazine, or what would most likely be safe is just to hold on and keep the weapon in a safe direction.

Side note: a free-floating firing pin (meaning one not held in its rearward position by anything, usually spring force) is considered a minor risk of allowing a firing on any bolt drop, as there is nothing preventing the pin from riding in its channel and tapping the primer. It looks like Colt actually tried out several different firing pin designs to keep this problem from occurring, deciding eventually on a pin that is light enough that it will not be able to carry enough energy to accidentally fire the gun under normal circumstances. You can however see evidence of the pin "trying to" if you let the bolt slam closed on a new round. If you load a new cartridge into a AR-15 magazine (or chamber), and let the bolt slam forward, you can then extract it without firing and note a small but visible dimple on the primer.

 

[arakboss]

1. No, the trigger only interacts with the fire-control-group, which is no longer involved in the case of a slamfire
2. Correct
3. No. If you were to hold the trigger continuously, the hammer would simply stay held in place by the trigger disconnecter after catching it at the end of the first cycle. Releasing the trigger during the firing would drop the trigger to its primary sear (its normal pre-firing position) but the bolt carrier would simply continue to ride back and forth on top of it.

As you indicated, the slamfire would be caused by the firing pin being jammed in the forward firing position. At this point, it's acting like a fixed-firing pin machine gun, but without a fire-control system designed to control that. Because the AR FCG isn't designed to catch the bolt at the rear of (like an open bolt machine-gun) you can think of this type of malfunction as effectively "short circuiting" the gun and running without the FCG being involved. I'm not sure the AR-15 system would keep sufficient inertia through the camming action of the rotating bolt to reliably detonate a primer, but I could imagine it happening.

Another thing to keep in mind is that if the above is all true and the system does have enough inertia, this firing is unrelated to the trigger, it would begin firing as soon as you drop the bolt on a loaded mag.

As others have said, the best mechanical ways to stop this (if you can) would be to drop the magazine or depress the "catch" nubbin on the bolt release, drop the magazine, or what would most likely be safe is just to hold on and keep the weapon in a safe direction.

Side note: a free-floating firing pin (meaning one not held in its rearward position by anything, usually spring force) is considered a minor risk of allowing a firing on any bolt drop, as there is nothing preventing the pin from riding in its channel and tapping the primer. It looks like Colt actually tried out several different firing pin designs to keep this problem from occurring, deciding eventually on a pin that is light enough that it will not be able to carry enough energy to accidentally fire the gun under normal circumstances. You can however see evidence of the pin "trying to" if you let the bolt slam closed on a new round. If you load a new cartridge into a AR-15 magazine (or chamber), and let the bolt slam forward, you can then extract it without firing and note a small but visible dimple on the primer.

Well explained thank you.

If the firing pin was stuck in the forward firing position, what are the chances that it would set off the primer before the bolt was locked in the chamber?

 

[Ginocide]

Well explained thank you.

If the firing pin was stuck in the forward firing position, what are the chances that it would set off the primer before the bolt was locked in the chamber?

Not sure. There's not a lot of reports around this which would lead me to think it's unlikely, but I also am not able to think of anything that would necessarily prevent it, other than the full weight and impact of the bolt not occurring until full lockup, before that it's being transformed into a rotating force by the cam pin and channel, but before that rotating occurs (while unlocked) there is certainly a significant amount of force that occurs as the bolt is forced into the chamber.

What I can say is that a lot of this is theoretical. The AR-15 design is such that during cycling the firing pin and bolt face are separated, and if there was a bit of carbon fouling holding them together I would predict that they'd still be ripped apart. You can test this by removing your BCG and holding your finger against the firing pin. Note that while the bolt is in the rearward (locked) position it protrudes only slightly, about a millimeter. Now with your other hand, pull the bolt forward and note how the bolt moves away from the frontmost firing pin position. Consider that this action would be happening under the full operating force and momentum of the bolt so that even if they were bound together temporarily, I would anticipate that they would be violently decoupled beyond the binding strength anything you're likely to encounter in normal operation. To have that pin still protruding in the unlocked position means that the firing pin has significantly more-likely broken somewhere somewhere along its length (a"stretched" firing pin could touch while unlocked, but would also likely spear into the primer while firing and cause other issues).

So I guess there's actually several possible types of "slamfire" we're talking about. In all of these scenarios the bolt-catch becomes your "trigger" while your regular fire-control-group is basically excluded from the process. The bolt catch would also be acting like the opposite of a conventional trigger; the spring force on the bolt catch works to keep it "open" or firing until the magazine follower or user stops that process.

1. Normal operation, but the floating firing pin has too much inertia:
   I don't see this as being predictable or reliable with the AR design. It was unlikely to begin with and Colt/ArmaLite design iterations made it much less likely.
2. Firing pin coupled to bolt-carrier, stuck in forward position (firing pin only protrudes upon lockup):
   This would be like a fixed-firing pin, gas operated, rotating bolt machinegun. The pin would not protrude until lockup had either completed or progressed by a large amount. I don't know that the carrier would have enough momentum to reliably ignite, but the lore and rumors about slamfires would lead me to believe it may.
3. Firing pin coupled to bolt face (firing pin protruding at all times):
   This is where you run into significant danger, because the firing pin is protruding at all times. At this point it is acting somewhat similar a straight-blowback, fixed firing pin, open bolt machinegun, and the AR-15 is not designed to do any of these three things. As the bolt strips a cartridge from the magazine it's acting on the rim, but during the closing and locking action the firing pin may be able to contact the primer prior to full lockup and force an out-of-battery explosion. The action of snapping through the extractor and the pushing force of the ejector would attempt to prevent this but I personally do not know if that would be enough.

 

[arakboss]

Not sure. There's not a lot of reports around this which would lead me to think it's unlikely, but I also am not able to think of anything that would necessarily prevent it, other than the full weight and impact of the bolt not occurring until full lockup, before that it's being transformed into a rotating force by the cam pin and channel, but before that rotating occurs (while unlocked) there is certainly a significant amount of force that occurs as the bolt is forced into the chamber.

What I can say is that a lot of this is theoretical. The AR-15 design is such that during cycling the firing pin and bolt face are separated, and if there was a bit of carbon fouling holding them together I would predict that they'd still be ripped apart. You can test this by removing your BCG and holding your finger against the firing pin. Note that while the bolt is in the rearward (locked) position it protrudes only slightly, about a millimeter. Now with your other hand, pull the bolt forward and note how the bolt moves away from the frontmost firing pin position. Consider that this action would be happening under the full operating force and momentum of the bolt so that even if they were bound together temporarily, I would anticipate that they would be violently decoupled beyond the binding strength anything you're likely to encounter in normal operation. To have that pin still protruding in the unlocked position means that the firing pin has significantly more-likely broken somewhere somewhere along its length (a"stretched" firing pin could touch while unlocked, but would also likely spear into the primer while firing and cause other issues).

So I guess there's actually several possible types of "slamfire" we're talking about. In all of these scenarios the bolt-catch becomes your "trigger" while your regular fire-control-group is basically excluded from the process. The bolt catch would also be acting like the opposite of a conventional trigger; the spring force on the bolt catch works to keep it "open" or firing until the magazine follower or user stops that process.

1. Normal operation, but the floating firing pin has too much inertia:
   I don't see this as being predictable or reliable with the AR design. It was unlikely to begin with and Colt/ArmaLite design iterations made it much less likely.
2. Firing pin coupled to bolt-carrier, stuck in forward position (firing pin only protrudes upon lockup):
   This would be like a fixed-firing pin, gas operated, rotating bolt machinegun. The pin would not protrude until lockup had either completed or progressed by a large amount. I don't know that the carrier would have enough momentum to reliably ignite, but the lore and rumors about slamfires would lead me to believe it may.
3. Firing pin coupled to bolt face (firing pin protruding at all times):
   This is where you run into significant danger, because the firing pin is protruding at all times. At this point it is acting somewhat similar a straight-blowback, fixed firing pin, open bolt machinegun, and the AR-15 is not designed to do any of these three things. As the bolt strips a cartridge from the magazine it's acting on the rim, but during the closing and locking action the firing pin may be able to contact the primer prior to full lockup and force an out-of-battery explosion. The action of snapping through the extractor and the pushing force of the ejector would attempt to prevent this but I personally do not know if that would be enough.

Good stuff, did you work in a armory or in gunsmithing?

 

[powersbj]

Why would you want to stop it?

 

[ob1]

Interesting conversation.

I recall an incident that happened locally back in the nineties ( insert old fud meme here ) where a malfunctioning early model sp-1 was blamed for multiple shots causing a fatality. The theory from the defense team during the trial was that the weapon went into full- auto during an argument causing unintended multiple hits on the victim.
A subsequent test firing by a defense forensic examiner did indeed show the weapon going into a short ( 4-5 rounds ) burst, which was documented on video.
While the incident didn't get the defendant off, the case did contribute to the permanent closing of one of the last open public shooting spots in Multnomah County.

 

[Member 18201]

If you load a new cartridge into a AR-15 magazine (or chamber), and let the bolt slam forward, you can then extract it without firing and note a small but visible dimple on the primer.

This is also the case with many rifles that do have firing pin springs.

 

[Quirt Evans]

if you let the bolt slam closed on a new round. If you load a new cartridge into a AR-15 magazine (or chamber), and let the bolt slam forward, you can then extract it without firing and note a small but visible dimple on the primer.

Yes, it's easy to see, and because of this CCI 400 primers are NOT recommended for AR use, as the cups are thin, and the risk of one of them detonating does exist, although it's a small one.
Early one, Colt lightened the FPs to decrease the risk, but it's still a heavy pin.
A friend of mine did experience a slam-fire, but it was in a Bullpup design.
I put a Titanium FP in my AR, and now an extracted/unfired round shows no sign of any dimple on the primer.
If an AR slam-fires before the bolt is locked,, you're gonna have a real bad day.