320 Theory -- Mag Pressure "Jacking" the Slide Upward?

Posted by T20_puddlejumper@reddit | Firearms | View on Reddit | 17 comments

It seems like every 320 incident we've seen happens when a fully loaded firearm has been staged for use (cop, competition, mil) and is either in a holster or has recently been placed in a holster, with it possibly being jostled but without the trigger being touched.

Here' my theory:

That upward mag pressure jacks the slide up, thins out the sear engagement to a hair’s breadth, and bypasses the safety-margin gap. Every "test" is focused on mechanically defeating the gun's safety by manipulating the slide, playing with the trigger, etc. but I don't think anyone realizes that the slide and frame are further apart when fully loaded than they are when empty.

Once the mechanical safeties have been bypassed by the gap, the gun is a ticking time bomb and goes off when jostled or moved, or at any other time. Then the user IMMEDIATELY unloads the gun, the tolerances go back to an acceptable margin, and the "uncommanded discharge" can't be replicated (even with the same gun).

Thoughts? Can someone with a better mechanical understanding of the pistol theorize with me?

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Leafy0@reddit

It could be part of it. But remember 2 things need to go wrong in the gun for it to fire from the holster. The firing pin safety lever needs to be pushed up and the firing pin needs to come off the sear. You’ve come up with a way the firing pin comes off the sear, but what is causing the firing pin safety to also fail? We haven’t heard many reports of people drawing their gun and having a dead trigger which is what falling off the sear but being caught by the safety would do.

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WindstormMD@reddit

There are cases of the FP safety sticking in place from lubricant contamination and other factors, so that combined with OP’s theory would set up a perfect storm for a gun where the tolerances on the slide to the frame rails was sufficiently loose

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WrathfulMechanic@reddit

During a regular cleaning on mine I lost the firing pin safety spring and didn’t notice till I found it after I had assembled the pistol. I can imagine there are some P320s out there sans a working safety.

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SilenceDobad76@reddit

If that was it Sig would have reported that quickly on examined guns.

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Leafy0@reddit

There most likely are, but a missing or misplaced spring would probably have been found in any of the investigations so far. My theory on that is that the fp safety lever spring can slip off that post and allow the lever to hang up, but when the nd happens the recoil causes everything to fall back into place.

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raz-0@reddit

Anyone not a moron who is trying to figure out what is going on is fully aware of what a loaded mag does in terms of physical positioning of parts. People who are trying to replicate the issue, even with guns that have been involved in an incident, are using loaded magazines when they fail to reproduce it. The minimal amount the slide to frame fit changes with a loaded magazine is nowhere near sufficient to bypass safeties.

The vast (if not all) incidents with the 320 are the result of someone or something operating the trigger.

The situation you describe of partial sear engagement CAN occur if you have the wrong slide catch in the gun (10mm/.45 catch in the 9mm/.40/.357 sig frame). But this is a pretty verifiable situation after the fact, and there has been no confirmation this has actually happened in any actual incident out in the wild. You still have to have a non-functioning striker safety tab though.

It can also occur with a foreign body in the gun's action. I've had this happen (overtravel screw on an aftermarket trigger worked itself loose and went into the gun) But at least in my situation, you'd have to reholster a loaded gun that was clearly having problems and should not be trusted at that moment. But essentially what can happen with a foreign body in the action is equivalent to having the trigger partially pulled. The safeties are disabled and the sear is not properly engaged. There was one military accident with no injuries where it was also effectively a foreign body. Their summary indicated the foreign body was a damaged striker safety, so no functioning striker safety either. It can also happen with an aftermarket trigger with pre-travel adjustemt where someone who doesn't know what they are doing has adjusted so much pre-travel out that they have no drop safety anymore and have started also taking out sear engagement.

You can create incorrect sear engagement by reassembling the gun correctly. But I have yet to see anyone do that and be able to chamber a round from a magazine. this situation also does not disable the striker safety.

Some guns DO have some tolerance stacking issues that can result in sear travel disabling the striker safety. See everyone doing the pencil test and tripping the sear through the back of the gun. but you need gross motion of the sear, which nobody has provided a credible scenario for that happening in actual use. I don't believe anyone has demonstrated it happening with small incremental motion like you'd get with vibration causing sear creep (also something nobody has managed to reproduce).

Now if someone can come up with a scenario that results int he gun being in battery with a round chambered, with partial sear engagement, I DO have a very mundane, plausible path to the striker safety being disabled. Namely that if you are the kind of person who cleans everything way too much and too often, and does not do a proper safety function check on reassembly, you can wind up with a non functioning striker safety tab. If you pull out the striker assembly and clean it with a toothbrush, you can lift the spring leg off of it's perch and it no workie no more. This is not terribly easy to do, but with a brush with stiffer bristles than a toothbrush, it is much easier. But you also have to not be function checking it on reassembly.

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T20_puddlejumper@reddit (OP)

Is it that implausible to think that your process defeats the striker/firing pin safety, and my process later defeats the sear/firing pin engagement?

Together, there's your explanation.

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raz-0@reddit

No. Your process is "loaded magazine pushes up slide and creates unsafe minimal sear engagement". That does not happen. Nobody has demonstrated an out of spec/tolerance stacking sample that does that. The only case in which that happens with a box stock gun is during reassembly. Following the reassembly procedure properly will return it to full engagement. Following the reassembly procedure improperly will make for YouTube click bait, but should result in a gun you cannot insert a magazine into until you manipulate the slide release and return the gun to full sear engagement. In a gun where sear engagement is compromised for whatever reason, inserting a loaded magazine has not been sufficient to trip the sear unless inserted quite vigorously. In which cases it is less a matter of the range of motion, but the momentum imparted to things. Inserting a magazine cannot overcome the range of motion imposed by slide rail to slide fitment, it just shims it up to (or nearer) one end of that range of motion. The full range of motion is accessible with no magazine in the gun as well. There's just very little likelihood it is significantly contributory and it demonstrably has not been overlooked in testing and attempts to reproduce failure.

What you suggested could be the case if the action was designed for tight tolerances and we got sloppy tolerances in the actual product. But it is not. As designed, with the slide movement the design permits, there is similar amounts of sear engagement to other striker fired pistols regarded as safe. There have been people who have put samples through pretty extreme vibration testing, and it has not caused issues.

If one views the protection from discharge as a belt and suspenders issue (i.e. sear is belt, striker safety is suspenders for my purposes), there are very limited paths to an unintended discharge that don't require removal of the suspenders first. They are 1) unintentional operation of the trigger. and 2) foreign body intrusion to the action. On pre-upgrade guns, you can add 3) sufficient impact to the rear of the gun at the correct angle. Although I'd argue that it too is technically unintentional operation of the trigger.

I do believe there was a factory configuration that could be dangerous. There was a revision of the sear that seated the sear springs in sockets instead of on nubbins. Some people had sear springs become displaced and interlocked. Some instances of that caused dead triggers. Anything that is a path to a dead trigger is effectively impairing the belt part of the belt and suspenders safety. Which means anything that messed with the suspenders part could potentially let the gun go band when you don't want it to. I will also point out here that in a quest to create something akin to the glock $0.25 trigger job, some people out there put forth deliberately crossing and interlocking the sear springs to reduce the effective spring rate. This was deliberately affecting the belt part of the equation, and stupid as all get out. It fairly rapidly got abandoned as stupid, but it was because it sacrificed reliability for little to no benefit. Nobody brought up the safety issues at the time as far as I could tell.

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906Dude@reddit

That is truly an interesting line of thought. What about the striker safety though? Shouldn't that still function? (Although, tbh, the design of it on that model doesn't inspire me with confidence).

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T20_puddlejumper@reddit (OP)

Another commenter mentioned how improper reassembly can disable the striker safety. Seems like a disabled striker safety + a fully loaded "mag slam" knocking it off the sear could very well be it

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Alita-Gunnm@reddit

And people test with a single (blank) round chambered, or nothing chambered, and an empty mag. This would be a good thing to test; fill the mag all the way with snap caps.

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T20_puddlejumper@reddit (OP)

Right! exactly

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ASnakeNamedNate@reddit

Just chop the stupid reverse leg off the sear and add a trigger safety to compensate for drop safety and call it a day. The whole system relies on the striker safety lever not depressing the striker safety lock, there’s no reason the sear should move the trigger.

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