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A pacifist pyrokinetic in my story is accosted by several armed men. They have handguns, but they are currently holstered and do not have rounds in their chambers (these nameless thugs are all about firearms safety). The pyrokinetic's ability is pretty minor (limited to creating small sparks).

She believes that if she can create a spark inside the first cartridge in each magazine, the cartridge would fire. Because the bullet has nowhere to go, it doesn't shoot out of the pistol (avoiding a potentially lethal ricochet) but does jam the whole contraption and make a scary noise.

So there are a few questions here:

  • Can a spark (as opposed to a regular strike by the firing pin) ignite a cartridge's powder, causing it to fire?
  • Would such a bullet penetrate the magazine and pistol grip, and shoot out into the world?
  • Would a bullet that shot out in such a way still have enough kinetic energy to ricochet off the floor and then injure or kill someone nearby?

I assume the power of the cartridge depends on the type, so please let me know if different types of guns would behave differently (say, a .44 Desert Eagle vs a 9mm Luger).

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    $\begingroup$ There should be zero problem if the spark magically appears inside the cartridge. $\endgroup$ – Xandar The Zenon Dec 29 '16 at 6:21
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    $\begingroup$ The real threat here is making a CHAMBERED ROUND go off. That would effectively fire a bullet straight through the bottom of the holster and may hit the thigh, leg, or foot of the person wearing the holster. $\endgroup$ – Jason K Dec 29 '16 at 15:19
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    $\begingroup$ @Jason K: But these thugs are into firearms safety, and so don't carry their guns with a round in the chamber. $\endgroup$ – jamesqf Dec 29 '16 at 18:03
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    $\begingroup$ Are you asking or is your pacifist pyrokinetic asking? Because I have this funny picture of her being accosted by the armed men and thinking to take a time out to ask WorldBuilding SE before proceeding.... $\endgroup$ – Michael Dec 29 '16 at 23:06
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    $\begingroup$ Sorta OT: in most circumstances it's safe to carry with a live round in the chamber. $\endgroup$ – Eric Lagergren Dec 30 '16 at 15:56

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On the whole your idea seems perfectly sound. Let me go through your individual questions.

Can a spark ignite a cartridge?

Probably yes, though it depends on where the spark is located and how hot it is. Cartridges are surprisingly robust and well-sealed, so an arbitrary spark nearby will almost certainly not ignite it. However, they can "cook off" in sufficient heat, so if the "spark" is a localized heating, and you can localize it either inside the cartridge or in the primer, then yes, it will absolutely go off.

Would such a bullet penetrate the magazine and pistol grip, and shoot out into the world? Could it ricochet and injure or kill someone nearby?

Probably not. Magazines and gun frames are generally fairly robust, and without a barrel to contain and focus the energy the cartridge won't do a lot. Here's a video from the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) on the subject of "what happens when ammunition burns". The video is a great watch, but the first few minutes will show you all you need to answer this question. Without a barrel to contain them, most cartridges (even rifle and shotgun cartridges) won't blast through plywood, let alone a metal gun frame.

Would different calibers behave differently?

Absolutely, but probably not enough to matter. The more powder is present, the more pop there will be when it goes off. I wouldn't want to have this happen with a gun I was holding or had in my pocket or waistband, but if it had to happen, I would much prefer this happened with a 9mm than with a .44 magnum!

You didn't ask this, but also note that setting off rounds in a revolver cylinder has different results than in the magazine of a semiauto. Unlike a box magazine, the chambers of a revolver are built to contain and direct the pressure, so a lot more damage could result.

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    $\begingroup$ Have to slightly disagree. A spark inside a cartridge would ignite the propellant. See various muzzleloaders which are fired by e.g. sparks from a flint. Also, the cartridge probably would blow the magazine apart, perhaps with enough force to injure the shooter's hand. It's not that uncommon for an obstructed barrel to burst, e.g. wideopenspaces.com/good-guns-go-boom-catastrophic-gun-failures and this would be similar. It's not the bullet that does the damage, but the explosion - like a big firecracker or small grenade. $\endgroup$ – jamesqf Dec 28 '16 at 19:13
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    $\begingroup$ @jamesqf A spark inside the cartridge will not cause an explosion. Smokeless powder is not black powder; it has a controlled rate of burn and will not detonate. The relatively slow expansion is only useful if it is controlled in a confined space, like a magazine chamber. It will not 'blow the magazine apart.' $\endgroup$ – kingledion Dec 28 '16 at 20:06
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    $\begingroup$ @kingledion A quick spark might not, but this person is a 'pyrokinetic' so a small flame inside a cartridge (much like that from a primer that is used in most pistol ammunition these days) would indeed cause the cartridge to ignite. The bullet igniting would definitely cause some kind of damage to the mechanics of the gun (a jam at the very least, and irreparable damage at worst). I don't envision the entire magazine exploding though. $\endgroup$ – BaseHobo Dec 28 '16 at 21:09
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    $\begingroup$ @kingledion: A spark will ignite ordinary gunpowder - I've done it, in my younger days. All powders have a controlled rate of burn. For instance, you use a faster-burning powder for pistols than for rifles & shotguns, because for effectiveness it should all burn before the bullet leaves the barrel. I think the clip & chamber should provide enough confinement for an explosion, though I admit I've never tried it myself, never having had the urge to wreck a perfectly servicable pistol. Cort Ammon's answer gives a better explanation, $\endgroup$ – jamesqf Dec 28 '16 at 22:41
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    $\begingroup$ @Daerdemandt Magazines are not nearly as gastight as a chamber. It doesn't take much for the gases to escape. I wouldn't want to be holding a weapon when a round went off in a magazine, but the OP's question states the gun is holstered. With the force blocked by a holster and some blue jeans, it would be unusual circumstances if the gun-haver was affected by anything more than surprise. $\endgroup$ – kingledion Dec 29 '16 at 3:18
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The key to this question is to note that gunpowder doesn't technically explode -- it deflagrates. It doesn't have a super-sonic explosion, but rather a sub-sonic burn. To get the powerful kick needed to project a rifle or handgun bullet, we rely on the fact that gunpowder burns faster in a confined space. The tighter the space, the more temperature and pressure it can achieve.

Fired properly, the gunpowder is confined by the barrel, permitting it to reach the high pressures of a gun shot. Outside of a barrel, the brass case holding the gunpowder and the lightly set bullet provides surprisingly little containment. One can "cook off" a bullet over a fire, and the result is sudden and surprising, but far from lethal.

A single spark on the outside of the case would have a hard time setting off a bullet. Having done this once in a controlled setting to test the safety of such a bullet, it can take several seconds over the top of a torch to reach the critical temperature to deflagrate. A lone spark may have trouble. However, if your pyrokinetic can put the spark on the inside of the bullet, that'd be a very different story. That would be remarkably similar to what the primer actually does when firing the gun!

As mentioned earlier, the bullet would most certainly not escape the magazine. Most magazines are made of steel, and many tests will show just how little momentum the bullet actually picks up. Most of the time it's just shoved out of the case just far enough to give the gunpowder room to burn. However, we have to recognize that this is still a confined space inside the handle of the gun. While it's not as small and well structured as the space inside a barrel, the gunpowder is still going to have to find an exit. It will build up pressure until it does find enough of an exit. This could be enough to cause damage.

The particular behavior is very dependent on the particular handgun and its construction. A revolver would most likely just shove the bullet out of the front of the cylinder, with little to no damage. An all steel handgun like a 1911, however, may contain the pressure better. This means it may fail in a more spectacular way. The small clip that holds the magazine into the gun would be my guess for "first to fail," causing the entire clip to pop out of the gun. If you had a "plastic" gun like a Glock 19, you could be in worse trouble. The bullets in the magazine are held in by a similar pin, but there's open access from the magazine to pressurize plastic all over. There's a decent chance that the force of the powder could rupture the plastic around the bottom edge of the slide (which is typically at a particularly nasty position for spraying plastic bits all over the gun's wielder).

Another question would be what happens to the other bullets? Depending on the exact mechanics of the rupture, you might push one of the other bullets out of the way, exposing another case full of gunpowder. This would create a much larger effect, though it's not immediately clear what sort of mechanical topologies might cause this.

Ironically, rifle bullets might have a less extreme effect than handgun bullets. Many rifle calibers involve a large chamber for powder necked down to a smaller bullet. In a barrel that is shaped for this, this allows for devastating power. However, in a magazine, that space would simply be expansion room for the burning powder. That extra expansion room may keep the pressure down enough that the rifle round may never reach the high pressures that could cause serious damage to the handgun, despite having more gunpowder to work with.

All in all, I don't recommend experimenting to find the answer =) While there's still squabbling over whether guns kill people or people kill people, everyone agrees that a misfired gun is a dangerous device and must be treated with respect until the misfire is resolved.

Edit - From a long discussion in comments, it looks like the question of revolver rounds is of interest. Thanks to Deolater and Supercat for tugging at this thread, and Supercat for bringing data to the table!

The key equation for determining the speed of a bullet is $F=p\cdot A$, the force propelling a bullet forward is the pressure behind the bullet times the cross sectional area of the bullet. Using the formula for work: $W=\int_0^LF\, dx$ where L is the length of the barrel, we can do some comparisons. Then, knowing that $E=\frac{1}{2}mv^2$, we can back out the velocity by noting that the velocity is proportional to the square root of E ($v\varpropto \sqrt E$)

We can consider two idealized cases for the powder burning. The first assumes constant pressure, and the second assumes the powder burns all at once, maximizing pressure at first. A realistic bullet will fall between one of these two extreme cases based on how fast the powder burns.

In the case of a constant pressure, we see $W=\int_0^LpA\, dx$ and thus $W \varpropto L$, where L is the length of the barrel. This means that $v \varpropto \sqrt L$ for the constant pressure case. In the case of an instantaneous burn, the pressure behind the bullet will obey some $p(x)=\frac{P_0}{x+C}$ where $P_0$ is the pressure at the start and $C$ is a constant capturing how much space is behind the bullet where pressure can be built before the bullet starts moving. This gives $W=\int_0^L\frac{P_0}{x+C}A\, dx$. If we cleverly choose units of length such that $C=1$ and thus $W\varpropto \ln(L+1)$.

Now we can put some numbers to this. Thanks to supercat's find, we have a table for a .375 magnum. Now .357 is rather convenient in that $C$ is roughly 1 inch (the case is 1.29" and the bullet rests a bit inside, so 1" is actually probably very close to correct). If the powder were to burn with a constant pressure, the energy after moving 1/2" (escaping the edge of the chamber) would be 1/12th that of the energy when escaping a 6" barrel, and thus a velocity that was .00694 that of the 6" barrel shot. This would put its velocity around 10fps. If we instead assume an instantaneous burn, we can use the second set of equations to see that the energy would be about 20.8% of the energy escaping a 6" barrel. This second equation would instead put its velocity at 65fps. The actual speed would be somewhere between these extreme assumptions.

Sure enough, if we graph muzzle velocity from the page of .357 data, we see a sharp knee in the curve as the barrel length gets smaller. The data doesn't go below 2", but extrapolating the lines confirms that the velocity exiting the chamber would be very small with respect to that of a properly fired bullet.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Serban Tanasa Dec 31 '16 at 0:50
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As mentioned, the unconfined explosion of the round will burst the cartridge case. Depending on where the case actually is, you may or may not damage the weapon itself (best case scenario, worst case you blow out the bottom of the magazine so the bad guy ejects the magazine and reloads a fresh one), and there is always a possibility the hot gasses and projected particles of metal will cause injury to the person carrying the weapon, but if it is properly holstered (as implied in the question), the weapon's frame and the holster material will likely catch everything.

The shooter is likely to be startled, and if you are in some sort of life and death confrontation (hence being surrounded by armed men and using pyrokenisis to defend yourself) this might actually be the trigger for the armed men to leap into action. They may well draw and attempt to use their firearms, but on discovering the weapons are jammed they may transition to secondary weapons (a holdout pistol, a taser, a knife or extendable baton) or even simply wade in with fists and attempt to fight it out hand to hand. The more trained these people are (i.e. ex cops, ex SoF mercenaries working for the drug cartels) the faster they will transition so the window of time to escape is very limited.

Your pyrokenisis person might actually have better success attempting to ignite disposable butane lighters in their pockets. The unconfined explosion will not be surrounded by a metal and polymer gun frame or enclosed in a holster, and the liquid butane remaining will ignite the bad guy's clothing.

If they are non smokers, search for the one carrying a Samsung Galaxy 7 mobile phone and go after the battery.....

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    $\begingroup$ Blowing up their phone batteries, brilliant! Maybe that deserves another question... $\endgroup$ – SPavel Dec 28 '16 at 19:46
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    $\begingroup$ Yeah, it works with any common lithium battery, every phone, camera, USB battery, lots of other stuff. The problem with the S7 was just that it blew up even without a pyromancer igniting it. :-) $\endgroup$ – Nobody Dec 28 '16 at 21:14
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    $\begingroup$ @SPavel Going after lithum batteries isn't paticularly pacifistic: lithum batteries go off like a model rocket engine. See "bonus fire" electronics.stackexchange.com/a/230164 $\endgroup$ – Schilcote Dec 29 '16 at 2:20
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    $\begingroup$ Suppose the magazine is wrecked, they eject it, and the gun is still good. First, the bad guy won't KNOW the gun is still good, and won't TRUST the gun to fire reliably. Second, if the bad guy thinks this was done by the victim's superpower, he WON'T load another mag, because he'll assume the vic will simply do it again to his next mag, and his next mag, until he's out. And he really doesn't want to be out, in case another enemy shows up (there's always the police). $\endgroup$ – Harper Dec 29 '16 at 4:02
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    $\begingroup$ @Thucydides: But the OP states that these are "thugs", not well-trained soldiers. They probably won't even be carrying spare clips, being "trained" on Hollywood action films where shooters never have to reload. $\endgroup$ – jamesqf Dec 29 '16 at 4:46
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Round
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Can a spark (as opposed to a regular strike by the firing pin) ignite a cartridge's powder, causing it to fire?

The wording is a bit conflicted. You state inside in text and then a in the question.

The function of the primer is to create a spark inside so yes if you create a spark inside it will ignite the gunpowder. But I cannot imagine how you would create a spark inside the (sealed) round other than the primer.

An external spark is not going to ignite the gunpowder in the round.

Would such a bullet penetrate the magazine and pistol grip, and shoot out into the world?

The bullet as in head - no. The gunpowder is a propellant. It takes the chamber and barrel to harness the propellant and transfer that energy to bullet.

That casing is very light compared to the chamber of the gun.

It would just fracture the casing and propel the bullet a (relative small) bit.

I seriously doubt the bullet would exit the gun.

The casing might fracture the handle.

Would a bullet that shot out in such a way still have enough kinetic energy to ricochet off the floor and then injure or kill someone nearby?

I don't think it would even have enough energy to exit the gun.

I assume the power of the cartridge depends on the type, so please let me know if different types of guns would behave differently (say, a .44 Desert Eagle vs a 9mm Luger).

Bigger rounds have more powder but they also have thicker casing. But at that .44 Desert will likely be more carnage.

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    $\begingroup$ I think its a super power, so should be able to create sparks anywhere? $\endgroup$ – axsvl77 Dec 29 '16 at 0:50
  • $\begingroup$ @axsvl77 Fine a super power creates a spark inside the casing. How does that change my anwser? $\endgroup$ – paparazzo Dec 29 '16 at 1:13
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    $\begingroup$ Not sure; I am ignorant about this topic, and learned the most from your answer. You wrote "I cannot imagine how you would create a spark inside the (sealed) round other than the primer. " So I'm saying what happens if the super power can do it? $\endgroup$ – axsvl77 Dec 29 '16 at 2:13
  • $\begingroup$ If it can create sparks anywhere then alveoli would be a nice target. Almost any assilant carries those. $\endgroup$ – Daerdemandt Dec 29 '16 at 2:14
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    $\begingroup$ @Paparazzi The OP indicates that the pyrokinetic person is a pacifist. It is the second word in the question; so killing prolly isn't what the story is looking for. $\endgroup$ – axsvl77 Dec 29 '16 at 2:51
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This is a highly firearm specific question. Ammunition in a magazine is not confined in a reinforced tight space like a chamber so the bullet won't fire forward very far.

BUT the front-back tolerances of the magazine may be tight enough that the ignited bullet can NOT push out of the cartridge because it hits the front of the magazine. See how little forward room is available in a 1911 magazine.

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This will block the release of gas and allow for a "kaboom!" (KB) inside the magazine which will bust the case and send gas and case fragments laterally. This can be a problem in magazines with ammunition witness holes or lightening slots on the sides of the magazine, now there are weak spots for gas to vent out of.

enter image description here

Pair this with a pistol that has open spaces around the magazine (like the 1911 if you take the pistol grips off) and you have an additional weak spot for expanding gas and fragments.

enter image description here

Most grips on steel framed pistols are wood or resin/plastic and can fragment.

enter image description here

So with a sufficiently large caliber round, a magazine with a tight front/back fit that traps the bullet into the cartridge (remember that the bullet is seated a good ways back into the case), and an open sided magazine and pistol frame, you COULD have a KB that cracks the grips and sends out some fragments into the immediate area (like the back of the guy wearing the pistol in a waistband holster). If someone was holding the pistol then their palm or fingertips could be injured (depending on which side blew out).

In general I think most magazines don't have tolerances tight enough to keep the bullet inside the case (but I'd have to check) and they have enough space inside the magazine to absorb all the gas (double stack mags especially), OR the bottom of the mag would blow out first, since the magazine is enclosed all around or the pistol frame is, OR the cartridge won't have enough energy to do any damage regardless. BUT you could have a unique situation where this tactic would work pretty well, like if the bad guys are carrying this pistol...

enter image description here

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    $\begingroup$ Even if the bullets weren't allowed to move at all, brass cases are generally not strong enough by themselves to hold a good seal with the bullet once fired (instead they are generally designed to expand slightly so as to form a seal against the chamber). I would not want to be holding a firearm when the rounds in the magazine went off, but the effect would be far smaller than if the same amount of powder were set off in a pipe bolb. $\endgroup$ – supercat Dec 29 '16 at 21:34
  • $\begingroup$ @supercat I agree, the effect won't be catastrophic, possibly not even noticed, except in a fringe case where a mild injury could incur. Would definitely prevent loading the round though, forcing a magazine change. $\endgroup$ – Jason K Dec 31 '16 at 17:42
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Having reloaded ammunition for over 40 years, I'd like to correct/clarify a few statements from other answers.

First, the spark would have to be inside the cartridge and either 1) be equivalent to that of the primer, or 2) detonate the primer, not ignite the powder. Primers are made with explosive materials, making the flakes/balls/rods of powder ignite essentially at the same time, whereas a small spark would ignite some powder, spreading to the rest at a rate much slower that a fired round and result in lower pressure.

Second, the gasses will take the path of least resistance. In the open, a cartridge "fired" would propel the brass much faster and further than the relatively heavy bullet, as described by Newton's laws (momentum, equal opposite). A contained cartridge will split the case and pressure would be released through the ruptures, not by propelling the bullet or casing held together by the magazine. The split casing could, of course, become hazzardous shrapnel, depending on the magnitude of the pressure developed before the casing ruptures.

Third, rifle cartridges, in general, produce much higher pressures than pistol cartridges. Pistol cartridges, including the former "most powerful handgun in the world" .44 Mag, have a very large amount of empty space. When reloading handgun ammo, one should always carefully check the powder level in each cartridge prior to seating a bullet, because it is quite easy to put a double load of powder in a pistol cartridge without it being visually obvious. Rifles, by definition, have longer barrels, and so require more, and differently formulated, powder to get a consistent burn throughout the travel down the barrel. Many rifle loads, especially Weatherby magnums, fill the cartridge to capacity. Cartridges used in both pistols and rifles, such as the .44 Mag, can be loaded "hotter" for use in rifles, but these loads can not be used in pistols. The upshot of it all is that pistol cartridges will, in general, in this scenario, result in a lower pressure "explosion" than a rifle cartridge.

Now, a plastic shotgun shell should give a more impressive, if not more dangerous, display of pyrotechics, as I think it likely the shell would rupture and release still burning materials. It would have more flare, and perhaps more flair. :)

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Can a spark ignite a cartridge's powder causing it to fire?

The way you have described the pyrokinetic's ability (i.e. 'limited to creating a small spark'), no. Modern cartridges are well sealed in metal, and are specifically designed not to cook off. In fact, according to the gun nuts I know, a lead bullet will melt (327 C) before the cartridge cooks off. The cartridge is specifically designed to go off only if the ignition cap is hit by the firing pin. I have only ever heard of cook off in a raging house fire or when a weapon has been shot excessively and has a smoking hot barrel.

Would a bullet penetrate the magazine and shoot out?

If you throw a .22 cartridge into a campfire, you get a little pop like a firecracker. Other cartridges are more powerful, but the effect is the same. As mentioned before, the temperatures that cause a cartridge to cook off will cause lead to melt. Thus, you are more likely to have a half-hearted pop due to the neck of the cartridge no longer being tightly sealed around the bullet. You will certainly not get enough energy to blow up your magazine, or even to cook off adjacent cartridges in the magazine.

Would a bullet that shot out in such a way be able to kill someone?

No.

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    $\begingroup$ What if the pyrokinetic places the small spark inside the gunpowder or the primer? That would have the exact same effect as if the firing pin hit the cap, making the bullet go off. So whether it's possible or not depends on the pyrokinetic's ability to place the spark with high accuracy (which might be easier said than done, seeing that it's a fairly small volume localized somewhere out of plain sight). $\endgroup$ – Mrkvička Dec 28 '16 at 19:43
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If the spark is created inside the cartridge the powder will go off. Without being properly confined by the gun the imparted velocity is low (and is mostly to the cartridge rather than the bullet), about the only way you could get a serious injury is if a piece went into an eye.

However, there are two other factors at work:

1) It's still fire. The energy released will find a way out, period. The strongest part of the gun (the firing chamber itself) will blow up if the pressure can't be vented normally by propelling the bullet. With a semi-auto gun the extra energy is ejected out the front and normally harms nobody. With a revolver the seal between the part holding the bullets and the rest of the barrel isn't perfect and enough energy can vent there to cause serious injury if you have your finger in the wrong place when you fire it. In this case the energy is either going to burst the magazine or vent where the magazine connects to the gun. In either case I would expect at a minimum second degree burns to the person with the gun.

2) Again, the energy must go somewhere. The weakest point will give, I would expect the magazine to be ejected with considerable force. Nothing lethal but I certainly wouldn't want to be the guy in it's path.

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As already noted in other answers:

  • No rounds will "fire" unless the spark is created inside the metal cartridge of ammunition, and possibly not even then.
  • When a round "fires" in the magazine, the bullet will separate from the cartridge, but not with much energy.

I've read that if a modern cartridge goes off while it is lying about (not in a tight container), the casing (aka bullet case) is more dangerous than the bullet, because the casing can be propelled like a little rocket.

In the confines of a magazine, neither bullet nor casing can go very far unless propelled by great force. As soon as there is a sufficient gap between the casing and the bullet for the gas inside the cartridge to escape, out it goes. The bullet and casing don't even have to move; the gap could occur simply by the expansion of the mouth of the casing.

So if you can get a cartridge to go off, the effect will be a lot of hot gases squirting out of various orifices and joints of the handle and frame of the firearm. Whether they do any harm depends on what they hit next. I suppose a military-style leather holster with a flap over the handle of the firearm would probably completely absorb the energy of the gas; it might end up a little singed.

If the round goes off while someone is holding the firearm by the handle it might be a very different story. When a revolver is fired, some gas escapes through the gap between the cylinder and the barrel, which can be very dangerous. The gases escaping the round that went off in the handle would be as hot as the gases from the revolver but propelled by less pressure. I'm not sure what the result would be; something between mild burns and amputation is my guess.

In any event, if a round went off you'd have a loose bullet and deformed casing in the top of the magazine, which seems pretty sure to cause a jam if the user attempts to load the chamber in the usual way without replacing the magazine first.

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As stated in most of the other answers, a spark outside of the case will have no effect on the ammunition inside the magazine. A spark inside the case would likely be identical to the effect the primer has, the round would go off. The round would deflagrate as others have mentioned, but would have little force. The brass casing would be the piece that would move or deform the most (the Mythbusters did a great job of showing this). The bullet (the front part inserted into the casing) would not move much at all.

What I'd expect to happen would be the top round in the magazine would go off, bursting the brass casing and pushing down on the rest of the rounds in the magazine (but not with enough force to cause them to fire as well unless some shrapnel struck the primer(s) of the lower round(s)). Since the bolt would presumably be in place above the affected round (who carries a gun with the bolt back?), the force has nowhere to go but down through the magazine well, this would most likely blow the bottom of the magazine off causing little to no damage to the firearm, but would render it inoperative until the operator cleared the damaged magazine from the well, and checked that no damage was done to their weapon. As pointed out by an earlier post, if the magazine has many perforations (pretty common, think 1911 as shown in the example above), the gasses would be expelled from those, lessening the potential damage to the weapon, and making it uncomfortable if the operator had their hand on grip of the weapon as the gas escaped. If the magazine didn't have many perforations (think Glock, XD, etc. most of the "plastic" guns), most of the force would be on the magazine base plate, which could quite likely be blown off.

If the plan was to make their weapons inoperative, this would do it, at least for a short time, maybe enough for the pyrokinetic to get away.

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Quick and simple answer. Modern day guns and ammo are too well sealed to be subject to the kind of damage your heroine is looking for. What you describe is a very real phenomenon and common occurrence with black powder revolvers which are still sold today as reproductions. A spark can jump from one cylinder to another in these types of guns if the projectile ball is not snug enough. The powder in 1 or more cylinders then explodes, sometimes forcefully enough to expel the projectiles. So to paraphrase Dirty Harry: "Being that this only works for a .44 caliber cowboy gun, you've got to ask yourself one question. Do you feel lucky steampunk? Well do you?"

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    $\begingroup$ I don't think you fully read the question, the protagonist will magically place fire inside the cartridge. How will modern, well sealed ammo be protected against that? $\endgroup$ – Mrkvička Dec 29 '16 at 8:26
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Creating a spark inside the cartridge would ignite the propellant. However in the magazine of a semi-auto pistol, without the confinement of the gun's chamber, the bullet will not move very much. The relatively soft brass at the mouth of the case will expand and open a gap around the bullet, and the hot gasses will escape around it. There might be enough force to dent the magazine or possibly jam the round in it, but not much beyond that.

If the frame of the pistol is made of a polymer material, the hot combustion products could conceivably cause a great deal of damage to the firearm. It might melt.

If the pistol is metallic, however, there wouldn't likely be any permanent damage. Given what I've seen happen with bad reloads, however: if the cartridge mouth is expanded by this, it would probably jam when the operator attempted to feed or extract the cartridge by racking the slide. The slightly oversized case or the loose bullet could get stuck. Ejecting the slide and reloading with a fresh magazine would result in an operable firearm. Whether in the heat of the moment someone would have the presence of mind to eject and reload rather than try and quickly arm their weapon is another story, especially given that rounds simply don't ordinarily go off on their own in a magazine.

Also I'd say it's highly unlikely that everyone would be carrying in an unchambered/safe mode. Setting off a round in the magazine would likely not prevent the one round already in the chamber from being fired, but I think it would be very likely the weapon would jam as it tried to chamber the damaged round.

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To answer as an engineer: If your pyrokinetic can project his spark into the PRIMER of the bullet, it will go off explosively and ignite the powder just as if struck by the gun's hammer. However, the bullet has very little room to move forward before it presses against the steel wall of the magazine. This wall is stronger than the brass wall of the cartridge. Therefore, the cartridge most likely bursts open inside the magazine with considerable force. How much force? Well, a .357 cartridge, once ignited, releases an energy in the vicinity of 850 Joules. Look at it this way. Suppose you took a gun, and shot a bullet into an IDENTICAL gun.
The burning powder releases 850 Joules of energy. The bullet carries the 850 Joules through the air. It hits the identical gun and transfers 850 Joules into the body of that gun, totally shattering and destroying it. That energy-level is more like a baseball bat hitting something. A .357 bullet will knock a grown man right over. It is not like some wimpy firecracker going off in your pocket as some posters would have it.
If your pacifist-pyrokinetic character wants to set off "just the first" bullet in each gun, they may have an unintended consequence, the bad guys might catch shrapnel from the sides of their plastic-stocked guns blowing up at their sides, but then, the excess heat in the magazine might start setting off the other bullets, catching fire in a chain reaction. The bad guys might have to tear off the useless weapons and throw them aside.

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    $\begingroup$ The impact of the bullet isn't what knocks people over though, otherwise the person firing the gun would be thrown backwards rather than just experiencing a kick. $\endgroup$ – Tim B Dec 31 '16 at 10:57
  • $\begingroup$ A .357 magnum bullet will not "knock a grown man right over", the difference in mass is too great. The Mythbusters proved this multiple times by shooting into pig carcasses. It might stagger them, but it's unlikely to knock anyone down. Even 12 gauge slugs fired at close range won't knock someone down. Unless the deflagration is contained in firing chamber (the brass is completely surrounded by the bullet, the chamber and the rear of the frame), the brass will move far more than the bullet and the resultant explosion will be far less productive than a round fired normally. $\endgroup$ – delliottg Jan 17 '17 at 22:49
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I guess my answer will be the less technical.

Just how realistic this has to be? I think that if you establish that sparks can pop bullets inside a cartridge without dealing any damage, this won't be a problem.

Your character is pyrokinetic, so suspension of disbelief is already in action.

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the spark would likely have little effect since the sealed cartridge has little to no oxygen contained within. The difference between a "little spark" and a primer being discharged (with listed pressures of between 8,000 to 10,000 psi depending on the manufacturer) is substantial. It's the pressure of the primer "explosion" that sets the powder off along with the flame involved. Can't imagine the "little spark" doing much more that causing the powder to smolder. Smokeless gunpowder open to the air will burn with a small spark to ignite it but not in an enclosed chamber without the added pressure from the "starter explosion" caused by the primer being set off.

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    $\begingroup$ Smokeless powder is self-oxidizing and does not need any oxygen to go off. A gun will fire in a anoxic atmosphere, or even in vacuum. $\endgroup$ – kingledion Dec 30 '16 at 2:22

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