I have a character who can trigger and inhibit combustion; I know him using his power to “cook off” the bullets in someone’s magazine would be a bad thing for the person holding it, I’m just wondering as to how bad.
This is going to depend on the weapon, and exactly what your character is doing.
It’s worth remembering that the normal meaning of, “cook off” doesn’t apply here. Under normal circumstances, a gun cooks off its ammunition when the chamber becomes hot enough (from sustained use) to ignite the powder in any freshly loaded round. On most automatic and semiautomatic weapons, this will cause a “runaway gun,” and it will proceed to dump its magazine quite quickly.
However, it’s nearly impossible for this to occur with a handgun. The fire rate in these situations would inversely related to the bolt weight. (The lighter the bolt, the faster it will cycle.) I’m guessing here, but most handguns would cycle at over 1k RPM if they could cook off. (For reference, the Glock 18 has a cyclic rate of around 1,200 RPM, so this number isn’t implausible.) The main issue with this is, that it’s not possible for a handgun’s chamber to get that hot (and it’s nearly impossible on any semi-automatic weapon.) You’re most likely to encounter runaway guns after heavy use of full auto weapons with deep magazines.
The absolute best case for spontaneous powder combustion, are single shot firearms with no spare ammunition. Breach loading shotguns, and some varieties of hunting rifle come to mind. Though, a pepper box style pistol, or double barrel shotgun would be a similar situation. In these cases, spontaneous combustion would basically just produce an accidental discharge.
Of course, if you combine this with poor muzzle discipline, you have a recipe for disaster.
This is also true for any muzzle loaded flintlock design. Though, the odds that a character wasn’t also carrying powder would be low. When dealing with black powder, I’m unsure how much benefit your character would really get from setting someone’s powder magazine ablaze. At least, in comparison to simply setting some of their foe’s internal organs on fire and calling it a day.
More modern weapons can become really problematic.
With revolvers, causing all six shots to cook off at once could have a variety of ill effects, and it would depend on the specific revolver. In particular, any round in line with the frame (specifically the bottom chamber in the cylinder in almost all cases) would cause catastrophic damage to the gun itself. This will render the weapon inoperable. This kind of a failure can result in shrapnel injuring the user (though, that’s usually more of a consideration with the cylinder failing due to overpressure rounds.) Bullets ejected down the barrel would behave (mostly) normally. Bullets ejected from any chamber with an unobstructed exit would tumble after exiting the gun, have relatively limited range (because they wouldn’t have had the entire barrel to build pressure and speed), and poor accuracy (as the round would be tumbling in flight, and not spinning, because it hadn’t passed through any rifling.)
What I’m not sure about with revolvers is, if the bullet strikes the frame (which could happen with all 5 out-of-battery rounds) whether the resulting back pressure would be enough to crack the cylinder. The thought process here is that the bullet would strike the frame and stop (causing some damage), but then the gas pressure behind the bullet would continue to build, and if it couldn’t force the bullet out of the cylinder, that force would then push outward against the sides of the cylinder. It is quite possible, particularly with high power cartridges, for that pressure to be enough to crack, or explode, the cylinder itself, causing the gun to literally explode in the user’s hands.
I’m not sure what would happen with tube magazines. Usually you see these on shotguns, though rifle and handgun examples of the design do exist. My suspicion would be that the magazine would (roughly) direct force away from the user, but I’m not sure exactly how that pressure would shake out. The other thing about shotguns is that they’re relatively low pressure weapons. It doesn’t take a lot of force to get shot or slugs moving, so they don’t. When you get into exotic rounds, such as dragon’s breath or flares, there might not be enough powder to cycle the bolt on a semi-auto shotgun. This means, while I’m not 100% sure, it’s possible if your character cooks off the magazine of a Benelli M4, all they’ll actually achieve is slagging the mag tube itself.
Of course, if you did load your shotgun with something like dragon’s breath shells, cleaning that mag out would be really obnoxious.
Detachable box magazines are norm for most modern firearms. Depending on the magazine, and the cartridges loaded in, this could get bad.
First, I need to explain something about basic firearms engineering. Firearms work off a basic, “path of least resistance,” principle. When you ignite a cartridge’s powder, you rely on the chamber to direct that force into the bullet, and push the bullet down the barrel. The entire mechanical system is designed to direct the force of burning powder. If you don’t do that, then the force will be (mostly) wasted. It will push against the least structurally sound part of the cartridge (which is the shell casing), and will barely move the bullet, (because that is the heaviest part of the cartridge.)
In fact, if you were to replicate the skillet scene from John Wick (which you really, really should not), you would likely end up with the bullets still in the skillet after they’d cooked, while the shell casings would have sprayed brass shrapnel in random directions, and probably bounced out of the pan. The dangerous thing about ammo in a fire is, often, the casings, not the bullets themselves.
With a revolver, bullets are always held in the chamber they’ll be fired from, and the gun rotates those cambers into line with the barrel. However, with autoloading designs, the cartridges are not secured in a chamber until they are ready to fire (and moved into battery.) This means, if you detonate those rounds, the casings will explode, while the bullet will (mostly) remain in place.
Importantly, what will not happen, is the gun firing all of the rounds forward from it’s magazine, tearing apart the user’s hand. Instead, the force will be directed outward, and the it may still tear apart the user’s hand.
Most modern box mags are made from aluminum, steel, or plastic. Which one you get will depend on the gun, and, in many cases, particularly rifles, you can pick which you want. Detonating the rounds within will probably (irreparably) damage the magazine. Possibly blowing out the sides. Depending on the specific example, this may be enough force to explode out of a lighter weight magazine. It wouldn’t surprise me if it’s enough to blow out the floor plate on some mags. If the magazine remains structurally sound, there will probably be a minor shrapnel blast out of the feed lips. (This is the only part I’d be seriously concerned about, without knowing what kind of gun we’re talking about.)
If the magazine is loaded into a rifle (so, most of the mag is exposed), the structurally weakest part (the floorplate) would be what I’d expect to see fail, blasting shrapnel down, and probably away from the shooter. If the floorplate doesn’t fail, then you might see the magazine mushroom, expanding with the igniting gasses. Depending on the rifle, this may force brass up into the chamber, jamming the action. But, without knowing exactly what we’re talking about, it could be difficult to know for certain. It’s also possible, depending on the exact mag release mechanism, for the expanding gasses to pop the mag lose, which would forcibly eject it from the weapon. But, you’d need to know exactly how the mags on that specific weapon are retained to judge how this would work.
If the magazine is held in the grip, (so, most autoloading handguns), if the magazine mushrooms, it’s possible that it will also deform the grip, making it impossible to remove. Again, I would expect the floorplate to fail before that happened, but it really does depend on the gun.
There are two special cases that deserve mention.
On some drum mags, it’s possible the front plate would break off, and that the combined force of the tightly packed rounds would direct a significant portion of the force forward. (Of course, once the front or back plate failed, I’d expect the wall to fail as well, so the the whole thing could come apart. This isn’t that outlandish, when you know that drum mags tend to be a mechanical nightmare under the best of circumstances.
The FN P90 is a very unusual gun in where it stores it’s ammunition. The magazine is stored flush across the top of the gun, with an unusual turret design at the back. Fifty rounds of 5.7mm are stored horizontally down the length of the gun, and physically rotated 90 degrees when they’re loaded into battery. It’s a very unique firearm. It also means that the magazine is right next to your face, and the only thing separating you from your ammunition is a sheet of translucent polymer. If those rounds were to spontaneously detonate, it would be very unfortunate.
So, while you could do a considerable amount of damage to the gun, and this dangerous, it’s not like you’d suddenly have all of those bullets spraying all over the place turning everyone in the area into chunky salsa. It just doesn’t work that way. At the same time, getting hit by brass shrapnel can injure or kill you. It’s not a bullet, and doesn’t have the range of one, but it doesn’t need to be.
Ironically, the more valuable thing here might be the ability to suppress combustion. As I’ve mentioned before, gunpowder isn’t, actually, explosive, it just burns extremely quickly. This means, your character could potentially prevent rounds from firing, and while it wouldn’t damage the gun, it would also create an obnoxious situation, as the operator would need to manually clear the dud round. Most firearms rely on the recoil generated from the previous shot to cycle the action, so when a cartridge fails to fire, you have to manually cycle the action.
For a more sadistic bent, this also means that the ideal time to detonate that round would be as the operator is removing it from the gun. As they cycle the bolt open to extract the “dud” cartridge, it detonates in their face.
Though, obviously, that would depend on how fine their control was. If they could simply cause things to ignite in an area, that’s going torch the people there as well. If they can suppress combustion over an area, then they could basically negate firearms when needed.