Disclaimer

This seems a particularly stupid question, but the more I think about it, the less trivial I find it. Worldbuilding possess the most appropriate tags. It does not fit in physics-network usual problems, and we don't have a gun-enthusiast network that I know of (other than in a theoretical manner here, especially when we have to wipe out civilizations.).

Context

(Edit 2) This is an explanation as to why I need such a specific information. I've been advised to do so by Revetahw to avoid a closure for being off-topic. I first abstained to write it as not to drown the question in too many information. You can skip to the next part if it does not interest you:

You're still reading? Then hang on, cause you will need to handwave quite a few things. My main character is a mutant in a near sci-fi future. Among other things, he possess the ability to absorb shock. His body dampen more efficiently kinetic energy and actively transform it in another kind of energy. He is more resistant than a human, but not bulletproof. Thus, he needs to wear a costume made from magically-genetically-modified-spider silk. Thing is, spider silk reacts badly to heat, and albeit the enhanced silk is designed to withstand more heat, it's highly unadvised to stay near a fire. He is going to fight crime and military groups who may eventually use grenades. I need to know if I'm going to kill my character outright or not. Plus, I like being thorough in the non-"it's magical, just read on" aspects.

Question

How does a gun kill you? It accelerates a bullet to a supersonic speed by igniting powder in a controlled environment. The bullet shape allows it to impact you with all its momentum on a very restricted area, perforating the target (overly and deliberately simplified). TLDR, a fast bit of metal flies from the end of the cannon and punctures you.

How does a grenade kill you? Some mean person throws an explosive device at you. When the thing goes off, the powder ignites and breaks the outer shell, sending shrapnel flying in every direction in a big boom.

The boom is the part bothering me.

Against small caliber, the most common protection is a bulletproof vest, one that does not rupture when the bullet impacts it, allowing it to spread the kinetic energy on to a wider area. You do not get perforated, thus you do not die (at least not directly).

The suit of a defusing squad is far heavier and thicker and protects the whole body, not just the torso (but then again, why would you need your head?). I'm wondering if there is more to it than just stopping all the "bullets" (which shrapnel kind of is, just thrown all around).

• Are the shrapnels hotter than a bullet from the explosion, thus impacting their penetration power? (A quick internet research indicates a small caliber round reach around 200°C, but the heat seems to vary a lot between the exit of the cannon and its eventual impact) Would that make the shrapnels burn their way through a regular bulletproof vest?


• Is the blast from the explosion powerful enough to outright kill you, even without the shrapnel?

I do need this kind of information for a worldbuilding purpose, and since I'm not too keen on using myself as a guinea pig, I'd appreciate someone explaining to me what to take into account in case of grenade explosion, especially regarding their impact on usual protections in a gunfight.

PS: Keep in mind that I grossly simplified the interaction. I'm aware of the physics involved but in no way proficient in ballistics. I know how to operate a handgun, the way it works, but couldn't write the equations for the life of me.

PPS: I think it's worth adding a few things.

I can reasonnably assume different types of grenades kill you in different manners. For the sake of simplicity, let's keep to fragmentation grenade (example used in the question) and concussive grenade (pertinent to the shock wave problem). I can make an educated guess at how an incendiary 'nade would kill you. (The answer is: very unpleasantly).

Second, me joking about EOD having full body protections is merely to keep you awake during the long question. I understand someone wearing a bulletproof vest can not always wear a full body protection cause they need mobility. Whereas an EOD capable of outrunning the shockwave seems pretty unlikely.
(Note that I'm not implying that the vest guy will outrun the bullet, but will attempt to get out of the way). However, I was unaware of ceramic being used in body armor, and will look into it.

What a wonderfully macabre question! This calls for cited sources!

First off, I'd like to point out that you are correct to notice the similarities between a grenade and a rifle round. Both have the same gross pattern: they set off some explosives near some metal that is going to be accelerated by the explosive. So if we start from that very simplified model, we can cover both the similarities and the differences between the two.

In both cases we start with a detonation. This is a combustion reaction which is fast enough to propagate a shock wave in front of it. The first thing we can do is contrast this with deflagration, where the combustion proceeds slower than the speed of sound. In both grenades and modern firearms (which use a nitrocellulose based explosive typically called "smokeless powder"), the reaction is a detonation. However, in black powder based firearms from previous eras or lower quality improvised grenades, the reaction is a deflagration. I'll assume that we are interested in the modern military grade equipment, given the wording of your question, so if you're interested in the deflagration case, simply skip the sections dealing with the direct consequences of detonation.

When you detonate an explosive, you create a shock wave. Shock waves are interesting little beasts.

If you want to skip the next few paragraphs on the physics of a shock wave, you can. However, I find its very helpful to understand what a shock wave actually is. That helps in understanding why the line between deflagration and detonation is such a big deal.

A shock wave is required because the simplistic laws of physics that we're used to break down. Usually we say that information about an object propagates ahead of it at the speed of sound. What this really means is that there are gas atoms which hit the object and are bounced in the opposite direction, traveling faster than the object itself. These eventually collide with other gas atoms, sending them scattering ahead of the object and so on and so forth. In normal every day circumstances, this process involves so many collisions that we can model it statistically. The result is that we can talk about "pressure" meaningfully, and we can talk about a "pressure wave" which propagates ahead of the object. All objects moving through the air have a pressure wave in front of them, though it is not always obvious. A fast moving modern car doesn't have very many insects striking its windshield because the pressure wave in front of it pushes the insects upwards over the windshield. An older car with worse aerodynamics may not generate a sufficient pressure wave to force the insect over the top of the car, and the result is... well.. messy.

When we get to events that move at the speed of sound or faster. As this happens, our nice clean statistical model of the gas breaks down. In the nice clean model we're used to, we say that every small region has a "pressure," and it pushes outwards in all directions equally. This works because, over the distances between collisions (the "mean free path length," on the order of 68nm), the velocity of particles on both sides of the object are close enough to use easy statistical distributions. However, as particle speeds approach the speed of sound, this changes. The differences in velocity get more and more pronounced until we can't sweep the differences under the rug with a simple differential equation. We have to account for more of the actual particle physics in this regime.

If you're skipping the physics lesson, this is a good time to rejoin the answer

So why is a shock wave such a big deal? Well when we account for the particle physics of objects at these speeds, we have to allow for what are basically discontinuities in pressure (modern measurements suggest a shock wave is about 200nm thick). The pressure can rise almost instantly. This matters because many objects, including human bodies, are very sensitive to sudden changes in pressure. Under normal conditions, such rapid pressure changes would call for an enormous amount of energy. However, if you can create a shock wave, you can create a rather large pressure differential with a much smaller explosive.

So what happens when a shock wave hits the body? For the most part, it can pass through freely, but if we find regions where the speed of sound changes dramatically, these sharp pressure changes can do damage.

The primary effect of such shockwaves is on the lungs. This effect is dominated by the impulse of the explosion (generally proportional to the integral of overpressure over the duration of the shock). In the case of the lungs, this impulse imparts a velocity to the cells lining the alveoli. Because they are thin, designed to stretch as we breathe, and right along an impedance boundary between flesh and air, they are very susceptible to this. The result of this damage is the rupture of the capillaries in the lungs, called pulmonary contusion. This is what medical doctors would call "Bad News." The damage can cause the alveoli to collapse, no longer participating in breathing. It can also cause pulmonary edema, filling the lungs with fluid and causing suffocation.

Other gas filled organs can be affected similarly, but in the literature, lung damage due to shocks is the primary issue.

Ear drum damage also occurs in response to such shocwaves. Being thicker than the walls of the alveoli, they respond to slower effects. The damage to the ear drum appears to be more associated with the actual overpressure than the impulse of the shockwave. A perforated eardrum is typically not fatal, but it can cause sudden disorientation and vertigo. Given that one is in an environment where there are grenades and/or flying bullets, this disorientation can lead towards a fatal incident shortly thereafter.

So this shows a major difference between grenades and bullets. In a bullet, the shock wave occurs quite far away and has rather low impulse by the time it arrives. The shockwave plays a very minor part. There is an argument that a supersonic bullet hitting flesh can generate its own shockwaves which can disrupt neural activity, known as hydrostatic shock, but it is a disputed theory.

Thus we see a major difference between the bullet proof vest and the EOD suit. You noticed that it covers the whole body, but it also covers it in a different way. The layers of an EOD suit are also designed to redirect and damp the shockwave. They do this using layers of varying acoustic impedance. EOD suits are also designed to protect in other ways, such as cushioning the spine so that an EOD expert thrown back by an explosion is unlikely to suffer catastrophic spinal injury.

Now some grenades stop here. Concussion grenades like the MK3 do their damage with these effects. A fragmentation grenade like the M67 adds a layer of metallic shrapnel. This shrapnel operates like a bullet. In fact, it is reasonable to model the effects of shrapnel exactly like we model the effects of bullets.

Bullets are really straightforward. Shove a metal slug through someone's body, and you force the bonds that hold their body together to give way. If any of those bonds were critical, the opponent is incapacitated.

Bullets come in supersonic and subsonic varieties. The fundamental difference between them would be that a supersonic bullet could cause a shockwave to propagate through the body. However, given that hydrostatic shock is a disputed theory, we can reasonably ignore that difference. Instead, we can just look at all bullets as the same sort of thing. Their damage is based on shape, energy, and momentum. Naturally, supersonic bullets can have substantially more energy, but other than that they aren't special.

A bullet entering a wound basically generates (subsonic) waves, pushing the flesh out of the way of the bullet just like the air was pushed around our windshield in the car example at the beginning of this answer. This pushing effect can tear tissue, and that's the primary cause of damage from a bullet (or grenade fragment).

If arteries, veins, or capillaries burst, blood loss will occur and may cause death. Damage to nerves can cause paralysis of the innervated region, and obviously damage to the brain can cause death. A bullet may break a bone, in which case those muscles can no longer effectively use that bone to create motion. It may also tear tings like tendons, which also prevent motion.

If a bullet or fragment strikes any area, it may cause infection. This is a major factor in abdominal wounds. Our intestines are quite full of bacteria kept safely within the body of the intestines. If the intestines are torn, they will spill this material out, creating a substantial risk of infection.

The kevlar and/or ceramics found in both bullet proof vests and EOD suits is focused on dealing with these objects. Both materials are very good at arresting physical objects before they enter the body. Bullet proof vests have a smaller coverage area because of tradeoffs. Those who wear bullet proof vests must move quickly and care about minimizing burden. Thus the vests only cover the regions where the lethality of a bullet wound warrants the burden of protecting it.

In the case of the EOD suit, mobility is less of a concern. The EOD technician is already where they need to be (which would be the place everybody else doesn't want to be). They do care about mobility, don't get me wrong, but the tradeoffs for someone intentionally going to the wrong sort of place are different. It's worth it to them to have full body coverage.

Which leaves me with a gem of wisdom I got from the comic Schlock Mercenary, by Howard Taylor. His The Seventy Maxims of Maximally Effective Mercenaries includes two which I am yet to find a veteran or active duty member who doesn't agree with, or at least have to give a nod at the wisdom of it all:

2. A Sergeant in motion outranks a Lieutenant who doesn't know what's going on.


3. An ordnance technician at a dead run outranks everybody.

The M67 grenade contains 180 g (6.5 oz) of composition B explosive.

The amount of explosive in a gun or rifle is way less than that, just few grams.

It follows that, close to the explosion, the damage of a grenade is dealt both from the fragments ejected in all the directions and from the shock-wave produced by the explosion.

When an acoustic wave propagates through different media, the amount of transmitted energy depends on the relative difference in acoustic impedance between the two media: the closer they are (i.e. water-flesh), the better energy is transmitted, else (i.e. air-flesh) it is reflected back in larger amount.

The human body is mostly made by flesh but has also some hollow places, like the intestines, lungs and stomach. The interface between these and the flesh can be approximated, from an acoustic point of view, to an interface between air and liquid.

The explosion happens in air, and when the shock-wave impinges on the body it is mostly reflected. But the amount which is transmitted through the body will be basically trapped between the outer shell (the skin) and the inner shell (lungs, stomach, intestine), wrecking havoc there.

This explains why explosions are way more lethal in water than in air (there is less reflected energy on the skin), and why the effect of the shock-wave is the more significant the closer the target is to the explosion point (the intensity attenuates with the square of the distance).

Incidentally, this is also way whoever gets an echo scan is spread with gel in the point where the scan is taken: to improve the acoustic coupling between the scanner and the body.

The shockwave from a fragmentation grenade is negligible. A concussion grenade, which has few fragments and which is specialized to kill directly through explosive overpressure, has only a 2m kill radius (https://en.wikipedia.org/wiki/Grenade#High_Explosive_(Offensive). A fragmentation grenade has less explosives than that, and would deal very little direct damage through explosive overpressure. It is the fragments that deal damage. There really isn't more to this than stopping all the fragments. A grenade explosion is not a particularly impressive event. There is just a puff of dark smoke and a bang, not a huge explosion.

There seem to be two misconceptions about body armor in your post.

1. EOD technicians do not necessarily wear better armor than regular soldiers. Most soldiers in modern militaries wear hard body armor made of advanced ceramic composites over their torsos, not kevlar vests. Kevlar and other types of soft, fabric like body armor can only reliably stop pistol caliber rounds and are not of much use on the battlefield. EOD suits are made of many layers of foam, plastic, and soft body armor. They absorb explosions and fragments efficiently, but are not necessarily better against bullets or other kinetic projectiles (depends if they include ceramic components).




2. EOD technicians do not only go after hand grenades. Many of the threats they deal with are much larger bombs that need more protection, so they wear more protective armor. Even if all they had to deal with was hand grenades, they would probably still wear their special armor. The majority of regular troops in the modern day see little to no combat, and even combat troops deal with grenades on an irregular basis. For them, covering all of the limbs and head in armor would restrict movement and breathing. However, EOD techs deal with explosives all the time, and expect to do so, so they wear armor to be prepared. (In reality, from what I have read, EOD techs typically try to shoot bombs with anti-material rifles or to disarm them with robots unless absolutely necessary)..