Why don't guns kill the shooter?

[beamthegreat]

Since momentum is conserved, why doesn't shooting a gun cause massive injury to the shooter's hand? If this has to do with the surface area of the bullet vs the handle of the gun, would gluing a bullet to the opposite end of the gun, pressing the tip of the bullet on your chest, and firing the gun kill you? If not, why?

 

[hilbert2]

It has to do with the surface area difference as you noted, and probably also the relative velocities that the gun and the bullet get with same momentum, as a bullet penetrating biological tissue is affected by velocity-dependent friction forces.

 

[Vanadium 50]

Why don't hypodermic needs not pierce the flesh of the nurse using it?

 

[Nugatory]

Since momentum is conserved, why doesn't shooting a gun cause massive injury to the shooter's hand?

It's not the momentum that does the damage, it's the kinetic energy. Although the recoiling gun and the bullet have the same (strictly speaking, equal and opposite) momenta, their kinetic energy is different; most of the energy released by firing the gun goes into the bullet.

[Edit: There's more to it than just the kinetic energy; the bullet also concentrates the kinetic energy in a small region which increases the damage done. In your hypothetical about gluing a bullet to the butt of the gun, the recoil might give the shooter a smaller and nastier bruise. The damage is still limited because most of the kinetic energy went with the fired bullet]

 

[hilbert2]

Why don't hypodermic needs not pierce the flesh of the nurse using it?

Yeah, a sharp needle can pierce soft material even in a "quasistatic" way, where you can keep the velocity as close to zero as you want throughout the whole process.

 

[sophiecentaur]

It's not the momentum that does the damage, it's the kinetic energy.

It may be worth pointing out, yet again, that, whilst Momentum is P = mv, Kinetic energy is E = mv²/2 so, as soon as v gets big, E starts to get massive.

 

[automotiveadam7]

I think it's also worth considering the relative mass of the bullet to the gun when we treat the explosion initiating the gunshot as F. Using Newton's third law, we know that the force applied to the bullet by this explosion will be equal and opposite to that applied to the gun. This means that the resulting acceleration will depend on each object's mass.
Let's call F 100N, m of gun 1.18Kg, and m of bullet .0075Kg. Now using the formula: F=ma, we can determine that under these circumstances, the bullet will accelerate at 13333.3m/s^2, and the gun will accelerate at 84.75m/s^2. Although these values may not be exactly true to real life, the relationship they demonstrate is accurate and significant. The masses used reflect that of a generally realistic relationship using data found online (didn't bother with force which was constant in comparison), and taking that into consideration, the answer can be seen. If we choose to measure an object's relative ability to cause damage in the amount of kinetic energy each element carries, we can work out the following:
Ratio of mass of gun to mass of bullet: 157.333:1
Ratio of acceleration (therefore final velocity given constant distance) of gun to bullet: 157.325:1
K=.5mv^2
In the formula, since v in terms of K has a higher influence than m given its degree, the amount of kinetic energy the object in question possesses will depend most significantly on its velocity. For this reason, a bullet traveling at a higher velocity than a gun even with as little mass it has in comparison to the gun, will have more kinetic energy, and will generally cause more damage. The values found indicate this relationship.

 

[rumborak]

So many elaborate explanations here, but the real reason is that people aren't killed by the recoil of the bullet. They are killed by the effects of the path of the bullet, that is the tearing of blood vessels. Usually nobody dies of being shot in the leg, whereas a shot through the chest usually does you in. Physically, they are the same.

 

[Drakkith]

So many elaborate explanations here, but the real reason is that people aren't killed by the recoil of the bullet. They are killed by the effects of the path of the bullet, that is the tearing of blood vessels. Usually nobody dies of being shot in the leg, whereas a shot through the chest usually does you in. Physically, they are the same.

The implied meaning of the original question is why does a bullet tear through your skin and internal organs but the gun does not. The question of why this results in death is not the topic of discussion.

 

[sophiecentaur]

The situation with bullet and gun is directly equivalent to the situation with a rocket and its ejecta. The share of energy of the high speed (less massive) member of the pair is far greater than for the low speed (more massive) member. Rockets have 'pathetic' efficiency at the start, when they are near stationary. It gets better as they get faster because the incremental increase in Momentum produces even greater increase in Kinetic Energy, once the rocket has gained some speed.

 

[PeroK]

You could ask the same question about a bow and arrow. Why does the archer not get killed by the bow?

 

[beamthegreat]

Why don't hypodermic needs not pierce the flesh of the nurse using it?

I assume it would if you replaced the plunger with the needle and pressed it. The same logic doesn't apply to a gun.

It may be worth pointing out, yet again, that, whilst Momentum is P = mv, Kinetic energy is E = mv²/2 so, as soon as v gets big, E starts to get massive.

So if I'm understanding this right, the reason a bullet is deadly is that it gains more kinetic energy from the explosion than the gun?

Does this mean the smaller the bullet the deadlier it is? If you halve the mass of the bullet, the velocity doubles, so the kinetic energy quadruples.

 

[Filip Larsen]

Does this mean the smaller the bullet the deadlier it is? If you halve the mass of the bullet, the velocity doubles, so the kinetic energy quadruples.

Firstly, the energy released in the barrel is transferred mainly to the bullet since the mass of the bullet relative to the gun is very small. If the gun and the bullet had same mass they would each get half of the kinetic energy which also means the heavier the bullet the more kinetic energy the shooter would have to absorb. If the gun contains moving parts the transfer of energy and momentum from the gun to the shooter may be spread out over a longer period of time (compared with the time the bullet takes to transfer its energy and momentum to the target), making the kick longer but less hard than if there were no moving parts.

Secondly, the kinetic energy of the bullet as it leaves the barrel is determined (mainly) by the mass of the bullet, the barrel cross section area and length and the amount of energy released by the charge. For a bullet half the mass and everything else being kept the same, I would expect the kinetic energy of the bullet at barrel exit to be higher perhaps even doubled, but not quadrupled. I suspect an accurate answer will depend on the specific gun setup and will likely not be simple (e.g. involving a model of the gas expansion phase), but perhaps there is a "rule of thumb" for small variations of bullet mass.

Thirdly, making a bullet lighter but keeping is size and shape will also make the bullet have a lower ballistic coefficient meaning its power loss due to air drag will be higher and make its range shorter. So, even if a lighter bullet has higher energy at point blank range it will, everything else being equal, at some range have the the same kinetic energy as a heavier bullet, and beyond that even lower energy.

I believe some of this is also explained on https://en.wikipedia.org/wiki/Internal_ballistics.

(Edited for more precise wording)

 

[f95toli]

I assume it would if you replaced the plunger with the needle and pressed it. The same logic doesn't apply to a gun.

No it wouldn't since the "tip of needle" in that case would be have a very large diameter (the diameter of the plunger); i.e. it would be so blunt that it wouldn't penetrate the skin.
As noted above there are multiple contributing factors here; but the fact that a bullet is relatively small and typically designed to have a shape that can easily penetrate is an important factor. A much larger bullet (with the same kinetic energy) would in most cases not be as dangerous.

 

[pbuk]

If you halve the mass of the bullet, the velocity doubles.

If this were true then we would make bullets out of the lightest material available but we actually do the opposite, why do you think this is?

Spoiler: hint...

If a bullet goes faster it spends less time in the barrel.

Spoiler: further hint

The energy from the burning charge can only be transferred to the bullet whilst it is in the barrel.

 

[russ_watters]

So if I'm understanding this right, the reason a bullet is deadly is that it gains more kinetic energy from the explosion than the gun?

Does this mean the smaller the bullet the deadlier it is? If you halve the mass of the bullet, the velocity doubles, so the kinetic energy quadruples.

The attributes of an effective gun/bullet are a complex mix of physics, biology and tactics/usage. And "deadlier" is just one of many features (even as vaguely put as that is).

Also, the reason a bullet kills and not a gun is not the same as why one bullet might be deadlier than another.

 

[Nugatory]

Does this mean the smaller the bullet the deadlier it is? If you halve the mass of the bullet, the velocity doubles, so the kinetic energy quadruples.

That four-fold increased energy has to come from somewhere; we would have to increase the propellant charge to increase the pressure behind the projectile to make up for the reduced time that the faster-moving projectile is in the barrel - see @pbuk's post above. Practically speaking we can't do this because modern firearms are already designed to transfer as much kinetic energy as possible to the projectile - loading for four times the energy is more likely to burst the barrel than to double the muzzle velocity.

Now if you could arrange to fire a small-caliber projectile through a large-caliber gun barrel with a large-caliber propellant charge... That is the idea behind discarding-sabot armor-piercing rounds

 

[pinball1970]

This has been an interesting thread to me as a neutral observer (none physics/engineering)

So, intuitively...

1.Newton's third law at work. Recoil.
2.The explosion is contained in a chamber
3.The gun is a lot bigger than the bullet so it is only when the projectile is large (like a canon) where the recoil has to be considered more from an engineering POV. Ie the gun has to be fixed or have an inbuilt recoil mechanism.

In the terms of gun to cannon I thought of those missiles that are fired from the shoulder, they are not contained at the back. They are open to reduce recoil. Get behind one when it goes off and the explosion contained in the handheld gun chamber comes out the back.
Bazooka. (Edit, I don't know weapons) a quick Google says it has no recoil. So all that energy, force comes out the back?
Why does a gun not kill the shooter? Because the guy who gets hit is in the front AND the back if he does not get out of the way.
Contain it or get out of the way.
https://en.m.wikipedia.org/wiki/Bazooka

 

[jbriggs444]

In the terms of gun to cannon I thought of those missiles that are fired from the shoulder, they are not contained at the back.

A little Googling suggests that the rocket propulsion burns out at about the same time that the projectile leaves the launching tube. [I had suspected as much -- you don't want the infantryman firing the weapon to catch a face full of rocket exhaust].

"The T1 was a slim steel tube with its 54 inch length dictated by the usual ignition-to-burnout time of the experimental rockets, featuring a wooden stock and electrical triggering grip, all closely resembling what was to follow. Along with the launcher came a supply of factory built parts to convert existing supplies of the 2.36 inch diameter M10 shaped charge grenade into a rocket.

I also recently watched a YouTuber read a Reddit posting describing the effect of the back blast on someone standing less than 100 yards to the rear (malicious compliance comeuppance). Apparently that was an M136 AT4 (100 meter back blast area in a 90 degree cone) rather than an M76 LAW (15 meter danger zone with an additional 25 meter caution zone). The former is not entirely recoilless and has a chamber that is designed to rupture during firing.

 

[sophiecentaur]

Does this mean the smaller the bullet the deadlier it is?

Deadliness is hard to define. Very narrow and pointed would mean the bullet could go through and exit the other side, still at high speed. All that wasted / damaging energy! Not very pleasant, in any event, but 'a clean hole' that didn't actually pass through a vital part would be less of a prob than a big hole, torn out by a hollow point shell - or a sonic shock into the surrounding organs.

 

[Drakkith]

Not sure if this has been mentioned yet, but the recoil force of the gun is widely spread out, not concentrated into a small point. Replace the grip of the gun with a needle and it will absolutely pierce skin.

 

[Filip Larsen]

Not sure if this has been mentioned yet, but the recoil force of the gun is widely spread out

The impulse is spread out both in area but certainly also in time depending on barrel length and presence moving internal gun parts like recoil dampers or autoloading. I think this has been mentioned or at least hinted at earlier in the discussion.

 

[sophiecentaur]

Not sure if this has been mentioned yet, but the recoil force of the gun is widely spread out, not concentrated into a small point. Replace the grip of the gun with a needle and it will absolutely pierce skin.

The velocity of the recoiling gun will be a lot less than the velocity of the bullet because of conservation of momentum. This will also affect the damage done to the marksman.

But there are so many variables involved so a definite answer to the question is more or less impossible.

 

[vela]

Alas, you've all fallen victim to the false premise in the question. Guns don't kill people. People kill people.

 

[jbriggs444]

Alas, you've all fallen victim to the false premise in the question. Guns don't kill people. People kill people.

That is a false dichotomy. Events do not have single causes. Events do not have chains of causes. Events have trees of causes.

On the other hand, every sweeping generalization is wrong.

 

[pinball1970]

Not sure if this has been mentioned yet, but the recoil force of the gun is widely spread out, not concentrated into a small point. Replace the grip of the gun with a needle and it will absolutely pierce skin.

I looked for big guns and I found one.


So, I imagined sawing half the barrel off, bank robbers used to do this in the UK for concealment in the 50s-80s, in think it went out if fashion.

Taking it to its limits, the smaller the mass of the gun in the video, with larger parts taken away without compromising the integrity of actual function. Say halving the mass each time? Get to the point where the mass is equal to the round, the shooter would be injured. Or killed.
Even if he is built like Kentucky dad. Considering the original post.

 

[sophiecentaur]

Alas, you've all fallen victim to the false premise in the question. Guns don't kill people. People kill people.

False dichotomy, certainly. 'The gun lobby' use all sorts of arguments to avoid taking the one action that would definitely (as in the rest of the world) reduce the present US blight of killings, namely drastically reducing the number of guns available.
It's hard not to get into the politics of this when someone brings it up. The daft thing is that many enthusiastic gun owners have lost loved ones to some else's action. That connection is always ignored.

 

[Filip Larsen]

Get to the point where the mass is equal to the round, the shooter would be injured.

Assuming the shooter still holds the gun, he is going to be part of that mass to some extend, or perhaps more precise, while the bullet is accelerating in the barrel the corresponding opposite force is "already" being transferred to the shooters body. Finding just how much it takes to get a dislocated shoulder or similar from firing a heavy gun probably requires a rather detailed model of relevant parts of arms, tissue, muscles and bones.

But, sure, if you assume a gun with empty weight same as the bullet and you discharge it without actually holding it but so that the gun will recoil onto your body, then that gun is probably going to about as lethal as the bullet. But no one is supposed to fire a gun that way; there is a reason rifle shooters are instructed to keep the stock tight against the shoulder.

 

[vela]

That is a false dichotomy. Events do not have single causes. Events do not have chains of causes. Events have trees of causes.

On the other hand, every sweeping generalization is wrong.

False dichotomy, certainly. 'The gun lobby' use all sorts of arguments to avoid taking the one action that would definitely (as in the rest of the world) reduce the present US blight of killings, namely drastically reducing the number of guns available.

Please. Don't confuse the situation with facts.

It's hard not to get into the politics of this when someone brings it up. The daft thing is that many enthusiastic gun owners have lost loved ones to some else's action. That connection is always ignored.

I was just being a smartass. Probably best to nip this branch here before derailing the thread into politics.

 

[russ_watters]

Yup, locked.