Knockdown Power Explained

[TonyPoulos]

In some movies people are shown flying backwards when struck. Is this realistic? How much force does a bullet really exert on impact?

Here's the answer:

Impulse force is equal to the change in momentum:

(Integral) Fdt = m*(Delta)V
F*(Delta)t=W/g*(Delta)V

Where:
F is force in lbs
t is time it takes bullet to stop after striking the object
m is mass of the bullet in grams
V is the velocity of the bullet when it strikes the object
W is weight of the bullet in lbs
g is the acceleration of gravity, or 32.2 ft per second squared
7000 grains equal 1 lb
Sorry, integral and delta symbols don't work, so I wrote them out. Integral is an area under a curve (from calculus) and delta means change.
* is multiplication
/ is division

For example, a bullet weighing 115 grains strikes a person at 900fps. The bullet penetrates 6" and comes to rest in .01 seconds.

F*(.01) = (115/7000)/32.2*900
F = 45.9 lbs

So the person feels a shove of 45.9 lbs when hit. Not exactly enough to go flying backwards. You can calculate for different bullet weights and velocities as well as stopping times and see the outcomes.

Tony

 

[James K]

I don't doubt the math, but when it comes to handguns, I think the problem here is sustained force. A bullet moves so fast that it can totally penetrate a man-sized target without imparting enough energy to overcome the inertia of the target mass. Even a bullet stopping in a man will dissipate its energy too fast to overcome the inertia. When it comes to knock-down power, an 80 lb sack of cement, tossed at a man, will knock him down faster than a bullet. Many people who have been shot with 9mm, .38 Special, .357, .45 and the like have not only remained standing but have returned fire. I think performance on bowling pins is a good indicator and a bowling pin weighs a lot less than a man.

Of course, Hollywood goes in for dramatic effects. Showing a person hit by a bullet, and standing there looking puzzled is not great drama. Having the victim tossed back 50 feet is.

I still have to conclude that the main reason someone who is shot falls down is that he dies or is seriously wounded, not that he is knocked down by the power of the bullet. For rifle bullets, there is more energy and more effect, and the bigger the bullet the more energy so that some knockdown effect is present. But I don't think it is there in a handgun bullet.


Jim

 

[Big Lou]

You can safely assume that the majority of stuff from Hollywood (movies and TV) involving guns is the result of an overactive imagination, or someone that's been using too many drugs. Apparently, reality has no place in Hollywood.

Even is a guy is wearing a kevlar vest and is hit in the chest by a large caliber bullet, it won't blow him off his feet and 6 feet back (as is frequently shown). It'll feel more like getting hit in the chest with a fastball.

 

[bandaidman]

You must also consider the momentum of the target. I have a reference source that calculated the maximum momentum transfer from a 12 ga or large caliber rifle can only produce backwards motion of .01-.18 m/sec in a 80kg target. Knowing the velocity of walking is about 1 m/sec it seems unlikey that targets will move that much. Hunting experience supports this but real life results may of course vary.....

 

[Alaska Roy]

Horse Puckey -

I've never trusted the new fangled math and bullet energy formulas.
On another site (www.sixgunner?) I believe this gent (Taylor) says to get a good sized section of log slab and hang it up with a lag bolt and chain.
About 3 ft of wet pine will stop most handgun slugs and if you rig a pendulum you can measure movement..if any with a 9mm...Roy

 

[JohnKSa]

I recall an article about a police shooting that happened years ago.

Both cops were armed with hot handloaded .44 Special in large frame revolvers. Performance from both guns was supposed to be on a par with the lower .44 mag loadings. The cops were gun-nuts and practiced and loaded together. The result was that they were both excellent shooters.

They were called to a disturbance at a bar and arrived to find the focus of the call leaving the premises. As he exited the bar, he saw the cops and removed a gun from the paper bag he was carrying.

Both cops drew and emptied their guns, hitting him 11 times center of mass in less than 5 seconds (the two cops had a lifelong friendly disagreement on who missed a shot). The ex-disturbance fell FORWARD and expired promptly.

 

[lonegunman]

The main reason people fall down when they are shot with a handgun is because they think to themselves "oh God, I have been shot, I am gonna die, I better lay down"

 

[444]

IMO, one problem with any of this stuff is that each shooting is different. I think it would almost be impossible to purposely set up two shootings to be the same. I am sure there is someone on here that is familiar with scientific method or whatever it is called. But I do know that in order to do scientific testing you have to have conditions exactly duplicated in order to get meaningful results. Anyone who has seen some shooting victims or even reasearched the matter to any great extent knows that a given caliber will perform well in on case and horrible in the next case. We may see a person poleaxed with a .22LR and next see a person laughing at a wound recieved with a .45 ACP. What all this as to do with the topic, I don't know but...... I have shot several animals while hunting that appeared to be knocked down from the impact of the bullet. These animals didn't just fall over, they didn't tetter. They simply vanished from sight the minute the firing pin hit the primer. I know this may have been an optical illusion, but it appeared that way to me. One was a mule deer buck hit behind the front shoulder with a .30-06 150 grain bullet at close to 3000 fps. The other was a coyote hit with a 185 grain XTP out of a Colt Officers Model .45 ACP. I have shot numerous game animals and these two stand out as examples of what I can only describe as knock down.

 

[WESHOOT2]

PHYSICS 101

If a bullet strikes with enough force to knock someone down how much force did it launch with?

 

[dZ]

i just saw on "Maximum Exposure" Richard Davis from Second Chance shoot himself in the chest with a 44

he did not budge backwards at all

they also showed him taking multiple hits from an AK from 5 feet

http://www.secondchance.com/

you would think the video would be on the web somewhere

 

[lonegunman]

Good point Weshoot2!

 

[Wild Romanian]

Many years ago a simple explanation was given. A bullet weighing 230 grains cannot knock down a man weighing 230 lbs.

People fall down because they may become incapacitated by being struck in the head or having their spinal cord severed.

In all my years of hunting I have never seen anything knocked off its feet. Yes sometimes even a very big animal will fall down quickly but it is not picked up and blown off of its feet.

If high power rifles are incapable of doing this then it is rather ridiculous to expect a pistol bullet to do this.

Lets face it if pistol bullets could knock someone down we would have proved that after the first bullet proof vests were invented. I once saw a picture of Chuck Taylor being shot point blank with a 9mm submachine gun. It did not knock him down. I have also seen the man who invented the second chance vest shoot himself with a .44 magnum. He did not even budge when being shot with this large handgun caliber.

There are many real life stories of men in combat who were shot and did not even realize it until they became weak from loss of blood.

I have seen deer run for 100 yards after being hit by an extremely high powered magnum rifle. Most of the time they do not even know they are hurt until they loose a lot of blood.

Knockdown power is a myth and relying on a pistol of any caliber to defend your life with is the height of lunacy. Ask any combat vetern that has seen a lot of action if you do not believe this. W.R.

 

[Digger]

Well,

Recoil = Knockdown + air resistance

If a bullet is going to "knock down" it's intended target, it's going to come close to knocking down the person shooting it!!!

A bullet expends LESS energy in the target than it does in recoil energy (Losses due to air resistance.) So if you want a bullet to hit like a baseball bat, you'd better pick a pistol that KICKS like a baseball bat.

Obviously, we complain about excess recoil, so the knockdown power of a baseball bat in handgun is not a reasonable expectation. So what does that leave us????

Shot placement and penetration. Since you won't knock 'em down, you must damage their inards!!!

 

[MK11]

Years and years ago (when the U.S. military had just converted to 9mm) someone in one of the gun mags made the analogy of jabbing someone in the ass with a needle. You're gonna jump, but it's not the force of the needle that propels the body into the air.

 

[Jack19]

I read an excellent article, a few years ago, about the psychology of being shot. The writer opined that, mostly, people do this because that's what they've been shown happens in the movies when you're shot.

With regard to knocking the target down with a bullet, if every action has an equal and opposite reaction, Newton's Third Law, whouldn't the shooter fall down as well?

 

[Covert Mission]

I remember reading about an African professional hunter (maybe Ross Seyfried, who's also an expert pistol shot?) who, iirc, had a standing bet for $500 or more: If a shooter could knock a 100 lb bag of sand off a table with his .458 elephant gun or the equivalent, he got the dough. No one ever won. Knockdown power from any gun that a man could tolerate shooting is a myth--- the recoil would be intolerable, and it's preposterous regarding handguns. The knockdown power comes from the immediate damage done by the energy imparted to the target, the hydrostatic shock, the immediacy of the fatal damage, and even then, the animal (or assailant) just may not know they're dead yet, especially if not hit in the central nervous system (my friend stopped a charging cape buffalo at about 40 yards with a perfectly placed CNS shot from a .458... lights out!).