Hollywood Gun Myth on Mythbusters... NOW!

[Fed168]

Remember the old video Deadly Weapons? Richard Davis stood several feet away from a guy with an FAL to prove that point (of course he was armored up).

The shooter hit him with several rounds, he just stood there. And then he did it on one leg, and still stood there.

 

[possenti]

What If...

Forget about the handgun calibers they used, but what if they had used a highly frangible 12ga. or .308 slug that fragmented on impact?

Everything they shot at the pig looked like FMJ, and penetrated through the target. A lot of wasted energy hitting the ground behind it.

I wouldn't expect a frangible slug to knock it back any great distance, but the results may have been a little more dramatic. Maybe more "splatter" effect.

Did anyone else notice they called the MP5 an "M4"? :banghead:

 

[pignock]

Brett Bellmore said:

Energy doesn't throw you back, it just injures you. Momentum throws you back. The key is that momentum is proportional to velocity, and energy to (half of) velocity squared, so because the bullet is traveling really fast, it takes most of the energy with it when you fire the gun, and delivers it to the target

For those of us who managed to complete compulsory education without taking a physics class, is there a measurement standard for momentum that can be applied to firearm projectiles? For example, if I know my 230 grain projectile is travelling at 800 fps when it strikes a stationary target, say a 25 lb. steel popper, can I predict how much momentum will be imparted to the target?

Thanks,

Keith

 

[grimlock]

I just wanted to point out to Jim that Gen. Polk died about .5 miles from my house. Whee!

 

[Deavis]

I'm just being pedantic

hehe pedantic... that word never gets old.

 

[Molon Labe]

Conservation of momentum is probably the easiest physical laws to apply to ballistics explanations.

This is correct.

A lot of people assume kinetic energy must be conserved. This is incorrect; energy can take many different forms, and all forms must be taken into account. Any attempt to look at the situation from a total conservation of energy POV will quickly lead to an overly-complex situation.

When analyzing recoil and other factors of exterior ballistics, it is much easier to look at it from a conservation of momentum POV.

 

[HankB]

Newton's theory may be correct, but it just doesn't apply to firearms

The conservation of linear momentum applies EXACTLY to firearms. Momentum - the product of mass and velocity - will be conserved. In fact, the momentum of a gun in recoil will be slightly greater than the momentum imparted to a target because a) the bullet will slow down before hitting the target, due to aerodynamic drag forces, and b) the powder gasses from the propellent will impart momentum to the gun, but not the target. (Let's ignore muzzle brakes and muzzle-contact shots here.)

I remember testing this years ago - we took an 18" piece of railroad tie, stood it on end on a hard surface, and shot into it from about 3 feet away with a full power .41 Magnum. The wood weighed less than any man, but the bullet - which didn't penetrate all the way through - barely rocked it. Conventional "wisdom" would have had the wood blasted back ten feet or more, but it wasn't.

When a bullet hits a man or animal and a violent reaction is observed, it's because of the nervous system's reaction to the trauma. Think about it - if you're sitting on a barstool and someone sneaks up behind you and sticks you with a sewing needle, can you say the needle "knocked you down" if your legs get tangled in the stool's legs when you try to jump up and you end up on the floor?

For those of us who managed to complete compulsory education without taking a physics class, is there a measurement standard for momentum that can be applied to firearm projectiles? For example, if I know my 230 grain projectile is travelling at 800 fps when it strikes a stationary target, say a 25 lb. steel popper, can I predict how much momentum will be imparted to the target?

Assume it's an inelastic collision, i.e, the bullet sticks to the target and doesn't pass through. The momentum of the bullet before it hits will exactly match the momentum of the system after the hit. Do a search on "ballistic pendulum" and you'll see lots of info on this. (Ballistic pendulums were used 'way back in blackpowder days to compute bullet velocities.)

 

[Molon Labe]

HankB: 100% correct on all accounts.

It should also be mentioned that, yes, energy is conserved. But trying to analyze the situation from a conservation of energy perspective is a royal pain in the a$$. This is because energy can "hide" in so many different forms (potential, kinetic, chemical, heat, etc.). Trying to quantitatively account for all these forms of energy would be a nightmare. By contrast, momentum cannot "hide."

 

[armoredman]

Also remember that the bullet is an energy concentrating device, designed to dump all it's energy in a very small area very rapidly, such as a sword blade dumps all of it's swing or stab energy across an area measued on the blade edge. The bullet launcher spreads this same Newtonian energy across a much larger surface, with dampening devices absorbing much of it, and lessening the impluse with delaying tactics.

 

[mfree]

115gr 9mm ball striking the top of a foot long piece of 4x4 will knock it over, just a wee bit faster than poking it hard with your finger.

Poke a person hard in the chest, and they'll laugh at you, and probably take posession of your finger(s).

 

[CW-op]

I can't and won't get into an argument about physics or kinetic energy or any of that other stuff. But, I will tell of something I saw years ago. 1965-deer hunting-Iron County, Mi.. I was watching an X crossing of two old logging roads at a distance of approx. 35 feet. Heavy snow falling with flakes the size of horse apples. Suddenly, out of the snow, a buck appeared, traveling R to L, with the intent of turning that corner. I raised my rifle [Rem. Mod. 141 cal.35Rem] only to find the peep sight blocked by some snow. I first blew into the sight without any luck. Then, I pointed the rifle at the deer and let fly. I saw snow fly just under the deer, so I knew I had shot low. At the same time, I noticed the snow had left my sight, so I planted one right in the boiler room. Now it gets interesting. When I fired the shot, the deer immediately turned 90 degrees in mid-air so that all four of his feet were facing me. He seemed, for just an instant, to be levitating. He then dropped to the ground.
Say what you like, that is how it happened, and I had never seen anything like that before and haven't since. Did the force of the round cause this? Of course it did. Have at it.

 

[TimRB]

"For example, if I know my 230 grain projectile is travelling at 800 fps when it strikes a stationary target, say a 25 lb. steel popper, can I predict how much momentum will be imparted to the target?"

Pignock, momentum is just mass times velocity. In your example, you would be able to predict the velocity of the target by first calculating the momentum of the bullet (convert units as required) and then divide that momentum (which must be conserved) by the mass of the target. This presumes that the target is being held by some frictionless device that allows it to move freely after being hit.

Tim

 

[sturmruger]

CW-op something similar happened for me once. I was using a Marlin 336 chambered in .44 mag. I was 12 years old and sitting in a makeshift blind opening morning in NW WI. I had a hunter choice that year so I was planning to take anything that came along. About 8:15 I saw a nice fat doe walking along a trail that I knew would pass within about 30 yards of my blind. I put the scope on her and pulled the trigger. The deer slammed down like someone had kicked her. I put the 320 gr HP right through both lungs. She wasn't much of a fighter she just laid there and died. I still vividly remember how that deer was knocked over.


That wasn't the last time I shot a deer close up, but it was the last time I saw such an imediate reaction like that. The last couple of deer I have shot had no visible reaction at all of course I was using a 75 gr hp .223 so I wasn't expecting anything. I wish I understood physics better to argue one way or another, all I can look at is my firsthand expierence.

 

[ZeroX]

To think my post sparked a physics debate...

 

[Ktulu]

Did the force of the round cause this?

No, the bullet didn't knock the deer over. One must remember not to change the facts to fit a strange situation... Bah... this says it better then I ever could...

"It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts." - Sir Arthur Conan Doyle, Sherlock Holmes, A scandal in Bohemia (1891)

 

[unreal45]

What happens when a 10yr old shoots another 10yr old with a .470 nitro express?

The parents go to court.

 

[BluesBear]

Psssst... over here

Back to the MythBusters eposode that started this.


I thought someone would have already commented on this but since they haven't I shall.

Even if the force of a bullet could knock a man over, their method of testing would not have been able to prove it.

The pig carcass they were originally shooting did have the weight and size of the "average" human torso. The fly in the ointment was that they had it HANGING.
It was suspended from the tripod. In essense it was acting like a ballistic pendulum. And when using a ballistic pendulum you shoot towards the bottom of the pendulum. Not at the uppermost point.
Ask any metallic silhouette shooter how many rams they can knock over by hitting three inches above the base as compared to hitting three inches from the top.

MythBusters were shooting in what would be the high COM area of a human torso. They would have gotten more motion if they had been shooting towards the bottom of the swinging meat.

Also since the pig had been dressed out there were no internal organs and corrosponding fluids to interact with the projectiles.

It's like the difference in shooting an empty gallon milk jug and one full of water.


So their testing was flawed from the get-go.

Which is not unusual for MythBusters.

 

[at-home-daddy]

I was too busy watching Kari, the redhead assistant, to fret over the physics of it all...

 

[BluesBear]

I didn't notice any flaws with her physique either.

 

[DRZinn]

hehe pedantic... that word never gets old.

That's because it never gets used enough.

RE: the deer turning in the air

Remember that the deer was already in mid-turn, so probably shifting body weight towards the turn, and the bullet accelerated that process.

 

[carebear]

three inches above the base as compared to hitting three inches from the top.

If we're talking about the lovely assistant, we need to change our target areas.....