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Old May 11, 2005, 04:33 PM   #26
Gewehr98
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Think of the physics involved.

If there's enough energy in a bullet to knock somebody down off of their feet who was downrange from the gun that launched it...

Why is the shooter still on their own two feet?




(Welcome to Hollywood physics, btw!)
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Old May 11, 2005, 04:43 PM   #27
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Chris in VA,
you have a major units problem here:

"So say the recoil at the Kimber is more like 10ft/lbs, and the bullet has what...450ft/lbs kinetic energy? "

Momentum is the product of velocity and mass, and has the units of kg x (m/s), or pounds mass x ft/sec.
Energy is mesured in ft x pounds force (often seen as lbf).
The conventional unit system creates the impression that a pound is a unit of mass. It is not. It is a unit of force (correctly labeled lbf). The unit of mass in the conventional systemis the 'slug'. To convert 'pounds' of weight (a force) to units of mass, divide by the acceleration of gravity (32.174 ft/s^2).

"But what if the bullet stayed stationary and was fired...wouldn't the force of recoil be horrendous? What I'm getting at is I don't believe the forces are equal here. I would think being hit by a .45 bullet would have a lot more force than just the simple recoil felt at the gun...no?"

The fact that the bullet moves has no effect on the recoil of the gun. If the gun weighs 2 pounds, it has a mass of 14,000 grains. With a 230 gr bullet at 900 ft/s the gun has a recoil velocity of 14.8 ft/s (about 10 MPH).
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Old May 11, 2005, 05:30 PM   #28
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What gun do you think can come close to doing this?
Panzerfaust
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Old May 11, 2005, 05:34 PM   #29
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"What I'm getting at is I don't believe the forces are equal here."

Believe it or not, they actually are. It is a zero sum game, Newtonian Physics. In cases such as this, the vector sum of all forces involved is zero. It has to be. Yes, there is a difference in mass. Yes, there is a difference in velocity. Yes, there is a difference in surface area. No, there is not a (big) difference in energies. Ok, so there are some added variables that skew the matter, like velocity of the exhaust gasses and such, but in general it pushes just as hard backwards as it does forwards. If anything, those extra variables skew it towards having that much more recoil.
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Old May 11, 2005, 06:02 PM   #30
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Ok, so there are some added variables that skew the matter, like velocity of the exhaust gasses and such, but in general it pushes just as hard backwards as it does forwards. If anything, those extra variables skew it towards having that much more recoil.
Just about everything you have said sounds right to me from what I have read. I just wanted to mention the exception I believe exists to your last two sentences....a muzzlebrake. But I guess that is what you meant when you said "in general"?
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Old May 11, 2005, 06:12 PM   #31
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About the muzzle breaks: Different designs work differently, but in general they move exhaust gasses upwards and backwards towards the shooter. The upwards part of the gas flow exerts a downward force on the gun, overcoming "Muzzle Flip". Backwards exhaust redirect pushes the gun forward, which has the effect of retarding or lessening of the recoil. The gas that exits the muzzle is moving forward, and so pushes the gun backwards, increasing recoil.
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Old May 11, 2005, 06:24 PM   #32
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Yes it can!

I have seen it in movies, and movies never lie
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Old May 11, 2005, 06:32 PM   #33
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gb_in_ga,
The only thing I have a question about what you have said that I might dissagree with (I might not dissagree if enlightened though) is the bullet staying still idea. This may be hard to imagine from the way I describe it, but lets say you had, instead of the stationary bullet, a tunsten alloy rod attached on one end to the side of a mountain and the other end sticking in the barrel all the way to where a bullet would normally be seated in the cartridge. If you fired the gun, wouldn't all of the force and energy (ignoring energy lost from the backwards force by heat, friction, compression of crystal structures in the metal, barrel expansion, etc.) be directed onto the gun and the shooter holding it and wouldn't the energy that would normally be in the bullets momentum (usage?) be transferred to the backward force of the gun? (I am trying to make the idea of a stationary bullet imagined by using the incompressible, immovable rod in it's place.)
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Old May 11, 2005, 06:34 PM   #34
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I have seen it in movies, and movies never lie
Must have been a Michael Moore movie
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Old May 11, 2005, 06:42 PM   #35
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Goddard amply proves the fallacy of "knock-down power" by calculating the heights (and resultant velocities) from which a one pound weight and a ten pound weight must be dropped to equal the momentum of 9mm and .45ACP projectiles at muzzle velocities, respectively. The results are revealing. In order to equal the impact of a 9mm bullet at its muzzle velocity, a one pound weight must be dropped from a height of 5.96 feet, achieving a velocity of 19.6 fps. To equal the impact of a .45ACP bullet, the one pound weight needs a velocity of 27.1 fps and must be dropped from a height of 11.4 feet. A ten pound weight equals the impact of a 9mm bullet when dropped from a height of 0.72 inches (velocity attained is 1.96 fps), and equals the impact of a .45 when dropped from 1.37 inches (achieving a velocity of 2.71 fps).30

A bullet simply cannot knock a man down. If it had the energy to do so, then equal energy would be applied against the shooter and he too would be knocked down. This is simple physics, and has been known for hundreds of years.31 The amount of energy deposited in the body by a bullet is approximately equivalent to being hit with a baseball.32 Tissue damage is the only physical link to incapacitation within the desired time frame, i.e., instantaneously.
This and other interesting facts @ http://www.firearmstactical.com/hwfe.htm
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Old May 11, 2005, 07:16 PM   #36
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"This was taken from that Compton California shooting report on a CBS website

Note that a policeman was KNOCKED DOWN when a bullet struck him"

A national network says it? It must be true.

Physics-wise a gun is heavier than a bullet, a gun has a larger surface area than the bullet, and the gun travels mroe slowly than a bullet. that's the only differnces between the gun and bullet. Amount of force is the same both ways- energy cannot be created or destroyed.

If anyone doesn't want to believe the physics, hunt with your 45 and see what it does to a man-sized deer or hog.
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Old May 11, 2005, 07:23 PM   #37
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I just remembered where I heard that before- knock a man off his feet. I had an old guy shopping next to me at a gun shop. The taurus dealer was there and I was looking at the 2" 45acp tracker when it came out. The old man turned to me and started talking in a wannabe old west cowboy drawl.

old guy: "That 45'll knock a man off his feet"

me: "really..."

old guy: "Yup."

I just couldn't argue with that.

old guy: "Remember on (some old west show that was on when I was like 3), when Marshal Dillon shot (some other guy)? It took him right off his feet."

me: "That was on t.v."

old guy (unfazed by the sarcasm): "It took six men pulling than actor back with a rope to simulate the effects of that 45 caliber bullet."

me: "on t.v."

old guy: "Six men!"

Old guy puffs his chest, holds head high, hitches his belt like Barney Fife and walks to the next case. Taurus guy, still silent, rolls his eyes.

Lessons learned:

People sitting around watching too much t.v. is NOT a new phenomenon.

A 45 WILL knock a man down, but only if you can tie a rope to him and get six men to stand behind him and pull when you shoot him. Of course, I think this method also works with 9mm, putting that whole 9mm vs. 45 debate to rest...
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Old May 11, 2005, 07:25 PM   #38
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Novus:

I see your point, but the answer is still, well, no. The point is that the zero sum game still applies. It turns out that the tungsten rod attached to the mountain moves ever so slightly (look at the mass involved!), but it still moves, mostly via compression. That satisfies the zero sum. Now, what happens to that energy afterwards is another zero sum -- it may be that the tungsten rod is accelerated by crystal expansion/contraction -- think "elasticity". The tungsten rod is pushed by the mountain back towards the gun, and equally pushes the mountain. The energy transfer will be nowhere near perfect, there will be much in the way of heat loss as well as loss through vibration, and lost through crystaline stress fractures. But while all this is going on, the gas hasn't had opportunity to escape, and it's gotta go somewhere. It can't go forward, the titanium rod is blocking it, it can't go to the sides due to the barrel. The gun is still locked up, so it can't go backwards. The gun isn't designed to hold this pressure indefinately. Ka-Boom. Just like trying to shoot a gun with a barrel obstruction, because that's exactly what is happening.

The recoil will be the same whether it is a bullet travelling at bullet type velocities, or if it is a mountain being shifted or compressed by a miniscule amount. The zero sum game still applies. In the one case you have little mass and high velocity, in the other case you have humongous mass and miniscule velocity. Same principle applies.
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Old May 11, 2005, 07:51 PM   #39
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"A 40 mm with bean bags dosent have much recoil and its still not going to knock them down
There have been lots of people take numerious hits from 40 bags with little or no noticeable effect"

Oct 30, 1997 we had been 39 days on a stand off with a barricaded woman armed with a 12 ga shotgun loaded with Federal sabot slugs. When she came out she was hit repeatedly 3 or 4 times in the upper torso with a SAGE 40mm firing rubber bullets. (attached is a pic of one of the recovered rounds.) These things are faster than a well pitched, major league fast ball. Somewhere around 125 mph. She never went down. She was wearing insulated coveralls at the time and just received some bruises. On Sept 26, 1997 we had hit her with several 12 ga bean bags and never phased her.
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Old May 11, 2005, 09:04 PM   #40
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Well, guys, I'm sorry, . . . but Newton, and all the rest can go be hanged.

A 40 mm grenade fired from either a "thump gun" or slung under the M-16 will not only knock the recipient off his/her feet with a good COM shot, . . . but it will also relieve them of about 1/3 to 1/2 of their body weight when the round goes off.

Even if the round doesn't detonate: they still go down, . . . and all your fancy theories ain't worth spit. Ask any grunt from 'Nam, . . . that carried a thumper, . . . man mountain Dean himself goes down when that 40mm contacts his sternum, . . . or worse, . . . his flack jacket.

For the record: that "down" is also part of "down and out" for the final count.

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Old May 11, 2005, 09:27 PM   #41
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Actually if you watch Last Man Standing with Bruce Willis you will find that it is absolutely true. Because it was Bruce Willis it must be true!!!!!!
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Old May 11, 2005, 09:37 PM   #42
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Knock down?

A 155mm howitzer will do it!! Well actually it converts the body into very small particles! But you get the message.

A school trustee, who stole money, committed suicide on TV a few years ago, stuck a big revolver barrel in his mouth, fired the shot, using a good VCR unit, I was unable to catch movement, one frame there, next frame, gone.

Instant, gravity collapse, no stagger, no step, no nothing, collapse, that is knockdown power, clean off the brain stem whilst standing.
Same as dropping a 200 lb bag of flour.
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Old May 11, 2005, 09:42 PM   #43
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They tested this on Mythbusters.

They hung a man sized pig from a toggle that could easily be dislodged by a light push.

Then they shot it with various firearms. 9mms, .45, various submachine guns, rifles, etc. At one point three of them were shooting it with submachine guns simultaneously.

The only thing that dislodged it from the toggle was a 12ga shotgun. Even then, the slow mo didn't show it getting knocked back, it just got bumped enough for it to pretty much fall straight down.

They decided that too much energy was getting wasted by bullets going stright through the pig so they put a bullet proof vest on it and repeated the test.

Same results. Only the 12 ga dislodged it and it fell pretty much straight down--no significant movement backwards.

Amazing that experiments bear out what the mathematics and science predict, eh?
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Old May 11, 2005, 10:33 PM   #44
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I remember seeing an old video where a guy with a bullet-proof vest allows himself to be shot.

He is standing on one leg and is shot with a .308 rifle from a few feet away. It did not knock him down! I don't think a .45 is going to pull it off if a .308 don't have the juice. The video was Deadly Weapons: Firearms & Firepower, they used to have it playing, on a loop, at a gun store here years ago.
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Old May 11, 2005, 10:41 PM   #45
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Dwight55:

No, actually that's in line with what I've experienced. After all, I actually have shot M203 with grenade rounds, I know what the recoil is like from them. So, yes, I can well believe that there is actually enough KE in a 40mm grenade to knock somebody down. Of course, if the grenade detonates as designed that's all she wrote dear John...


JohnKSa:

"Amazing that experiments bear out what the mathematics and science predict, eh? "

After all, that's why they call them laws. Not only that, these are really straightforward ones.
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Old May 12, 2005, 10:51 AM   #46
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“ No, there is not a (big) difference in energies.”


“…bullet simply cannot knock a man down. If it had the energy to do so, then equal energy would be applied against the shooter and he too would be knocked down.”

“energy cannot be created or destroyed.”

Momentum is NOT energy.
Energy is MV^2/2, and is measured in lbf*ft.
Momentum M * V and is measured in lbm * ft/sec
They are not interchangeable in calculations.

The ‘tungsten rod in the barrel’ is a poor example, since you are now involving energy, and not just momentum.
When an engineer analyses a problem involving forces a sketch called a ‘free body diagram’ is used to account for all the forces in a simple way. Think of the gun hanging from a thread and fired. Bullet moves one way, gun moves the other, momentum (NOT energy) must be conserved. If the gun weighs 60 times more than the bullet, the gun will have 1/60th of the bullets velocity.
This reminds me of the drawing showing a pair of mule teams pulling opposite directions on a pair of jeans. The pants see the exact same load as if one pair pulled and the other was attached to the mountain. Mule team pulls 1000 lbf North, mountain must be providing 1000 lbf South or the pants would be moving.

A 40mm will only knock someone over from surprise and not being set up to absorb the blow like the shooter was.
I fired a .375 H&H at a deer that ran across a private range where I was checking the rifle. I had already fired 3 shots with the accompanying recoil with out incident. A followed the deer and fired with the rifle 90 degrees to the line between my feet. And promptly was sitting on my but. No way to absorb the recoil.
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Old May 12, 2005, 10:59 AM   #47
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Nope.

In fact, I was disappointed after just having watched "Open Range" finally, which is overall a good movie - but there were two hokie occasions in scenes when Duvall's shotgun blew someone up off their feet and through the air 6 or 8 feet. A shotgun can't do that - not even close, and a handgun certainly can't.
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Old May 12, 2005, 11:08 AM   #48
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About the only shoulder fired weapon I can think of that can "knock someone off their feet"
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Old May 12, 2005, 11:22 AM   #49
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About the only shoulder fired weapon I can think of that can "knock someone off their feet"
What about: bazooka, panzerfaust, CHICOM type 36 57 mm recoiless rifle fired from the shoulder(and other recoilless rifles), and the TOW missile, etc.
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Old May 12, 2005, 11:25 AM   #50
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Knock off their feet

If you want to knock someone off their feet with a .45, hold it by the barrel and hit them with the butt. Hard. You watch too much TV.
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