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"Knock down" power

 
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  #1  
Old 11-30-2001, 09:22 AM
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"Knock down" power

Here's an item I get excited about, just saw another reference to it.

The statement generally sounds like this...

This rifle/bullet is SO POWERFUL that if "knocks the animal off it feet". These statements in most instances are taken as standard hunter exageration of power but there are folks that actually believe this happens.

The latest one I saw was about a round nose bullet out of a 7mm-08 physically "knocking" an aninal down.

It's just not possible for a 150 grain bullet fired from a sporting firearm to actually knock an animal the size of a deer off it's feet.

Hollywood does not help with this myth. They have stuntmen flying around from the impact of a 9mm or a shotgun blast, not too realistic. (I personally witnessed a person being hit at point-blank range with a 230 grain 45 cal bullet and the person didn't initially even realize they'd been shot.)

As I understand it and attempt to explain the problem is as follows.

One must convert the two objects to similar weight units. The bullet is in grains and the critter is expressed generally in pounds. 1 pound = 7000 grains.

A 100 pound live weight deer (teeny tiny Florida whitetail) = 700,000 grains

150 grain 30 caliber bullet.

Now we divide the energy from this bullet in the total weight of the animal (assume the bullet actually stops in the animal).


700,000 / 150 = 4,666 times the mass for the deer as compared to the bullet.

3000 ft/lbs of energy divided by this 4,666 mass differential. 3000/4666 = .643 ft/lbs.



I'm sure Warren has the better physics explanation about why this can't happen.

[ 11-30-2001: Message edited by: Dave King ]
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  #2  
Old 11-30-2001, 09:51 AM
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Re: "Knock down" power

Dave,

I shot a Sika deer on the Eastern shore on Maryland 2 years ago with some of those 200 gr Ballistic tips that you cooked up for your old Lapua (103.4 gr of N165). My brother was in the tower with me and that were his exact words "you knocked it clean off its feet". I hit it at 100 yds and the bullet never exited. The deer absorbed everything. Actually what I think happened was total bullet failure. It was moving way to fast and exploded on impact. The off side rib cage looked like someone shot it with a shotgun....jacket fragments everywhere.

Matt
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Old 11-30-2001, 10:25 AM
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Re: "Knock down" power

Dave is right. "Knockdown" is an overused and misused term when it comes to lethality. In actual fact there is less energy, momentum and power at the killing end, due to velocity loss, than on your shoulder when the rifle is fired. Did it knock you down?

When these observations are made what is generally observed is the permanent or temporary shutting down of the central nervous system of the animal causing the animal to "just drop". It has been described as turning off a switch. If the animal is moving at all when this happens it can appear that it was knocked over.

This shutting down of the central nervous system is a complicated phenomena and is not always permanent. The classic case is the buck which is hit in the antler, knocked unconscious, until the hunter gets closer or even beginning to dress the animal, at which point it gets up and everyone is startled. Hits in the brain or spine can cause this. Hits near the spine with fast energy dumping bullets can cause this. Hits further away from the spine with bullets that have the right combination of expansion and penetration velocity can cause this. The further away from the brain that the hit occurs that "turns of the switch" the less the likelyhood that the animal will regain consciousness. Shoulder and body hits like this are very nearly always fatal. After much study of this phenomena it is my opinion that the right combination of expansion, velocity, wave effect, and penetration velocity combined with tissue structure and distance to the spine will cause an electrical, or electrical-like, impulse through the tissue and nerves that shuts down the central nervous system. On body hits this occurs most often on deer and elk in the higher shoulder region with the bullet passing directly through both shoulders.

I have witnessed this phenomena a number of times and the suddenness with which the animal collapses does indeed present the image that it was "knocked down".
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Old 11-30-2001, 11:02 AM
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Re: "Knock down" power

Warren,

Quick question. You stated that the rifle would have more energy on your shoulder than from the bullet. I did a quick calculation and a 10 pound rifle firing a 250 grain bullet at 3000 fps would only generate 60 fpe with a velocity of 19 fps. Maybe my calculations are wrong. Help us understand more. I agree with all of your statements about "knockdown" power. I am not trying to be arguementative, I just want to learn more about the physics in the area of recoil (actual and perceived).

Jim
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Old 11-30-2001, 01:05 PM
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Re: "Knock down" power

Thanks very much for the reply Warren.
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Old 11-30-2001, 02:16 PM
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Re: "Knock down" power

Warren,

Thanks for the explanation. I never considered body weight as another force vs. recoil. Your explantion was very well stated and very correct.

Perceived "Knockdown" plainly stated is the instantanous shutdown of the central nervous system of the animal. Though a brain shot may not cause this, as evidenced by the residual movement the animal maintains, i.e. leg kick etc. (the chicken with it's head cut off) from the electronic pulses left in the nervous system. When the animal goes assystoly (from a heart shot) there is still may brain activity but the central nervous system has not shut down (why an animal may run several yards from a heart shot). If the heart shot sends the brain into shock the animal may appear to have been "knocked down" by the shot but it is simply in shock from it. These may be instantaneous in their timing, therefore the myth of "knockdown".

Anyway, thanks again Warren for your very well stated explanation.

Jim
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Old 12-01-2001, 12:34 AM
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Re: "Knock down" power

I knew somebody would want to break it down.

You must add the weight of all of the ejecta which includes the weight of the powder combusted. Your Free Recoil Impulse is 5.0, you Free Recoil Velocity is 16.1 f/s, and you Free Recoil Energy is 40.5 ft-lbs. at the butt of the rifle. When calulating perceived recoil most figure the weight of the rifle, which is incorrect. The weight and resistance of your shoulder has to also be inputted, if the rifle is being held against your shoulder. But this will vary from shooter to shooter and rifle to rifle. If you are calculating this in terms of "knockdown" then you would have to assume movement of the entire body in which case the entire weight of the shooter must be added to the weight of the rifle for recoil calulations.

One question is the deceleration energy dispursed more quickly than the acceleration energy is accumulated, thereby making the terminal end more abrupt. You have to make several assumptions. First is that the bullet reaches it's maximum velocity as it exits the barrel. I have seen it argued that the bullet may actually accelerate for a short distance after exiting the barrel as the compressed gasses are accelerating past the bullet and before undisturbed air is encountered by the front of the bullet. If it does it is not much. Anyway, the distance traveled by the bullet from it's point of rest to it's maximum velocity is around 20" to 24" on 22" to 26" barrels. This distance is traveled in around 3 milliseconds, depending on cartridge, load and rifle. It is very seldom less than 2 milliseconds or greater than 4 milliseconds. On the receiving end (the animal) the deceleration occurs in approximately the same time and distance. The deceleration time and distance can sometimes be twice or greater than that, which will serve to extend or dilute the energy dissipation. It is very seldom half the 3 millisecond and 20" to 24" value with big game rounds. The point is the acceleration time and deceleration time are relatively equal, ergo there is no disparity between these to separate energy impulse.

The point is if you and your rifle weigh approximately 200 lbs. and the animal you are shooting weighs approximately 200 lbs., then if the bullet "knocksdown" the animal it would have to "knockdown" you when you fired your rifle. If it "knocked down" a 600 lb. elk, it would pert near have to flatten you.

[ 11-30-2001: Message edited by: Warren Jensen ]

[ 11-30-2001: Message edited by: Warren Jensen ]
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