Author Topic: Formula for Energy developed by velocity and bullet weight ??  (Read 624 times)

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Offline irold

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Formula for Energy developed by velocity and bullet weight ??
« on: September 01, 2007, 04:57:15 AM »
Can anyone give me some sort of formula for the "foot Lbs of energy" developed by certain load combinations.  I'm aware it depends on bullet weight and velocity at a given distance.  Surely there's a way to to figure it out?? ???

Offline Castaway

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #1 on: September 01, 2007, 05:18:51 AM »
The formula is:  1/2 mvv/32.16  where m = mass in pounds.  There are 7,000 grains in a pound.  Now, having said that, ft/ lbs. of energy is not a reliable indicator of killing power.  Think about the two extremes;  a sewing needle driven at 9,000 f/s vs. a big, fat, slow moving 45 caliber bullet mushing it's way through soft tissue and bone.  Then ask yourself, would you rather catch a fast ball thrown by Nolan Ryan or a 16lb bowling ball heaved by Hulk Hogan.

Offline gitano

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #2 on: September 01, 2007, 05:48:53 AM »
You can simplify the calculation by combining some terms. The kinetic energy can then be estimated with:

 (muzzle velocity squared * weight of bullet in grains) divided by 450250.

The constant 450250 is derived from the 1/2 and the 32.16*7000 terms.

So... a 500-grain bullet going 1650 f/s at the muzzle, will have a muzzle energy of (1650*1650*500)/450250 = about 3023 ft-lbs.

Paul

Offline LaOtto222

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #3 on: September 01, 2007, 05:56:41 AM »
There is something to be said about a large bullet with a large flat tip. They seem to kill beyond energy figures. There has been every North American game animal taken with a 44 magnum with heavy cast bullets including the big brown bears of Alaska. They seem to pole axe any thing that they hit. A 45 Colt loaded up stiffly would do a good job too. Not to mention the 45-70 or a 444 Marlin with heavy cast bullets. The cast bullets do not upset very much if any at all. With all but the largest game, they just tear a big wound channel and bust out the other side dropping them in their tracks. Of course you are not going to this at 300 yards because of the bullet drop, not because there is not enough killing power. Even though they are not going to get the shock power of a high velocity bullet, they seem to knock 'em over with ease. Like was said above, you are going to feel a bowling ball at a third of the velocity a lot more than baseball. We hunt with shot gun slugs around here a lot. Any deer hit in the vitals is dead. Slugs are soft, I saw one that hit a deer in the neck near the base, bang flop. The recovered slug flatten out to about the size of a silver dollar. They will still run with a "gut shot". I do not care what you shoot, if it is poorly placed it may kill eventually, but it will take a lot of time, more than likely resulting in a lost dead animal. A hard cast bullet penetrates deep. If the above mentioned deer was hit the same with a hard cast bullet it would have went right through smashing the neck bone in the process, still dead of course. What were we talking about?
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Offline irold

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #4 on: September 01, 2007, 07:39:33 AM »
Thanks.......I think ??   Someone figured out what I was attempting to calculate, just how effective my hand loads were at killing deer sized game.  Or maybe I should rephrase it , whether my hand loads were going to be effective at killing deer sized game.  (assuming good shot placement ).  Somewhere I read, the rule of thumb was 1000 FPS was considered minimum for deer sized game. I realize many deer have been taken with 357's, 38's, 32-20's....the list goes on and on.  During the development of my 357 Max loads, I just wanted to be sure the foot/lbs was sufficient.  Seemed like the 158 gr loads were beginning to pale as far as the foot/lbs goes....Again, thanks for the info...Mike

Offline Don Fischer

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #5 on: September 01, 2007, 08:26:21 AM »
I'm not sure what everybody said up there. Here's what works for me and it coincides with published figures.

Muzzle velocity times muzzle velocity. Divide the answer by 7000 (# of grs in a pound), divide that answer by 64.32 (specific wt of gravity), multiply that answer by the weight of the bullet.

IE. 180gr bullet @ 2700 fps:

.......2700 x2700 = 7290000 divided by 7000 = 1041.4285 divided by 64.32 = 16.191363 x 180 = 2914.4453 or 2914 ft lbs E.
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Offline irold

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #6 on: September 01, 2007, 12:39:00 PM »
Thanks ,  Don, I kinda like your way better.....my calculator goes into error mode with anything over 8 digits.  Although , the other way did seem to work also.   Thanks  :)

Offline Georgen71

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #7 on: September 01, 2007, 09:05:56 PM »
I use speer gold dot bullets in my 357 loads and have taken deer at various ranges. I feel pretty confident in my loads out to about 50 yards, then the energy does dissipate. I do head shots and they go down pretty good and don't seem to mind the lower energy and don't seem to sweat the formulas, although it is the last thing to go through their heads. Geo. ;D

Offline Lloyd Smale

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #8 on: September 01, 2007, 11:02:20 PM »
math doesnt kill deer. It may bore a high school student to death but thats as dangerous as it gets. There are some rounds that kill out of perportion to what the numbers say. Examples are the 44 mag 3030 .35 rem and about any big bore. A big slow moving bullet has a tendency to penetrate deap and penetrate straight causing a long wound channel the disrupts as much tissue as a high velocity bullet that creates a wider but shallower wound channel. It DOESNT take a 1000 fps of mathimaticaly figured energy to kill a deer. What it takes is a bullet hitting the vitals whether it be the heart lungs liver or brain. Ive kiilled and have seen killed truck loads of deer with rounds like the 3220 and 30 carbine and have never seen one lost that was hit properly. If you want to relate math to hunting the best equations is pratice=accuracy=success a marksman with a 223 is alot deadlier then a gun with a 300 short mag that shoots a half a box of ammo in his gun to sight it in and thinks hes ready to hunt.
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Offline victorcharlie

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #9 on: September 02, 2007, 02:33:13 AM »
Kinetic energy= 1/2 mass X velocity squared

Momentum = Mass X Velocity

Momentum, or the ability of an object to resist a change in motion is vastly under rated IMO and this is where the big heavy slow bullets shine.

We've all probably played baseball.....would you rather be hit by a wiffle ball at 80mph or a baseball at 40mph?

There are so many variables involved in internal ballistics and I certainly am not a physics whiz.

Racers learned along time ago that you can't beat cubic inches.
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Offline Castaway

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #10 on: September 02, 2007, 11:00:51 AM »
irold, just for giggles and grins, what caliber and what combination of powder and bullet were you thinking about shooting?

Offline irold

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #11 on: September 02, 2007, 02:32:40 PM »
Cast,  I recently acquired a 357 Max, in 10 " bull for my Contender.  I've been putting together some loads for it....using 296 and 158 gr Horn XTP,    also using same powder with 180 XTP.   I understand what Lloyd is saying about shot placement etc etc....using that mentality though , if legal, I could use my 22 mag for deer......no, sorry I won't do that.   Forgive me Lloyd, maybe I misunderstood you.  I was trying to figure out if the 158 XTP at just under 2000 FPS was enough.....according to the Sierra manual , they don't recommend anything smaller than 170 gr.  All the manuals seem to say the Max works better with heavier bullets...ie 180 gr.  I've already gotten some good advice from a few guys off another forum here at GB's.   Maybe you guys on the reoading side can offer your help also...it would be appreciated.    Oh yea, Cast, I'm using 20.5 of 296 with the 180's and 25 with the 158's.  The 25 gr in the 158's seem to be the most accurate....cloverleafs at 50 yds...2-3 inch groups at 100 with 2X Leo.



Offline Lloyd Smale

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #12 on: September 03, 2007, 12:59:30 AM »
Im sure the reason they dont recomend there 158 is that its not constructed heavy enough for those velocitys. A 180 grain cast slug in my opionion would be optimum  for deer hunting with your max. But id definately step up to at least 180 jacketed stuff. What you need to kill animals with a handgun is penetration not fps of energy. Handguns dont relate well to ft pds of energy calculations and in my opinion for what its worth rifles dont either. 10000 fps of energy isnt going to kill a deer unless you put the bullet into the vitals. you may laugh at the thought of shooting something with a 22mag but it will kill better then a bullet that blows up on the shoulder and doesnt penetrate. Ive personaly seen to cases of bullet failure on hogs. One was a 158 hp out of a marlin carbine and the other was a 170 sierra out of a 41 mag. Neither bullet penetrated the shoulder and in the case of the 41 it was shot twice and both bullets failed and it was put down by a 41 speicial shooting a 220 cast at a 1000 fps. Now deer arent as tough as hogs to penetrate but i wouldnt want to bet on hitting a shoulder bone even on a deer with one. To me its silly when theres proper bullets available. I allways relate ft pds of energy and its use in rating guns like this. If you were faced with a charging buffalo or brown bear and had your choise of guns which would you take. a 257 weatherby shooting 1oo grain bullets at 3500 fps or a 4570 with a 405 grain hard cast flat point at 1300 fps. No doubt the 257 has it beat to hell in ft lbs of energy but which is going to save your skin!!!!!
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Offline irold

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #13 on: September 03, 2007, 10:12:24 AM »
Lloyd,  I understand completely....good comparison.  Now without showing too much of my ignorance..I've never shot cast. I presently load 158 gr Hornady swaged ( sp) lead bullet in my 357 mag with unique for a practice load, everything else has alwas been a jacketed or solid copper bullet. Is that swaged 158 gr a cast bullet? Think I may be getting confused !  I'm not into making my bullets, nor do I have time,I can barely keep up with reloading and shooting, let alone juggling my job in there somewhere. Can you buy what your referring to ?  Do I need gas checks , etc ?  Whats the major difference between , say, the 180 cast and the 180 XTP ( besides the copper jacket) ?   Sorry for all the ?'s....I may be old but I'm still trying to learn.    Thanks

Offline AK737FO

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #14 on: September 04, 2007, 05:55:59 AM »
I use a slightly different formula that I got out of an old reloading book to figure out foot pounds of energy. 

FPS squared divided by 450400 times bullet weight (in grains) equals foot pounds of energy on target.

For my new Smith 500 load it is : 1650 X 1650 / 450400 x 440 = 2659.  About the same energy out of a pistol that I get from my 30-06.  Wow...


Offline davem270win

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Re: Formula for Energy developed by velocity and bullet weight ??
« Reply #15 on: September 04, 2007, 09:28:54 AM »
You may want to also look at the "Taylor Knockout Formula"

Energy in foot-pounds is a true measure or comparison of bullet 'power', but energy alone leaves a lot to be desired. It does not compare bullet effect on animals. As a result of this, many shooters have switched over to using the Taylor Knockout formula (TKO) as a much more realistic method of comparing bullet effect on game.

The Taylor K O Formula is an empirical measurement and is calculated as follows...

Bullet Weight (in grains) multiplied by Calibre (in inches) multiplied by Velocity (in fps) divided by 7000 = TKO rating.

This figure is basically the momentum of the projectile modified by a factor proportionate to the calibre.

A 22 LR = 1.5,  44 mag is 21, 45-70 is 35.