Please dissect - Bullets, Ballistics, & Stopping Power


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jscott
May 20, 2012, 10:42 AM
I thought about putting this in a number of different categories, so "General Gun Discussions" it is. If it needs to be moved, please feel free.

I know this is long (2 pages) but I am hoping that some ballisticians can dissect the information and tell me what is right/wrong with it. While I feel I have a pretty good understanding of bullets and ballistics, I'm not 100% about all of the info.

This was compiled by someone I know. There are no copyright/publication issues:

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56hawk
May 20, 2012, 11:05 AM
Well... The whole force part doesn't make any sense. Trying to convert velocity into acceleration doesn't work unless you know the time it takes for the bullet to stop. What is actually calculated by the equation mass times velocity is momentum not force.

Loosedhorse
May 20, 2012, 11:14 AM
+1. A bullet in motion doesn't have "force"; it has mass, velocity (both linear and rotational), direction, energy and momentum. It does not possess acceleration, though gravity and air resistance (friction) are changing its velocity.

And just using the term "stopping power" is a red flag to a bull around here.

Pigoutultra
May 20, 2012, 11:26 AM
I can't even tell what this is trying to say. It seems very dodgy to me.

jscott
May 20, 2012, 11:31 AM
A bullet in motion doesn't have "force"

I took "force" and "acceleration" as written in the text to mean once the bullet is acting upon the target and not just during flight. It would seem to me that there is indeed a force and acceleration (deceleration), and not just velocity, at that point, whether or not the calculation that got one there in the write-up is accurate. At least that is how I interpreted the leap being made between velocity and acceleration for the sake of the equation.

I get the whole foot/pounds of energy thing but I'm not sure I follow the "force" part, at least as stated in the write-up. How would one go about calculating the "force" of the bullet on the target?

The thing that caused me to hesitate and give the write-up the extra look is that the results do tend to be consistent with the proverbial idea of "stopping power," at least considering when you look at the numbers from the top down.

56hawk
May 20, 2012, 11:55 AM
How would one go about calculating the "force" of the bullet on the target?

It would be the mass times acceleration as listed. Problem is how do you figure out the deceleration as the bullet hits the target? It is going to be dependent on the target material and projectile diameter and shape. You could get a general idea from high speed photography of ballistic gel. I think I saw a post here where Brass Fetcher did exactly that.

Loosedhorse
May 20, 2012, 11:59 AM
How would one go about calculating the "force" of the bullet on the target?Tough to do, as the force can be applied collinearly, orthogonally (radially), and rotationally, depending on the nature of the target and the collision. The force applied to a thick teel target will be different than that applied to a piece of paper or to a long brick of ballistic gel.

Much easier to calculate the total energy the bullet transfers to the target, even though that also will take many forms.

Iramo94
May 20, 2012, 12:07 PM
I was always taught to not talk about force as a result, but as a cause. The bullet has no inherent "force".
We shouldn't say F=ma, but instead F/m=a.
More specifically, the deceleration of the bullet in the target is a function of the friction of the target. (divided by the inertia of the bullet, of course)
So, to find the "force" that happens during a bullet hit:
First, find the amount of time it took the bullet to stop. Then, the velocity of the bullet. From this you can find the average acceleration. Multiply by mass to get the frictional force needed to stop the bullet. Then apply the action-reaction law, and there you have the "force" imparted on the target when the target tries to stop the bullet.
That's my best guess of how it might be found.

ETA: this assumes no rifling and a constant friction, something found in ballistic media but not a body. Organs have different density than flesh.

56hawk
May 20, 2012, 12:10 PM
Found the post I was talking about: http://www.thehighroad.org/showthread.php?t=611748

They did it as a change in kinetic energy, but the principal is the same.

MachIVshooter
May 20, 2012, 12:27 PM
Kinetic energy is a good way to compare rounds and determine their ability to do work, but a whole lot more goes into terminal effectiveness than that.

As well, the force calculation doesn't mean much. A bullet striking a vest will impart more force on the target than one that passes through them, but is far less likely to cause severe trauma. Similarly, getting hit by an automobile traveling 10 MPH transfers a LOT more energy into a person that being struck with a bullet, but probably won't be fatal unless one is actually run over (crushed) by the vehicle.

Bullets kill by destroying tissue/organs and causing blood loss. It really is that simple, and projectiles were killing things long before people understodd the physics involved.

wally
May 20, 2012, 12:34 PM
Check out this one: "carry what you like, they all work or don't about the same"

http://www.buckeyefirearms.org/node/7866

56hawk
May 20, 2012, 12:43 PM
Check out this one: "carry what you like, they all work or don't about the same"

http://www.buckeyefirearms.org/node/7866

Thanks for posting that, saw it a while back and then couldn't find it again. The one graph I found interesting was the one below. There is a big difference going from 32 to 380.
http://www.buckeyefirearms.org/publicfiles/Ellifritz_Failure_to_Incap.png

MachIVshooter
May 20, 2012, 12:52 PM
56Hawk-

While I don't have difficulty believing such charts, it's anecdotal-even if done scientifically. Additionally, we often aren't privvy to the methodology and controls, and many such studies don't stand up to scrutiny. M&S is probably the most comprehensive study of GSW stats, but it is still the subject of much criticism and debate.

It still comes down to damage done being the only semi-reliable factor. Sometimes peope get lucky (or unluckly if the one shot) with small cartridges and sometimes people walk away from hits with big bullets that should have made them DRT. In the end, though, the bullet/cartridge combo able to cause more damage is more likely to stop the threat. It always has been and always will be that way, no matter how many whizz-bang formulas are created to make people feel better about carrying ice-pick rounds and the like.

jscott
May 20, 2012, 01:54 PM
Tons of good info – and links that I missed – don’t know how.

A long time instructor and student, I feel I have a pretty good understanding of ballistics. Here’s what brought me to my original post though and I realize that finding a concrete answer in a subject area marked by a combination of theory, science, and case study is an uphill battle.

I recently obtained a copy of my own police report wherein I shot a bad guy with a .223 caliber M-4 carbine at about 10 yards. He was at a very slight angle to me. The single Federal 55 grain hollow point penetrated the sternum and center-punched the heart. I am aware that a person can theoretically continue for up to 15 seconds or so with even such a devastating wound. However, in my case I can say that without a doubt life left the body while he was still fully erect. I have seen a lot of death. There was no delay. Eyes, instantaneously limp body, a literal sack of potatoes if you will. I have even considered the psychological effects of being shot and thinking you should fall but let me say this, he wasn’t thinking anything the moment that round hit his chest.

So what the heck was it? Blood loss couldn’t have been fast enough. 15 seconds hadn’t elapsed. Central nervous system disruption is a maybe.

While I am very familiar with temporary wound channels, permanent wound channels, the effects of blood loss, destroying vital organs, the theory of hydrostatic shock, and more, I have endeavored to put it all into a concrete “why.” Things such as the .357 having seemingly a greater ability to stop a threat, all being equal, than the .38. They can have the same bullet weight and often penetrate to nearly the same depths. The .357, having a higher velocity, has a much higher foot/pounds of energy result. What is it about the greater velocity and foot/pounds that makes it a better stopper? A lot of test data, including referenced graphs, seems to conclude that they are about the same but I would say in reality it’s the .357.

Both rounds (or many others you could compare) will do the things necessary to stop – destroy vital organs, blood loss, etc. Basically what it comes down to is, all else being equal, why is kinetic energy, foot/pounds energy, or something else entirely, so important? That is why the “force” and “energy” combination presented in the original write-up was appealing.

For reference, and I am sure not boasting for the experiences have been less than pleasant, but between the military and police work I’ve had a few shootings with the handgun and long gun. Those hit with rounds from the handgun didn’t seem to react as quickly and this makes sense to me, but that would seem to discount blood loss, organ damage, etc., to some extent because rifle or handgun, each caused significant damage and blood loss while the rounds all achieved adequate penetration. "Acceleration" is possibly a variable between the rounds but I'm not sure how that plays outside of the force equation. The graphs depicting kinetic energy are interesting but don't seem to indicate a large difference between calibers that are traditionally thought of as having a large disparity.

So, force, kinetic energy, momentum, what is it? Though it is much criticized, “hydrostatic shock,” in part contributed to the increased velocity of rifle rounds, seems to be consistent with my personal results, which taken in perspective I admit is a very small sampling.

I know, an age old discussion. All of the above I guess. But my intellectual side beckons for clarity.

Shawn Dodson
May 22, 2012, 11:45 AM
See: Extract from “Wound Ballistics Misconceptions.” (Duncan MacPherson, Wound Ballistics Review, 2(3): 1996; 42-43) at http://www.firearmstactical.com/tacticalbriefs/2006/04/03/0604-03a.htm

Open the link and scroll to the end of the page.

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