RFI: A Different Testing Medium

My RFI is for someone in the north Texas / southern Oklahoma area who can weld heavy steel.  Anyone out there interested in doing a long-term project with me?

Here’s my line of thinking.  When a bullet strikes a soft target, you’ll get penetration to varying degrees (as we’ve been seeing here, for example).  That kind of measurement and analysis is made  possible with the use of ballistic gel.

I want to measure something a little different:  kinetic energy.  I know that ammo manufacturers usually supply this information in ft/lbs for their products (at least, most of the centerfire rifle stuff does);  but I want to try it for myself.

Here’s what I want to do.  We’re all familiar with the tractor-pull thing, where trucks compete to see who can shift a specific weight the furthest (with a shifting weight which increases drag over distance).  I want to apply that same principle, only using a weighted sled running on rails.

The methodology would be to have a stout piece of steel, e.g. a 1-ft x 1-ft x 2″ thick steel square — the target (solid, to avoid any thought of penetration) — welded to a  weight with four wheels (like below) attached.

Ideally, the whole weighted/wheeled target would weigh about 100lbs.

Then I’d want to get two lengths of steel I-beam laid on their side, upon which the wheels would run, set on level ground.  (I don’t know how long the beams would have to be;  10′? 15′?  We’d have to see.  Or if we needed shorter channels for ease of use, remove the wheels and replace with skids instead.)

  

The we could shoot the 1′-square steel target, and see how far the bullet(s) pushed the sled along the rails.

This all came about when I was talking to someone about the wisdom / folly of hunting a Cape buffalo with a .45-70 Govt vs. the Usual Suspects (.375 H&H, .458 Win Mag etc), and the guy (a seasoned hunter) said that it was all very well to use a round which penetrates a buffalo, but if it went all the way through, it was wasted energy;  he’d prefer to dump all the energy into the animal, to “knock it on its ass”, as it were.  A buffalo’s hide / body is tough, all right:  but the old “needle vs. bowling ball” argument always rears its head.

My goal in this is not to test rifle ammo, but to test self-defense pistol  cartridges.  I believe that if you were to combine ballistic gel-penetration numbers with the sled’s momentum / ft.lbs data, you’d be able to add yet another dimension, and judge a cartridge better than simply relying on the Lucky Gunner formula of muzzle velocity / bullet expansion / gel penetration.

If someone (e.g an engineer) has experience doing this kind of thing and wants to scope / design the project, please let me know.  Right now, I’m just blue-skying the thing out of ignorance.

Or has this, or something similar, been done elsewhere and I just missed the party?

Your thoughts in Comments.

14 comments

    1. Re-reading your post. You misused “kinetic energy”. The pendulum was invented because folks hundreds of years ago wanted to figure out velocity. Just like electricity calcs, all these equations are tied together. Figure out momentum and mass you can find for velocity. Soooo today we have excellent chrono’s. No real need to build something. Or just accept the fps by manufacturer on factory ammo and the bullets given weight…

      Momentum can be computed : Momentum in lb-f/s = bullet weight (in grains) divided by 7000 times velocity (in fps).

      Now you have everything..fps…mass…kinetic energy…and the big mo….

  1. I think you’ll find that a 100 lb target and 10 – 15 ft rails to be too big, especially for handgun levels of energy. Mythbusters did an episode similar to this idea. The myth was that shooting a playground merry-go-round with a handgun would cause it to rotate, which was a plot point of some movie or other. Even with a lubricated and balanced merry-go-round they couldn’t get it to move more than a fraction of an inch with any handgun round. To get it to move even a few inches they had to use increasingly larger rifle rounds, culminating (IIRC) with a .50 BMG which still only moved it < 1 foot.

  2. In addition to the above, bear in mind that only a portion of the bullet energy will be transferred to the sled as momentum. Part of the energy will be transferred into heat. And part of the energy dispersed as the round disintegrates and shrapnel splashes off the solid metal target. I’ve no clue as to what percentage is actually transferred as momentum, but I doubt you’d move very much weight.

    If you do create something of the sort, at best it will be useful only as a comparison measure.

    1. “…but I doubt you’d move very much weight.”

      So, In Open Range, when Boss Spearmint shot the bad guy through the window with a 10 gauge coach gun and he flew across the alley, it was pure, unadulterated bullshit? What in the name of all that is unholy…..?

  3. Two words: “Ballistic Pendulum”

    The math and science has already been done. This is an old technique to calculate kinetic energy and velocity. The modern chronograph now takes care of the velocity part. That leaves kinetic energy. It will save you from re-inventing the wheel.

  4. I’m curious, Kim. You stated you want to measure kinetic energy; precisely, to what end?

    “Kinetic energy” is mass in motion over a distance over time (remember that “distance over time” is actually a velocity measure) and; the motion and time parts are usually expressed as a velocity component (which is inaccurate, to a certain degree in the ballistics world, because velocity is not a constant, except at one very small physical point; even very high end chronographs measure average velocity over a certain distance because there will be distance between the start and stop sensors and unless conducted in a vacuum (which would actually be really interesting) velocity will decline over even that small distance).

    If it’s “kinetic energy of the moving bullet” it can be calculated, and thanks to Mr. Newton and his Third Law of Motion, can be measured (but not without some difficulty): the bullet is being pushed away from the gun with exactly the same force with which the gun is being pushed away from the bullet, so measure that force at the gun.

    Actual velocity data – which depends on lots of different factors – is either available, or can be easily determined; bullet expansion can be measured using a “standard medium” which, unfortunately, is randomly inaccurate enough in actual practice (eg., defensive gun use on the street); bullet performance for the task of self defense can be tabulated from historical record,which brings us back to the “randomly inaccurate” problem due to lack of experimental controls. And, as Mr. Curton (above) points out, some of the energy will be dissipated as heat and used for metal deformation of both target and projectile.

    If it’s simply measuring “energy” a chronograph, grain weight scale and a calculator are all the tools one needs, but I sense there’s something else you’re seeking here that you perceive as lacking in current ballistic analysis. I can’t figure out exactly what it is though.

    1. I wanna shoot something and watch it move. Then shoot a different bullet at the same thing and see what happens. Repeat ad infinitum.
      Out there in YouTubeLand, there are hundreds of videos of guys shooting different things: gel, water-filled milk bottles, concrete breezeblocks, etc. I just wanna add to the fun.

      1. In that case, this is probably the wrong approach.

        From a purely technical standpoint, a chronograph and bullet weight provide all the data you need. The chrono for velocity and bullet weight for mass. Then work out momentum and kinetic energy…watch the units.

        Now, if you want to put on a show, I’d take a look at the Capandball YouTube channel. I know the man who does that, he’s got serious credentials in both the shooting and historical arenas.

  5. I wanna shoot something and watch it move.

    I’d guess most of us are more interested in the opposite effect when we shoot something, but, well, OK. At least your approach probably won’t result in the gendarmerie becoming involved.

    It’s an interesting engineering problem. Assuming accurate capture of maximum energy is the objective, the medium used needs to allow non-destructive containment of the projectile, preferably without allowing the divertion of any energy into deformation of the projectile. Someone above suggested using gelatin as the energy-absorbent media and measuring the thrust applied to it.

    Gelatin’s properties vary based on mixture and temperature, and it isn’t dense enough to absorb the energy involved within reasonably small volumes. It’s also a bit limited – I’ve got some loads for my .45-70 that if used with ballistic gelatin would produce results better measured with a mop than a yardstick.

    I may be over-thinking this, but maybe a piece of ballistic plate is the answer; some versions are available with anti-spall coatings that seem to do a reasonable job of projectile containment. I’d guess a 4″X 4″ or 6″X 6″ piece, perhaps with a double layer of anti-spall, mounted on a rolling base (which would require bearings as frictionless as possible, plus a base as smooth as possible, maintained precisely level, etc.) could produce the visual effect you’re looking for. Probably FMJ bullets, too, but a comparison between HP, SP and FMJ might be interesting. Still needs a chrono, though, because velocity is the major component of energy, and some sort of conversion algorithm to compensate for velocity variations. I suspect, though, especially with handguns, there’s not enough energy available to produce the sort of dramatic visual effect that would capture YouTube headlines. Bottom line is, again, Newton’s Third – no more energy can be imparted to the measurement gizmo than is simultaneously imparted to the projectile expulsion device; since a 3 lb gun doesn’t push the user across the room even with 44 mag, enough mass in the target to avoid deformation (because deformation=energy consumption, never mind possible destruction of the target) it won’t move all that much, either.

    Hmmm…..a suitable absorbent medium vertical to a flat plate that’s supported by air bearings (think “air hockey table”) might be frictionless enough. Maybe. Would still need something to control movement in the Y axis so that tangental movement is eliminated and X axis movement is reliably measurable, maybe teflon rails, or
    magnetic Y positioning (my brain hurts, can I stop now?).

    The water jug thing has been done to death, wet newspapers is just soggy cellulose, brightly dyed gelatin, mixed thick enough to stand up by itself but thin enough to blow apart isn’t different from water jugs with food dye, and Billy Bob and Cletus probably aren’t interested in ballistic pendulum numbers. Using gunfire to push an energy-absorbing target X feet across an air hockey table, with bright colors, cogent narration and good music, maybe…..find a way to incorporate scantily-clad (or naked) women, and you just might have a winner.

  6. In Range (Forgotten Weapons) did a video using handguns with a swinging/rotating pendulum target. How far can you displace the target plate, and how many rounds does it take to get it to flip over vertically. They were playing with .50GI 1911’s and Glocks, and other lesser caliber handguns. Ian and Karl were having fun.

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