That Word Doesn’t Exactly Fit

Some time ago, I posted an essay concerning how bullets fired from derringers can tumble in flight.

engraved derringer

tumbling bullet trajectory

Long time reader Fruitbat44 left a comment that has been nagging at me.

“Please correct me if I’m wrong, but I always thought that a bullet that tumbled would do more damage to an attacker than one which didn’t.”

What Fruitbat44 is referring to is a claim made during the vicious and heated debate that ensued when the US military started to replace their front line battle rifles with the M16.

m16

The new rifle was chambered for the 5.56 NATO round, which produced around 1,700 foot/pounds of muzzle energy.  This was to replace the 7.62 NATO cartridge, which produces about 2,500 ft/lbs of muzzle energy.  They were going to go from bigger to smaller.

762 next to a 556 round

The reaction of most people who had been in combat was swift and decisive.

HOLD THE PHONE!  The US military, in the middle of a war, was going to replace a proven and battle-tested rifle with one that was weaker?  Who the hell thought this one up?  THEY WERE SENDING OUR GUYS OUT TO DIE!

Not so, proponents of the new rifle said.  The new ammunition was designed to tumble, dontchya know.  It didn’t matter that it was smaller and weaker than the old ammo, because it produced even bigger wounds!  The tumbling bullets chewed through a human body like a buzzsaw meeting a damsel!

NellSawMill_8673

(Picture source.)

There is also no doubt that 5.56 rounds produce some impressive wounds for their size.

556 in ballistic gel

So the bullet from an AR design tumbles while it flies through the air?  Actually, no.  It flies straight and true in the air like any accurate round, but tries to swap ends when it enters something soft and squishy like a human body.  When the bullet turns sideways, the stresses produced tend to break up the round.  It is very common for 5.56 rounds to fragment into at least a few fragments.

So why does the bullet do this?  Have no idea, actually.  Depends on who you talk to.  Some people say it is due to the shape of the bullet, others say it is the way the bullet is balanced, and still others say that every bullet does this and the 5.56 is no different.  I’m just pretty sure that it does.

If anyone has the inside scoop, please share in the comments.  I’m going to go to the range and fire a few rounds.

11 Responses to “That Word Doesn’t Exactly Fit”

  1. mkl says:

    Seems like I read the original Vietnam rifles at the time had one in fourteen twist barrels which would just barely stabilize the issued 55 grain bullet in flight. Once the marginally stabilized 55 grain pill hit anything it would begin to tumble and inflict major damage. Any bullet over 55 grains would not stabilize with that twist rate, which is the reason today we see a one in nine twist most common in the 5.56 rifles designed to use the 63 grain or heaver bullets.

    This is from memory which may be poor these days; I’m sure someone will correct me if I am wrong.

    • Bram says:

      Yes – The originals were 1/14 and the original ammo was 49-grain. So it was really moving. It also had a reputation for ricocheting off of anything – leaves, bones, whatever. A relatively short ranges against NVA soldiers without body armor it was pretty effective.

      Now with 62-grain bullets, double the twist, and shorter barrels the round is very stable – so a less impressive wound.

      I have heard that the Russian 5.45×39 round was purposely designed to yaw or tumble.
      http://en.wikipedia.org/wiki/5.45%C3%9739mm

  2. A tumbling bullet will do more damage IF IT HITS. But a tumbling bullet will veer off, and your chance of hitting is lower.

    If it’s a derringer, fired from a range of 5 feet, it won’t veer enough to matter. But if it’s a rifle fired from a range of 50 yards, this is very important.

  3. Aerodynamically, the ideal shape is blunt in the front and strongly tapered in the rear. (I know that’s counterintuitive, but it’s true.) Which is to say that the normal firing position of an FMJ bullet with a point is exactly backwards, and there are strong forces trying to turn it around. Rifling stops that; the spin prevents the bullet from swapping ends. But it isn’t too much of a surprise that it flips once it’s inside a body.

    • Fruitbat44 says:

      So that’s why you get target shooters loading wadcutters?

      • knirirr says:

        I always thought that it was because they punched nice neat holes in the target which make it easier to score; the wadcutters I have certainly do that (they are flat at both ends).

        • Fruitbat44 says:

          I’ve heard that one too. And there was that S&W(?) auto designed to feed .38 Special wadcutters for target shooters.

          Hmm . . . armchair gunfighter again, but I dimly record reading about some handloaders using wadcutters for self-defence load. And also the Webley .455 “manstopper round” which looked something like a wadcutter.

    • Brother Pilot says:

      I’m a bit late to the discussion but I find it fascinating. I’m a pilot and aeronautical engineer, and I like to shoot. This discussion hits 2 of those three things.

      Steven, what you say about shape is true aerodynamically for subsonic rounds and minimizing drag. For supersonic rounds you want a pointy shape more like the typical rifle bullet to decrease the strength of the shockwave and thus decrease drag. Also you want a boat tail on the bullet (best would be if you could taper it to a point in back but that’s not practical for size and weight limitations usually). Counter-intuitively some rifle rounds are hollow point (or open tip is slightly more accurate a term…pardon the pun) for improved accuracy. The open tip allows more uniform construction of the bullet for reasonable cost. There are also tipped match bullets. Sierra, for example, has their Tipped Match King bullets, which are open tip bullets with a plastic tip inserted into the opening. Sierra’s claim is that the tip improves ballistic coefficient. If you look at the picture on Sierra’s website you’ll see that the very point of the bullet is rounded. I suspect this is to make the manufacturing process reasonable, and to keep it from being fragile and susceptible to damage. There are also some very small scale aerodynamic effects that benefit from a rounded leading edge.

      Stability is affected by the relation of the center of gravity of the bullet vs the center of pressure (where the aerodynamic forces act). If I recall correctly you generally want the CG in front of the center of pressure for a stable round. The closer together the two get the more nuetrally stable the round will be and if the CG is behind the center of pressure you’ll have an unstable round. Unfortunately I can’t recall the relation of shockwave shape to center of pressure, and my books aren’t available to look it up, but I believe that the more raked back your shockwave is, the farther back the center of pressure will be. All that to say that most rifle bullets are actually fairly stable aerodynamically, as long as they stay supersonic. Transitioning to subsonic will often do strange things to stability.

      In the case of the 5.56 FMJ round, you can still get an unstable round by playing with the exact position of the center of pressure and shifting the center of gravity. Which is true of any bullet…the combination of weight distribution and shape are what make it stable or unstable. So our host’s last couple paragraphs are basically on target (again please pardon the pun). It’s not shape, it’s not weight, it’s the combination.

      You can overcome stability issues by spinning the bullet and getting gyroscopic rigidity in space, as Bram noted above with the different twists. Again, you can tailor the twist and weight and resulting forces to be very close to unstable. Or to be quite stable.

      I’ve rambled on quite long enough (possibly far too long). But hopefully with some, ahem, accuracy. Where I’m full of baloney please feel free to tell me so.

  4. Fruitbat44 says:

    So I ‘m now a “long term reader?” That’s cool James.

    So the point about tumbling with Derringers was that the bullet lost accuracy and velocity *in* flight, rather than the, alleged, extra-damage a tumbling bullet does to a bad guy.

    As to the M16/5.56mm business all my knowledge of its terminal effects are heard/read about. However Gordon L. Rottmann in the Osprey book ‘The M16’ gives annecdotes from personal experience which support both views of its performance.

  5. […] In a previous post, there was a question about how bullet shape increases stability. […]

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