How and why does spinning a bullet make it stable?

[TriPolar]

@Machine_Elf

OK, I looked around online. A number of references (such as this one) talk about the M16 bullet’s tendency to yaw/tumble after impact with a target, but not during flight. Supposedly the tendency for post-impact tumble was enhanced by shaping the bullet so that it is tail-heavy. It’s still spin-stabilized in flight for accuracy,and the spinning provides enough stability to counter aerodynamic forces, but not enough to counter the yaw forces presented when such a bullet strikes flesh: the nose of the bullet gets pushed to one side, which presents a longer lever-arm with respect to the bullet’s center of mass, which results in greater yaw, and so on.

The key assumption is that any fleshy target is almost never going to be struck at a perfect 90-degree angle of impact, so there almost always will be a tendency for the bullet to receive some initial yaw impulse when it hits.

If the bullet actually tumbled in flight, you’d expect to see the round hit its target straight-on only very rarely; most times it would be canted at some angle, or even completely sideways. If you were shooting at paper targets, you’d see a lot of very weirdly-shaped holes, and very few small circular holes. Instead, the opposite is true: you never see oblong holes in a paper target, just nice circles.

This all makes sense, I don’t recall where, but I saw some info in an article about the history of the M16. I know a guy who told me they used to use knives to bore out the end of their M16 barrels to get the tumble or wobble effect, but he’s usually full of shit.

[CalMeacham]

The history of spin stabilization might go back a long time. A 19th century article in a Classical journal (I don’t have the reference at hand right now) suggested that the reason that the Keraunos, the classical image of the Thunderbolt of the Gods, often had a characteristic “twist” depiction was because it was recognized that a spinning object will fly true.

[Bear_Nenno]

@dasilva94

The projectile from an AK-47 is deisgned to tumble inflictiing a larger wound than a direct firing bullet.
I don’t know if any current “assault weapons” replicate this.

Nonsense.

[Dallas_Jones]

@CalMeacham

The history of spin stabilization might go back a long time. A 19th century article in a Classical journal (I don’t have the reference at hand right now) suggested that the reason that the Keraunos, the classical image of the Thunderbolt of the Gods, often had a characteristic “twist” depiction was because it was recognized that a spinning object will fly true.

The fletching on arrows used to be feathers and feathers have a curvature depending upon which wing of the bird the feather came from, right or left. This imparts a slight spin on the arrow and aids stabilization along it’s long axis. Modern arrow fletching can be applied in a helical alignment to impart the same sort of spin, but you don’t want too much spin or it will slow the arrow.

And then there is the thrown football (American football) where the spiral helps to keep the ball on target. No quarterback is going to throw a long pass with a non-spinning ball. The ball is gripped, usually near the treads, and then spun off the hand upon release.

Sorry FeAudrey, I missed your post.

[Bear_Nenno]

@TriPolar

I know a guy who told me they used to use knives to bore out the end of their M16 barrels to get the tumble or wobble effect, but he’s usually full of shit.

I wouldn’t doubt for a minute that some ignorant GIs did something so ridiculous. I could fill a book with things stupid privates are capable of doing.
If he and his buddies ever actually did this, they were not accomplishing what they thought they were. It’s like leaning forward in the car to make it go faster. Sure, someone somewhere has done that… but it didn’t actually help anything.

[Machine_Elf]

@Bear_Nenno

Nonsense.

Thank you for this very informative contribution to the discussion.

[MichaelEmouse]

Bear,

APFSDS rounds use fins, as their name implies. How true do they fly?

Did you tank use HE rounds? If so, how were they stabilized given that the barrel is smoothbore?

[Alessan]

Smoothbore tank rounds fly so fast they don’t have time to destabalize.

[MichaelEmouse]

@Alessan

Smoothbore tank rounds fly so fast they don’t have time to destabalize.

APFSDS, perhaps, but I would be quite surprised if HE rounds travel at Mach 4/5/6.
In any case, a 5.56NATO round travels at about Mach 3 and still has time to destabilize over a few hundred meters (otherwise why bother to have it spin?). If a 5.56 at Mach 3 has time to destabilize over a few hundred meters, an APFSDS at Mach 5/6 would have time to destabilize over distances which are at least twice as great.

[smithsb]

Both the HEAT and Sabot rounds are fin stabilized for the M1 Abrams and Leopard Tanks. Also the multipurpose round.

[Der_Trihs]

@CalMeacham

The history of spin stabilization might go back a long time. A 19th century article in a Classical journal (I don’t have the reference at hand right now) suggested that the reason that the Keraunos, the classical image of the Thunderbolt of the Gods, often had a characteristic “twist” depiction was because it was recognized that a spinning object will fly true.

@Dallas_Jones

The fletching on arrows used to be feathers and feathers have a curvature depending upon which wing of the bird the feather came from, right or left. This imparts a slight spin on the arrow and aids stabilization along it’s long axis. Modern arrow fletching can be applied in a helical alignment to impart the same sort of spin, but you don’t want too much spin or it will slow the arrow.

I recall reading some years ago that the arrows found on “Otzi the Iceman” were made with angled fletching to impart a spin. So the principle was known over 5000 years ago by some peoples.

[Bear_Nenno]

@Machine_Elf

Thank you for this very informative contribution to the discussion.

Oh, but asking for a cite is a thorough contribution? I am not going to ask him for something I know does not exist. No cite for such a ridiculous claim exists because it is nonsense. If engineers want to improve or change the terminal ballistics of the round, they are not going to do it at the expense of external ballistics. No rounds are designed to tumble in flight. It’s a silly bit of misinformation that continues to be passed along and simply will not die.

But you just go ahead and wait for him to give you a cite. I wouldn’t hold my breath if I were you.

[Bear_Nenno]

@MichaelEmouse

Bear,

APFSDS rounds use fins, as their name implies. How true do they fly?

Fin stabilized rounds fly true. Because they have fins, they do not need to be gyro stabilized. Rounds without fins require rifling.

Did you tank use HE rounds? If so, how were they stabilized given that the barrel is smoothbore?

I don’t have a tank. Never been in one. But, yes, there are strictly HE rounds for the M1 Abrams. As you might imagine, they are also fin stabilized.
www.epicos.com/epicos/extended/switzerland/sn_technologies/imgs/pro12.gif

In any case, a 5.56NATO round travels at about Mach 3 and still has time to destabilize over a few hundred meters (otherwise why bother to have it spin?). If a 5.56 at Mach 3 has time to destabilize over a few hundred meters, an APFSDS at Mach 5/6 would have time to destabilize over distances which are at least twice as great.

I really don’t understand what you’re getting at here. There are two options for stabilizing a projectile. Spin it, or put fins on it. Pick one. If the round is so tiny that it would be impractical to use fins, then shoot it out of a rifled barrel to spin it. If the round travels so fast out of the barrel that rifling becomes impractical or useless, put some fins on it.

[Dr.Strangelove]

@Bear_Nenno

Rounds without fins require rifling.

In practice, yes. In principle, you could construct an ordinary-looking bullet that was stable without fins by adjusting the internal density distribution. An aluminum bullet with a tungsten slug near the tip might not fire very well, but it should be stable in flight.

You want the center of pressure to be well behind the center of gravity; fins are one way of ensuring this, but moving most of the weight to the tip would also work.

[Knorf]

@Quercus

Let’s just all agree that for any standard bicycle with a rider, gyroscopic stability is much less important for stability than front-wheel trail.

@TriPolar

Easily agreed.

Nope. Sorry for the hijack, but your statement is not up to date. The information on the website demonstrates that neither the gyroscopic effect nor trail are the critical factors for bicycle stability, but rather the bicycle’s inherent tendency to self-correct is: it turns into a fall, and only falls when it no longer has the momentum to stay upright.

[eschereal]

Try to throw a frisbee without putting spin on it.

[j_sum1]

@Knorf

Nope. Sorry for the hijack, but your statement is not up to date. The information on the website demonstrates that neither the gyroscopic effect nor trail are the critical factors for bicycle stability, but rather the bicycle’s inherent tendency to self-correct is: it turns into a fall, and only falls when it no longer has the momentum to stay upright.

Thanks for that. I will have to have a closer read later and watch the videos.
I has always assumed that castoring led to the phenomenon of trail and that in turn led to the tendency to turn into the direction of a fall. Simple things are often deceptively complex.

Anyway, back on topic – ballistics – I have nothing to contribute.

[Tangent]

Here’s a cool Mythbusters clip involving spinning bullets.

[Patch]

@Machine_Elf

OK, I looked around online. A number of references (such as this one) talk about the M16 bullet’s tendency to yaw/tumble after impact with a target, but not during flight. Supposedly the tendency for post-impact tumble was enhanced by shaping the bullet so that it is tail-heavy. It’s still spin-stabilized in flight for accuracy,and the spinning provides enough stability to counter aerodynamic forces, but not enough to counter the yaw forces presented when such a bullet strikes flesh: the nose of the bullet gets pushed to one side, which presents a longer lever-arm with respect to the bullet’s center of mass, which results in greater yaw, and so on.

The key assumption is that any fleshy target is almost never going to be struck at a perfect 90-degree angle of impact, so there almost always will be a tendency for the bullet to receive some initial yaw impulse when it hits.

If the bullet actually tumbled in flight, you’d expect to see the round hit its target straight-on only very rarely; most times it would be canted at some angle, or even completely sideways. If you were shooting at paper targets, you’d see a lot of very weirdly-shaped holes, and very few small circular holes. Instead, the opposite is true: you never see oblong holes in a paper target, just nice circles.

Bullets tend to be base heavy, and want to rotate 180 degrees and finish their travel base-first. The M-16’s 5.56NATO does it, the 7.62x39 fired by the AK-47 does it, a .30-06 does it, a .22 does it, etc. It’s not unique. Even the 6.5mm Carcano fired by Oswald does it.

I used to be into wound ballistics during the 90’s, and from what I’ve picked up out of the literature, because bullets tend to be base heavy and want to flip so they travel base first, the bullets are spin stabilized to keep them facing forward. However, when a bullet strikes a denser medium such as water or tissue, spin stabilization no longer is effective and the bullet will yaw 180- degrees and finish its travel base first, assuming the path in the target is long enough.

How long it takes to do this varies by bullet design. Some 7.62x39 designs can take over a foot of travel, others a few inches. The 5.56NATO will yaw after 4-8 inches, but this bullet has a thinner copper jacket than other bullets, and if it’s traveling fast enough (+2500fps), it will fragment. The 5.45x39 fired by the AK-74 series yaws after 2-3 inches, but with its thicker jacket, even though it’s traveling at a velocity comparable to the 5.56NATO, it won’t fragment (contrary to popular belief, it’s the bullet shape that’s responsible, not the air pocket at the tip). The British in WWII used a .303 bullet with an aluminum nose filler instead of lead, causing it to yaw early. There’s a bunch of useful information here, and also done diagrams showing terminal behavior of various bullets.

[TriPolar]

@Knorf

Nope. Sorry for the hijack, but your statement is not up to date. The information on the website demonstrates that neither the gyroscopic effect nor trail are the critical factors for bicycle stability, but rather the bicycle’s inherent tendency to self-correct is: it turns into a fall, and only falls when it no longer has the momentum to stay upright.

Which in no way contradicts our statements.

ETA: To be more specific, those attributes exist in regular bicycles. They may not be necessary for a bicycle to remain stable, but they contribute to that stability when they are there.