Falling bullets

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Someone wrote:

Bullets can spin upwards of 300,000 RPM + for rifle bullets, not sure about pistol bullets.

Click to expand...

Sorry, that just doesn't sound right.

Twist is expressed as 'one in ten inches' for instance. Just doing an order of magnitude estimate gives 10, 000 rpm. One twist in a ten inch barrel. The bullet takes a millisecond at a thousand feet per second to travel the ten inch barrel and acquire its angular momentum.

Further, and for this I have no 'formula' in my head, I doubt lead and likely steel would withstand the centripetal forces of 300,000 rpm and would disintegrate.

Lead yield strength is 12 MPa Alloy 514 yield strength is 760 MPa. '10^4 sec^-1' and '3 x 10^5 sec^-1'. R = 0.5 cm.

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To much trouble finding the bullet that way. I have enough trouble finding those small
estes rockets, and they are huge next to the bullet.
Guess if you used a tracer round you would have at least proof it went straight up.
I found the best way to get a feel for the reality of them is skip a bullet off of water.
The timing is just like a stone, but the skips are massive distances. Never got more
than 4 before running out of water.

Besides with the fantastic homeland security we have, they would probably charge
you with praticing for an airline terror attack.

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Doug Huffman wrote:

Someone wrote:

Bullets can spin upwards of 300,000 RPM + for rifle bullets, not sure about pistol bullets.

Click to expand...

Sorry, that just doesn't sound right.

Twist is expressed as 'one in ten inches' for instance. Just doing an order of magnitude estimate gives 10, 000 rpm. One twist in a ten inch barrel. The bullet takes a millisecond at a thousand feet per second to travel the ten inch barrel and acquire its angular momentum.

Further, and for this I have no 'formula' in my head, I doubt lead and likely steel would withstand the centripetal forces of 300,000 rpm and would disintegrate.

Lead yield strength is 12 MPa Alloy 514 yield strength is 760 MPa. '10^4 sec^-1' and '3 x 10^5 sec^-1'. R = 0.5 cm.

Click to expand...

OK. Let's take a common rifle bullet:

223 Remington (5.56x45mm) -- 62 gr FMJ (steel core), 3020 fps, 1255 ftlbs

We'll figure it fora barrel with a 1 in 12" Twist first to make the math easier.

1 twist per foot, x 3020 Feet per Second = 3020 Revolutions Per Second.

3020 x 60 (seconds per minute) = 181,200 RPM.

A 1 in 7" Twist (not uncommon for a .223 barrel) would push that same bullet to over 310,000 RPM.

I had no idea bullets spun at such a high RPM either, until I read John Ross's Unintended Consequences (I can't recommend that book enough). Fictional novel, but a lot of factual informationthrown in. One character has a custom varmint rifle that does have such a high twist and high velocity that about 1 in 10 bullets DOES just desintegrate mid-air.

...Orygunner...

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Orygunner wrote:

Doug Huffman wrote:

Someone wrote:

Bullets can spin upwards of 300,000 RPM + for rifle bullets, not sure about pistol bullets.

Click to expand...

Sorry, that just doesn't sound right.

Twist is expressed as 'one in ten inches' for instance. Just doing an order of magnitude estimate gives 10, 000 rpm. One twist in a ten inch barrel. The bullet takes a millisecond at a thousand feet per second to travel the ten inch barrel and acquire its angular momentum.

Further, and for this I have no 'formula' in my head, I doubt lead and likely steel would withstand the centripetal forces of 300,000 rpm and would disintegrate.

Lead yield strength is 12 MPa Alloy 514 yield strength is 760 MPa. '10^4 sec^-1' and '3 x 10^5 sec^-1'. R = 0.5 cm.

Click to expand...

OK. Let's take a common rifle bullet:

223 Remington (5.56x45mm) -- 62 gr FMJ (steel core), 3020 fps, 1255 ftlbs

We'll figure it fora barrel with a 1 in 12" Twist first to make the math easier.

1 twist per foot, x 3020 Feet per Second = 3020 Revolutions Per Second.

3020 x 60 (seconds per minute) = 181,200 RPM.

A 1 in 7" Twist (not uncommon for a .223 barrel) would push that same bullet to over 310,000 RPM.

I had no idea bullets spun at such a high RPM either, until I read John Ross's Unintended Consequences (I can't recommend that book enough). Fictional novel, but a lot of factual informationthrown in. One character has a custom varmint rifle that does have such a high twist and high velocity that about 1 in 10 bullets DOES just desintegrate mid-air.

...Orygunner...

Click to expand...

RPM =/= RPS My apologies.

I wonder what twist my .17 Remington had that shot at 5050 fps. The centripetal force goes down with radius of gyration.

I just looked through my Handbook of Physical Calculations and probably saw the solution but in too terse terms to recognize. Maybe I'll rummage though my Halliday, Resnick and Walker.

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The following is from Mythbusters;
[edit] Episode 50 – "Bullets Fired Up"

• Original airdate: April 19, 2006

edit] Bullets Fired Up



Myth statement
Status
Notes

Bullets fired into the air maintain their lethal capability when they eventually fall back down.
Busted , Plausible, and Confirmed
In the case of a bullet fired at a precisely vertical angle (something extremely difficult for a human being to duplicate), the bullet would tumble, lose its spin, and fall at a much slower speed due to terminal velocity and is therefore rendered less than lethal on impact (the Busted rating). However, if a bullet is fired upward at a non-vertical angle (a far more probable possibility), it will maintain its spin and will retain enough energy to be lethal on impact (the Plausible rating). Because of this potentiality, firing a gun into the air is illegal in most U.S. states, and even in the states where it is legal, it is not recommended by the police. Also the MythBusters were able to identify two people who had been injured by falling bullets, one of them fatally (the Confirmed rating). To date, this is the only myth to receive all three ratings at the same time.




http://en.wikipedia.org/wiki/MythBusters_%28season_4%29

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While the mythbusters show was interesting and informative as far as it went it has left some false impressions. There is no reason that the bullet would stop spinning just because it has stopped climbing. In fact it would not stop spinning and therefore would begin its return to earth butt end first. The tumbling they noted resulted from tipping on impact, combined with the fact that they were not fired stright up. As noted in this thread, that is not easy to do.

In addition to air resistance and gravity, the spin of the earth has an effect on the equations. If the bullet is fired to the east it will have a different velocity then if fired to the west relative to a stationary object on the ground. The earth spins at nominally 1000 MPH. Also in trying to fire straight up this rotation must be taken into account. The factor is small but it is there. Also the horizontal velocity will also be affected by wind restance, it will not remain constant as was implied earlier.

Most people think that earth rotation is too small to consider. Read up on Foucault pendulums. These can be used to actually see the effect of earth rotation.

The upshot of the discussion so far is that bullets fired just off the vertical are the most dangerous. That is the correct conclusion. But it should be noted that in WWI they actually made bullet sized "bombs". These were designed to be dropped from aircraft onto enemy trenchs. They were not explosive and they were about the size and weight of a 50 cal bullet with four small fins on the rear. They really look like little bombs. Dropped from anything over 1500 feet they could be lethal. They probably did not spin at all they just fell.

In no case would the bullet return to earth at the same velocity at which it was fired.


I too am to lazy to do the math. It is enough for me to know better than to shoot into the air. Stay under a roof at 12:00 midnight on new years eve.


Regards

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Hawkflyer wrote:

SNIP Read up on Foucault pendulums.

Click to expand...

You sneaked that in there just so everybody would be put to sleep by the big words and mathematical equations, didn't you? (yawn)

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Citizen wrote:

Hawkflyer wrote:

SNIP Read up on Foucault pendulums.

Click to expand...

You sneaked that in there just so everybody would be put to sleep by the big words and mathematical equations, didn't you? (yawn)

Click to expand...

Its a swinging subject:lol:

I am still laughing at the other thread.

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Orygunner wrote:

Doug Huffman wrote:

Someone wrote: 

Bullets can spin upwards of 300,000 RPM + for rifle bullets, not sure about pistol bullets.

Click to expand...

Sorry, that just doesn't sound right.

Twist is expressed as 'one in ten inches' for instance.  Just doing an order of magnitude estimate gives 10, 000 rpm.  One twist in a ten inch barrel.  The bullet takes a millisecond at a thousand feet per second to travel the ten inch barrel and acquire its angular momentum. 

Further, and for this I have no 'formula' in my head, I doubt lead and likely steel would withstand the centripetal forces of 300,000 rpm and would disintegrate.

Lead yield strength is 12 MPa  Alloy 514 yield strength is 760 MPa.   '10^4 sec^-1' and '3 x 10^5 sec^-1'.  R = 0.5 cm. 

Click to expand...

 

OK. Let's take a common rifle bullet:

223 Remington (5.56x45mm) -- 62 gr FMJ (steel core), 3020 fps, 1255 ftlbs

We'll figure it for a barrel with a 1 in 12" Twist first to make the math easier.

1 twist per foot, x 3020 Feet per Second = 3020 Revolutions Per Second.

3020 x 60 (seconds per minute) = 181,200 RPM.

A 1 in 7" Twist (not uncommon for a .223 barrel) would push that same bullet to over 310,000 RPM.

I had no idea bullets spun at such a high RPM either, until I read John Ross's Unintended Consequences (I can't recommend that book enough).  Fictional novel, but a lot of factual information thrown in.  One character has a custom varmint rifle that does have such a high twist and high velocity that about 1 in 10 bullets DOES just desintegrate mid-air.

...Orygunner...

Click to expand...

I knew a guy years ago who reloaded his own .223 ammo. He also swaged his own bullets, using lead wire for the core and spent .22 Magnum brass for the jackets.

He was gradually loading his rounds hotter and hotter, and suddenly he wasn't able to put holes on paper with his ammo anymore. He was very puzzled by this until he fired a shot while not looking through the scope, and happened to see the puff of his bullet disintegrating in midair.

I also had no idea they spun so fast.

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Hawkflyer wrote:

Citizen wrote:

Hawkflyer wrote:

SNIP Read up on Foucault pendulums.

Click to expand...

You sneaked that in there just so everybody would be put to sleep by the big words and mathematical equations, didn't you? (yawn)

Click to expand...

Its a swinging subject:lol:

I am still laughing at the other thread.

Click to expand...

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Well, there are cultures where celebratory gunfire is common practice. It should be noted tht these are all cultures in need of population control and also none of them is a dominant cuture in the world. Unless you are celebrating by firing at a target or hunting, "celebratory gunfire" is in my opinion the act of an idiot and/or a method of thinning the herd. Ballistics be damned, it is EXTREMELY F$%#ING STUPID!!

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Alexcabbie wrote:

Well, there are cultures where celebratory gunfire is common practice. It should be noted tht these are all cultures in need of population control and also none of them is a dominant cuture in the world. Unless you are celebrating by firing at a target or hunting, "celebratory gunfire" is in my opinion the act of an idiot and/or a method of thinning the herd. Ballistics be damned, it is EXTREMELY F$%#ING STUPID!!

Click to expand...

I can agree with most of your post, but you should be aware that a lot of people "right here in river city" do in fact shoot into the air on both New years eve and the fourth of July. I am not saying its right, but it does happen.

Like I said before, keep a roof over your head at midnight 12/31.


Regards

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FreedomJoyAdventure wrote:

I knew a guy years ago who reloaded his own .223 ammo. He also swaged his own bullets, using lead wire for the core and spent .22 Magnum brass for the jackets.

He was gradually loading his rounds hotter and hotter, and suddenly he wasn't able to put holes on paper with his ammo anymore. He was very puzzled by this until he fired a shot while not looking through the scope, and happened to see the puff of his bullet disintegrating in midair.

I also had no idea they spun so fast.

Click to expand...

I have heard of bullets disintegrating in midair before but have never thought about the spinning being the cause of it. 300,000 rpm is enough to rip about anything apart so I can believe it if that figure is actual. It would be interesting to see if anyone has actually measured the spin rate to see if that is the real figure. Doing the calculations of 1-10 or 1-7 gives a good figure but that doesn't take into account any slippageas does the bullet do the reverse ofa tire on a dragster and not get to the calculated spin rate?

I can imagine a lead bullet just vanishing into thin air like being hit with a phaser from Mr. Spock. :lol:

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Don't forget that the bars in DC will be open until 5 AM on Inauguraation Weekend. The MPDC is gonna be real busy. I am gonna knock off at midnight, if yu think I am gonna pick up vomity drunks off the Metro at 6:30 AM you got a nother "Think" a-comin.

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Hawkflyer wrote:

There is no reason that the bullet would stop spinning just because it has stopped climbing. In fact it would not stop spinning and therefore would begin its return to earth butt end first. The tumbling they noted resulted from tipping on impact, combined with the fact that they were not fired stright up.

Click to expand...

The bullet would tip over on its fall because of aerodynamics. The bullet would stop its forward progress so it would lose the stabilizing effect of the spin. It would destabilize on the reward fall resulting in a spinning tumble. As far as tipping after impact, the bullets penetrated the ground a couple of inches. If they had been vertical on impact they would have made round holes. If they hadhit, then tipped over before penetration, they would have stopped on the surface. The dry lake bed that they are on is very hard packed. The bullets would not be able to hit then sink into the surface after tipping over.

How do you figure that they were not fired straight up, they were on arig that had been checked for plumb. They were pointing straight up.

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Gordie wrote:

Hawkflyer wrote:

There is no reason that the bullet would stop spinning just because it has stopped climbing. In fact it would not stop spinning and therefore would begin its return to earth butt end first. The tumbling they noted resulted from tipping on impact, combined with the fact that they were not fired stright up.

Click to expand...

The bullet would tip over on its fall because of aerodynamics. The bullet would stop its forward progress so it would lose the stabilizing effect of the spin. It would destabilize on the reward fall resulting in a spinning tumble. As far as tipping after impact, the bullets penetrated the ground a couple of inches. If they had been vertical on impact they would have made round holes. If they hadhit, then tipped over before penetration, they would have stopped on the surface. The dry lake bed that they are on is very hard packed. The bullets would not be able to hit then sink into the surface after tipping over.

How do you figure that they were not fired straight up, they were on arig that had been checked for plumb. They were pointing straight up.

Click to expand...

You don't need forward progress for a spin to stabilize the bullet. Aerodynamics play a certain amount into what direction a bullet is pointing (just as a thrown football's orientation "arcs" with the arc of the throw), but if it's fired straight up, or even close to straight up, I believe it's going to come back down butt-first. It's not going to start tumbling while it's still spinning at 100's of thousands of RPM, when all that's acting upon it is air.

Look at a gyropscope. It will continue to hold the same orientation no matter which way you move it, and resists any attempt to change it's orientation. As long as it's spinning, it's not going to just start tumbling unless it actually hits something.

...Boy, this "armchair physics" stuff is fun ... Is there a real physicist in the house?...
...Orygunner...

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Orygunner wrote:

Gordie wrote:

Hawkflyer wrote:

There is no reason that the bullet would stop spinning just because it has stopped climbing. In fact it would not stop spinning and therefore would begin its return to earth butt end first. The tumbling they noted resulted from tipping on impact, combined with the fact that they were not fired straight up.

Click to expand...

The bullet would tip over on its fall because of aerodynamics. The bullet would stop its forward progress so it would lose the stabilizing effect of the spin. It would destabilize on the reward fall resulting in a spinning tumble. As far as tipping after impact, the bullets penetrated the ground a couple of inches. If they had been vertical on impact they would have made round holes. If they hadhit, then tipped over before penetration, they would have stopped on the surface. The dry lake bed that they are on is very hard packed. The bullets would not be able to hit then sink into the surface after tipping over.

How do you figure that they were not fired straight up, they were on arig that had been checked for plumb. They were pointing straight up.

Click to expand...

You don't need forward progress for a spin to stabilize the bullet. Aerodynamics play a certain amount into what direction a bullet is pointing (just as a thrown football's orientation "arcs" with the arc of the throw), but if it's fired straight up, or even close to straight up, I believe it's going to come back down butt-first. It's not going to start tumbling while it's still spinning at 100's of thousands of RPM, when all that's acting upon it is air.

Look at a gyroscope. It will continue to hold the same orientation no matter which way you move it, and resists any attempt to change it's orientation. As long as it's spinning, it's not going to just start tumbling unless it actually hits something.

...Boy, this "armchair physics" stuff is fun ... Is there a real physicist in the house?...
...Orygunner...

Click to expand...

Orygunner is correct. The spin creates a gyroscope. And with almost no resistance working to slow the spin the bullet will be spinning quite fast. the gyroscopic effect will be the major force acting on the bullet until it picks up a few hundred FPS of velocity on the return fall to earth. Even then it is likely to remain pointed butt end first as it falls. That gyro effect is very strong.

I know the bullets were not fired straight up in the testing done by Mythbusters for three reasons.

1) I actually watched the show.

2) They said that they could only get CLOSE to straight up and even that was difficult.

3) The bullet did not hit very near the launch point. While one would expect some drift, lets face it, straight up would produce a point of return very close to the point of launch.

Because the bullets were not fired at a perfect perpendicular and because a gyroscope can precess (look it up), the bullets hit the ground at an angle. AS the contact end of the bullet hit, it met the resistance of the ground causing the aft end to fall over making a keyhole. These bullets were NOT tumbling as they fell.

Any of you guys that have ever hear a bullet tumbling can chime in at any time. At 300-400 FPS you can hear a bullet tumbling.The sound made during aricochet is in part caused by the bullet tumbling. The mythbusters reported no such sounds.

Regards

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Hawkflyer wrote:

Orygunner is correct. The spin creates a gyroscope. And with almost no resistance working to slow the spin the bullet will be spinning quite fast. the gyroscopic effect will be the major force acting on the bullet until it picks up a few hundred FPS of velocity on the return fall to earth. Even then it is likely to remain pointed butt end first as it falls. That gyro effect is very strong.

I know the bullets were not fired straight up in the testing done by Mythbusters for three reasons.

1) I actually watched the show.

2) They said that they could only get CLOSE to straight up and even that was difficult.

3) The bullet did not hit very near the launch point. While one would expect some drift, lets face it, straight up would produce a point of return very close to the point of launch.

Because the bullets were not fired at a perfect perpendicular and because a gyroscope can precess (look it up), the bullets hit the ground at an angle. AS the contact end of the bullet hit, it met the resistance of the ground causing the aft end to fall over making a keyhole. These bullets were NOT tumbling as they fell.

Any of you guys that have ever hear a bullet tumbling can chime in at any time. At 300-400 FPS you can hear a bullet tumbling.The sound made during aricochet is in part caused by the bullet tumbling. The mythbusters reported no such sounds.

Regards

Click to expand...

I too watched the show and I agree with your report and conclusions.

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Hawkflyer wrote:

SNIP The spin creates a gyroscope...the gyroscopic effect will be...FPS of velocity on the return fall to earth...That gyro effect is very strong.

Because...a perfect perpendicular and...a gyroscope can precess...the bullets hit the ground at an angle.

Click to expand...

Zzzzzz. skynxxxxx. zzzzzzzz.

Say what?

PlainEnglish, please.

Next your gonna tell methat somehow involved the former vice-president's ability to stay with the timing while dancing.

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I watch Mythbusters regularly and find it for the most part entertainment. As a science show it is far from factual and sometimes quite comical. You have to remember that it is based on their special effects backgrounds and not of a scientific genre. If they would quit trying to bust the 007 or Dukes of Hazzard gadgets and stunts it would be more believable but about the only times I agree with their conclusions are when the prove a myth to be believable.