For ballistics nerds, hard to categorize, bullets colliding

[darthgently]

28 minutes ago, Codraroll said:

As far as I can tell, those balls were mostly made of iron

???

[mikegarrison]

On 6/14/2023 at 11:23 AM, JoeSchmuckatelli said:

That was cool. 

 

@magnemoe - good point about the rifling, they were clearly counterspinning and this could certainly be a factor... Except for one thing.  I went back to the video and it's a Minnie ball from 1862.  Meaning it was fired from a muzzle loading rifle. 

During the Civil War (1861-65), the basic firearm carried by both Union and Confederate troops was the rifle-musket and the Minié ball. The federal armory in Springfield, Massachusetts, produced a particularly effective rifle-musket that had a range of around 250 yards; some 2 million Springfield rifles were produced during the war

So the Smithsonian rounds would have been fired by rifled barrels, spin stabilized and thus something else has to be the key. 

I suspect they may be correct about the metal composition and velocity.  They're trying to adjust for speed with powder (the part about the long vs short cartridge) and I'm surprised they did not consider wadding in the longer cartridge to maintain a good seat (the inconsistent performance with the short cartridge is actually a sign of them creating a dangerous condition using an incorrect sized cartridge for the gun). 

As to the metal composition?  I don't know.  They used to use scrap metal back in the day, and varying degrees of alloy resulted. Tin and antimony are mentioned in the literature - but finding anything about the ductility / cold welding property of 1860s lead is beyond my Google - fu

The whole reason for the Minie ball was to allow it to be used in rifled muzzleloaders. The "ball" (not actually a sphere) is slightly smaller than the rifling so it loads as easily as a smoothbore muzzleloader, but when fired the base of the "ball" expands enough to engage the rifling. Note that this means the lead has to be fairly plastic, because it is designed to squash out to engage the rifling without fracturing the ball. That probably is why it was plastic enough to stick together in an inelastic collision.

Edited June 18, 2023 by mikegarrison

[farmerben]

5 hours ago, TheSaint said:

. A staggering improbability, but there aren't a whole lot of other ways to explain it.

I tend to believe they stuck together in mid-air but there are a couple other possibilities.  It could have happened inside a piece of wood and the shots need not even be simultaneous.  And, it could be faked.

[TheSaint]

4 hours ago, JoeSchmuckatelli said:

Law of large numbers - or some other way of saying monkeys and typewriters. 

I remember watching a BBC interview in the aftermath of the Westgate shopping mall attack in Nigeria. They were talking with an ex-SAS soldier who had responded to the attack and rescued some of the folks trapped in the mall during the shooting. The reporter remarked on the unlikeliness of him being there at the time of the attack, and he just sort of chuckled and said, "The odds of this sort of thing happening to you are extremely slim. Until it happens to you."

2 hours ago, Codraroll said:

@TheSaint made a great reply already, but let's do the math anyway.

So a Minié ball weighs approximately 1.14 ounces, that's 32.3 grams or 0.0323 kg. It flies at approximately 1200 ft/sec or 400 m/s. That gives it a kinetic energy of 2584 Joules.

As far as I can tell, those balls were mostly made of iron, which has a specific heat capacity of 0.451 J/(g*K). That's at room temperature, but we assume it stays somewhat constant. At 32.3 grams, the heat capacity of the ball is 14.57 J/K.  We can assume all 2584 J of kinetic energy are converted to heat. That ought to be enough to heat the ball by 177 degrees K. Of course, that assumes uniform heating across the entire ball, and the impact happens at such a short time span that that won't happen. If the impact only heats, say, a tenth of the ball, that tenth would briefly be heated by 1770 K. Then its temperature would go from battlefield temperature (~300 K) to the melting point (1811 K) in a flash, still with some left over to account for the bullet slowing down before collision or fragments shooting away with some of the kinetic energy. It's likely that spontaneous welding would happen in such an event.

Of course, there's more energy than that involved in the collision between two moving balls, but there's also twice as much mass to heat, so you can just assume symmetry and disregard the other ball entirely.

Thanks, that is way more effort than I would have put in. But I do have to point out that Minie balls were made of lead, not iron. But that makes it even more likely that the energy of the collision would have been sufficient to weld them, since lead melts at a much lower temperature.

[JoeSchmuckatelli]

3 hours ago, Codraroll said:

you can just assume symmetry and disregard the other ball entirely.

When people who can do the numbers in physics write stuff like this...  I get flummoxed.

[Codraroll]

14 hours ago, TheSaint said:

But I do have to point out that Minie balls were made of lead, not iron. But that makes it even more likely that the energy of the collision would have been sufficient to weld them, since lead melts at a much lower temperature.

D'oh, bit of a mistake there. To be fair, it was way past midnight. 2584 J/ (128 J/kgK * 0,032 kg) =630 K. Add the 300 K from the ambient air, and we're way above the melting point (600 K), even if some of the hottest flakes shatter away and disappear with a lot of kinetic and thermal energy.

14 hours ago, JoeSchmuckatelli said:

When people who can do the numbers in physics write stuff like this...  I get flummoxed.

Uh, see the bit above with the time of posting. Still, I don't quite see the problem. If we assume a symmetrical collision, what happens to each ball is exactly half of what happens to both. The energy of two identical balls that stop each other in mid-air, divided by two balls, equals the energy of one ball that stops by itself in mid-air. Disregarding spin, fragmentation, and other such effects, of course.

Edited June 17, 2023 by Codraroll

[farmerben]

I think the large size of Minnie balls makes them smush more than shatter.  

[JoeSchmuckatelli]

1 hour ago, Codraroll said:

Still, I don't quite see the problem

Grin - it's not a you problem, it's a me problem. 

I only got through calculus in college by taking a winter semester-break course on 'business calculus' - and about the only thing I remember from it was something about making the route walked by a postman more efficient. 

[Codraroll]

5 hours ago, JoeSchmuckatelli said:

Grin - it's not a you problem, it's a me problem. 

I only got through calculus in college by taking a winter semester-break course on 'business calculus' - and about the only thing I remember from it was something about making the route walked by a postman more efficient. 

Aha, I was afraid I had made some kind of horribly basic mistake. I read your post along the lines of "You know enough to do the numbers, but then you make this stupid assumption at the end?", and thought I had missed something obvious.

[Shpaget]

Metal doesn't need to reach melting temperature to weld.

https://en.wikipedia.org/wiki/Cold_welding

While this effect is primarily a concern in vacuum, and requires clean surfaces, I don't thing it's too big of a stretch to imagine that such a violent collision would evacuate most of the air at the point of impact, and possibly expose new, clean, material to contact.

[darthgently]

On 6/16/2023 at 8:46 PM, farmerben said:

I tend to believe they stuck together in mid-air but there are a couple other possibilities.  It could have happened inside a piece of wood and the shots need not even be simultaneous.  And, it could be faked.

I think if they'd both struck wood prior they'd have mushroomed more (much more) prior to getting near each other and there would be wood in between them making a weld difficult.  But am guessing

[JoeSchmuckatelli]

3 hours ago, darthgently said:

I think if they'd both struck wood prior they'd have mushroomed more (much more) prior to getting near each other and there would be wood in between them making a weld difficult.  But am guessing

The video suggested that as a possibility - the very end, where he's selling wallets - they shot the aluminum wallet and it looks like they got a weld. 

Entirely possible that the penetration / tunneling evacuated the target material (aluminum in the video, wood, possibly, back in the day) leaving fresh clean lead to weld / cold weld as they met. 

[kerbiloid]

What about piercing a wider bullet with a thinner bullet to make a tunnel in it?

[darthgently]

2 hours ago, kerbiloid said:

What about piercing a wider bullet with a thinner bullet to make a tunnel in it?

I could see a smaller caliber armor piercing tunneling through a big soft lead slug.  Like maybe a 20 caliber-ish steel core something against a 12 gauge 1 oz SG slug

[magnemoe]

11 hours ago, Shpaget said:

Metal doesn't need to reach melting temperature to weld.

https://en.wikipedia.org/wiki/Cold_welding

While this effect is primarily a concern in vacuum, and requires clean surfaces, I don't thing it's too big of a stretch to imagine that such a violent collision would evacuate most of the air at the point of impact, and possibly expose new, clean, material to contact.

Or the opposite spin might generate friction welding who is easy to do with lead, now this require the two bullets to stay together for a short time so the melt cooled. 

[mikegarrison]

6 hours ago, JoeSchmuckatelli said:

The video suggested that as a possibility - the very end, where he's selling wallets - they shot the aluminum wallet and it looks like they got a weld. 

Entirely possible that the penetration / tunneling evacuated the target material (aluminum in the video, wood, possibly, back in the day) leaving fresh clean lead to weld / cold weld as they met. 

I think it's much more simple than that.

He notes early in the video that they considered casting their own bullets but decided to just buy some. Commercial bullets are forged ("swaged") and not cast. Cast lead is softer and more plastic.

His bullets kept fracturing along what look to be grain boundaries. Perhaps hand casting might eliminate a lot of those.

[Codraroll]

On 6/18/2023 at 7:45 PM, kerbiloid said:

What about piercing a wider bullet with a thinner bullet to make a tunnel in it?

I'm not sure if that would even be possible. Any little bullet moving at extremely high speed would make a roughly spherical crater in the big one (which, I believe, has something to do with the spread of the shock wave in the material, which is why impact craters are circular even if meteorites hit at an angle), and if the crater were deep enough to pierce all the way through the big bullet, it would probably just tear the big bullet apart entirely instead of creating a tunnel. 

Then again, if the big bullet was made of some soft and foamy material, I guess it could be done. Nobody said the big bullet had to be a good bullet.

[kerbiloid]

The bullet doesn't need to have a meteorite speed, 2+ km/s.

[monophonic]

On 6/18/2023 at 2:55 AM, Codraroll said:

Aha, I was afraid I had made some kind of horribly basic mistake. I read your post along the lines of "You know enough to do the numbers, but then you make this stupid assumption at the end?", and thought I had missed something obvious.

To be honest, I gave a far more juvenile chuckle at something in that sentence in particular...

20 hours ago, Codraroll said:

I'm not sure if that would even be possible. Any little bullet moving at extremely high speed would make a roughly spherical crater in the big one (which, I believe, has something to do with the spread of the shock wave in the material, which is why impact craters are circular even if meteorites hit at an angle), and if the crater were deep enough to pierce all the way through the big bullet, it would probably just tear the big bullet apart entirely instead of creating a tunnel. 

Then again, if the big bullet was made of some soft and foamy material, I guess it could be done. Nobody said the big bullet had to be a good bullet.

A jacketed bullet might hold itself together better. One example pointing at that is the famous bullet cross from Gallipoli, although that one probably wasn't a midair collision.

https://steemit.com/photography/@dat1729/two-collided-bullets-from-the-battle-of-gallipoli-1915-16