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Old April 14, 2010, 12:28 PM   #26
ScottRiqui
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Another important consideration that I don't know the answer to is "how much spin does a round projectile have when fired from a smoothbore, and what direction is the spin?"

If a ball fired from a smooth bore has some amount of random spin just from imperfections in the ball/bore/etcetera, then that might cause problems from the Magnus effect or similar aerodynamic effects.

Spinning the ball around an axis parallel to the direction of flight (like the spin imparted from a rifled barrel) won't cause a Magnus effect unless there's also a crosswind, but a ball spinning about either of the other two axes (pitching or yawing) will cause a Magnus effect even without a crosswind. For instance, if the ball has a backspin, it will experience an upward force as it travels downrange. If it has a topspin, the force will be downward.

It may be that the benefit of rifling with a spherical ball comes not from a stabilizing effect of the rolling spin, but from the fact that it gives the ball a predictable, repeatable spin, rather than a potentially random spin each time the rifle is fired.
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Old April 14, 2010, 12:32 PM   #27
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What you say makes sense but I don't think a smoothbore has random spin from shot to shot. If they did they wouldn't be as accurate as they are within their given range.

Last edited by Hawg; April 14, 2010 at 12:50 PM.
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Old April 14, 2010, 12:46 PM   #28
ScottRiqui
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Entirely possible. In any case, I don't think it will ever be possible to experimentally answer most of my questions, because it's impossible to impart a spin on a perfectly round projectile without deforming it (since it has to deform to engage the rifling grooves.)

I got to thinking about this mostly for two reasons:

1) I see the simplified version of the Greenhill formula trotted out here all the time, but I suspect that if you look at how Greenhill actually derived the full version of his formula, he's assuming the projectile is cylindrical (longer than it is wide.) Once I get my hands on that article I found, I'm going to see if that's the case, and if it is, I'm going to re-derive the formula using the moment of inertia equations for a sphere rather than a cylinder. This may cause some terms to cancel out entirely and may change the final version of the formula.

2) I don't think that the traditional notions of "stability" apply to round projectiles, since a round projectile's center of mass and center of pressure are always co-located with each other. As such, I dont think they can be "stabilized" or "destabilized" under the traditional definitions.

I'll post any interesting results, but I realize that the simple truth is that even ball projectiles do better from rifled barrels. I'm just trying to figure out if that's because there's some inherent physical benefit to spinning a sphere in flight, or if it's just because the projectiles aren't perfectly spherical when they leave the barrel.

Last edited by ScottRiqui; April 14, 2010 at 12:59 PM.
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Old April 14, 2010, 03:28 PM   #29
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Proof is in the freezer.......



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Old April 14, 2010, 04:03 PM   #30
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Scott, just looking at the way that my revolvers work, the balls are not spherical once they're loaded - and they get even more deformed when they're shot.

That .457" ball goes into a .450" cylinder, then .440" lands. It's not a sphere when it comes out of the muzzle. Oh, it's not elongated like a contemporary bullet, but it's longer than it is wide.

Also, a sphere can behave like a gyroscope. With sufficient angular velocity, the sphere will resist torques along its axis - which tend to keep it moving along a straight path (this is an enormously simplified statement of what's happening, of course).

And, just ruminating here, I'd think that another factor that is required for outside forces to act upon projectile is time. A bullet isn't in the air for all that long before it either strikes a target or the ground.

But I'm no expert on the subject - I work with electricity; I play with bullets!
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Old April 14, 2010, 04:04 PM   #31
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Engaging the rifling

The most salient point yet is that the round ball is going to have to engage the rifling to test the theory, therefore it would deform the roundball.

I was thinking a round ball made of depleted uranium, and lands and grooves machined out of titanium or even diamond edged lands ...

And then I saw that really fine flintlock and thought about all those steaks, and stews and burgers and backstrap ... and raided the refrigerator!

Just a giggle that comes to mind. We print out targets we download from friends over the internet ... at which we shoot smoothbore flintlocks ... that tickles me to no end.
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Old April 14, 2010, 04:21 PM   #32
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If you have a polished tungsten carbide ball accelerated by uniformly compressed air out of a polished barrel into vacuum, I'm pretty sure the stabilized and unstabilized balls perform identically. In a real world scenario you are having a lot of imperfections and non-uniformities to deal with, and you need to average out these effects by rotation.
As for the golf ball dimples, the reason a golf ball performs better with dimples than without (by about a factor of 2 - 3 distance wise) is due to the speed regime. Usually, a sphere will have nice laminar flow around it right up to midpoint, where the laminar flow leaves the surface, and the resulting turbulence generated drag. The little dimples on the golf ball produce a small layer of turbulent flow on the surface (little more drag) but allow the laminar flow to stay close to the ball for most of its length (much less drag). Unfortunately that only works in the 150 - 250 ft/s speed range, too slow for bullets to be useful.
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Old April 14, 2010, 10:56 PM   #33
Hawg
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Quote:
The most salient point yet is that the round ball is going to have to engage the rifling to test the theory, therefore it would deform the roundball.
Not with a patched ball out of a rifle.
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Old April 15, 2010, 04:27 PM   #34
CajunPowder
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True

Quote:
Originally Posted by Hawg Haggen
Not with a patched ball out of a rifle.
So I'll need a kevlar patch for my DU or TC round ball ... hmmmmm
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Old April 15, 2010, 05:55 PM   #35
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I use Teflon myself
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Old April 15, 2010, 07:44 PM   #36
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I use pillow ticking. And so do, I believe, the vast majority of long gun shooters, rifled or smoothbore.
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Old April 15, 2010, 11:01 PM   #37
Hawg
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Quote:
I use pillow ticking. And so do, I believe, the vast majority of long gun shooters, rifled or smoothbore.
Likewise
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