Rifled barrels for spherical projectiles?

S

ScottRiqui

New member
Not sure if this is the correct forum, but I thought my "blackpowder brethren" might be the right people to ask:

Why would you use a rifled barrel to fire spherical bullets? I was looking at the Wikipedia article on rifling, and they had a picture of an old cannon that used round cannonballs with little "winglets" on it that were meant to engage rifling grooves in the cannon bore to make the ball spin in flight.

I can understand the need for rifling with pointed bullets, because imparting a spin on the bullet helps prevent tumbling and keeps the pointy end of the bullet facing forward. But round balls don't have a "front" or a "back", so I'm not sure why it would be necessary to have them spin during flight.

What am I missing?

Thanks,
Scott
 
Increased range and greater accuracy. A smooth bore is fairly accurate to 75 yards or so. Someone who really knows his rifle might extend it to 100. A rifled barrel is accurate well past 100 yards.
 
Thanks, although I'm still at a loss as to what exactly rifling does for a spherical bullet, since a round ball can't "tumble" the way a pointed bullet can. The aerodynamics would seem to be the same whether a ball is spinning during flight or not.

Is the ball deformed as it travels down the barrel so that it's not perfectly round when it leaves the muzzle? I can see where a spin would be helpful if the bullet isn't perfectly round during flight.
 
A round ball is patched. The patch grips the rifling and imparts spin to the ball. A round ball out of a smoothbore is much like a bb. It tends to curve on it's way to the target.
 
A round ball out of a non rifled barrel performs much like a knuckle ball in baseball. Anyone's guess where it will be at any given range.
 
Just because a projectile is round doesn't mean that it doesn't need to be stabilized.
But because a round ball is shorter in length than a conical bullet of the same caliber, it simply requires a slower rate of twist to be stabilized.
Look up the Green Hill formula and it will help you to understand that stabilizing a projectile is not about its shape as much as it's about its length.
 
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A round ball out of a non rifled barrel performs much like a knuckle ball in baseball. Anyone's guess where it will be at any given range.
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That's not entirely true. Some of these guys that shoot smoothies exclusively can turn in some amazing groups.
 
obturation - swaging

I've learned that many prefer to ram a round ball into the chamber of a revolver so that it leaves a ring of lead shaved off of the ball.

In order to do this one has to put a ball in the cylinder chamber that is a few thousand's of an inch to large for the chamber.

When you do this it changes the shape of the ball however minutely. As the ball is no longer "perfectly round", (and it never was), it can benefit from being spun as it engages the rifling.

If the boolit is to engage the rifling, then it must be larger than the "inner" barrel diameter described by the surfaces of the lands. So in order to engage the grooves the shape of the round ball must once again be "swaged" to fit through the barrel.

Ideally, ball continues down the barrel and is tight as tick in the barrel so that the force behind it, the explosion, does not leak out around the sides.

So there is some squeezing, or swaging, of the soft round lead ball in the ramming and loading phase as well as the firing phase.

And all of the previous posters who have told of the need to impart spin to even a perfectly round object are purely correct.

When we take into account the shaving and swaging of a ball every bit of help stabilizing that "round ball" helps.

Also, there is a big, huge explosion happening on one end of that ball and that does tend to obturate the back of the ball and now I seem to remember some very intense discussions and threads I have read on this very subject.

Rifling is good, and some can shoot smoothbore with fascinating accuracy.
 
There's no such thing as a perfect sphere in lead roundballs. One side may have a dent or a wrinkle that the other side does not have. By spinning the ball, the effect of these imperfections average out as the ball flies through the air.
The knuckleball pitch analogy is a good one. If a baseball didn't have stitching and was a perfectly smooth sphere, then throwing it without spin wouldn't cause the erratic flight that a knuckleball is known for.

Some smoothbore shooters have experimented with dimpling their roundballs similar to a golf ball by rolling them between two course rasps and have claimed good results.

If a non spherical projectile has its center of mass ahead of its aerodynamic center of drag, it will fly pointed end first even without rifling. Examples of such projectiles include arrows, darts, badminton shuttlecocks, hollow base air rifle pellets, shotgun slugs, and some Minié ball designs. Yet they are also more accurate if they spin a little.
 
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Spinning

I remember when I used to drink it imparted significant spin to ... me.

And I would hit the floor with great accuracy. That's not to say I am a round ball, but I certainly fight that tendency.

:eek:
 
Someone suggested looking at the Greenhill formula. I've done that, and part of the problem is that the typical form of the Greenhill formula that's used is a simplified version of his complete formula (which I haven't found yet), and the constant 'C' in the formula (usually defined as 150 or 180, depending on projectile speed) hides a lot of assumptions and approximations.

I've found an article from the November issue of the International Journal of Impact Engineering that contains the full derivation of Greenhill's original formula and not just the final simplified version, but it will take a few days to get the article in-hand. In particular, I think that the "moment of inertia" parts of the calculations assume a cylindrical projectile (longer than it is wide). If you were to replace those equations with the equations for a sphere (which has the same moment of inertia about all three major axes), I think the effect of rifling will diminish greatly, if not cancel out altogether.

I'll know more once I've looked at the full derivation of the Greenhill formula and dug back into my fluid mechanics textbooks, but I think that the only reason spherical projectiles benefit from rifled barrels is that a) they're not really spherical, and b) they're not perfectly smooth. I believe that if a projectile is in fact nearly round, and uniformly smooth over its surface, its exterior ballistics are not going to be affected by whether or not it's spinning during flight.
 
Don't forget mass distribution as well. If there are even small voids or impurities the center of mass won't be co-incident with the geometric center. Spin will add some gyroscopic stabilization to reduce the effects.
 
The flight of a perfect sphere is stabilized by rotating about an axis aligned with the direction of flight the same way a gyroscope is stabilized in the plane containing the axis of rotation. Gyroscopic stabilization occurs as a result of the angular momentum of the projectile, regardless of it's aerodynamic properties. The amount of stabilization does vary with the inertia of the projectile about the axis of rotation since that inertia affects the angular momentum, but it is present whenever there is rotation, even in a perfect sphere.
 
Correct, but why would it matter if a sphere were gyroscopically stabilized or not?

With a pointed bullet, any pitching or yawing means that the bullet will quickly begin to tumble and the aerodynamic drag will go to hell as the frontal surface area rapidly changes size and shape. With a perfectly round ball, both the frontal area size and shape always remain unchanged, no matter if the ball pitches, yaws, or rolls. So, I don't see how the exterior ballistics of a stabilized sphere would differ from an unstabilized sphere.
 
No ball that comes out the muzzle is round. While we can load a round ball,
we do not shoot round balls. All the round balls I shoot in rifle and pistol,
come out a enlogated bullet. They must be spun to stablize for accuracy.
This has been known for hundreds of years.
 
From The Complete Blackpowder Handbook: The Latest Guns and Gear
By Sam Fadala:

Forward velocity is killed by the atmosphere. Gravity plays a role in bullet flight, but it isn't nearly as responsible as the atmosphere for slowing forward progress. Rotational velocity, on the other hand, is much better retained........
........We also recognize by understanding rotational spin retention why the round ball continues to maintain its original attitude in flight to a reasonable distance. Losing 50 per cent of its original velocity within only 100 yards is common.


http://books.google.com/books?id=Dz...&q=stabilizing a round ball in flight&f=false
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The Magnus effect is the phenomenon whereby a spinning object flying in a fluid creates a whirlpool of fluid around itself, and experiences a force perpendicular to the line of motion. The overall behaviour is similar to that around an aerofoil (see lift force) with a circulation which is generated by the mechanical rotation, rather than by aerofoil action.

http://en.wikipedia.org/wiki/Magnus_effect

In external ballistics

The critical factor is the location of the centre of pressure, which depends on the flowfield structure, which in turn depends mainly on the bullet's speed (supersonic or subsonic), but also the shape, air density and surface features. If the centre of pressure is ahead of the centre of gravity, the effect is destabilizing; if the centre of pressure is behind the centre of gravity, the effect is stabilizing.
Click to expand...
 
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The Magnus effect isn't a significant player in bullet ballistics unless there is also a crosswind (this is discussed further down in the Wikipedia article you referenced.) In fact, this would be an argument *against* using rifled barrels, since eliminating the bullet spin would negate the Magnus effect of any crosswind present.

You're correct about the location of the center of pressure versus the center of gravity and its effect on stability, but both the center of pressure and center of gravity are independent of spin *for a perfectly spherical projectile*.

Also, while forward velocity drops off faster than rotational velocity, that's not really relevant - that's just because forward velocity is affected by drag from the relatively-large frontal area of the bullet, while rotational drag just comes from the interaction of the thin boundary layer with the atmosphere.

I'll wait until I've gotten a copy of the article I mentioned earlier and worked through the derivations for the full Greenhill formula, but the more I look into it, I think the only reason that spherical projectiles benefit from rifling is that as an earlier poster noted, the projectiles aren't really round (or smooth).
 
Factory swagged round balls are relatively perfectly made though.
And when wrapped with a patch they are cushioned as well.
They don't seem to get upset much at all during ignition so any imperfection may actually be negligible. Although if they do indeed become upset and that's the case then there's no sense arguing whether a perfectly round ball can be fired from a gun barrel accurately without being deformed at all unless it's a low velocity airgun and then what's the point? So then rifling would be required for best performance as current practices suggest and the question is very hypothetical.
What if a perfect lead round ball had dimples on it like a golf ball that would allow it to fly with much less air turbulence? Then it would be the smooth surface of the ball that would better define its imperfection and not that its shape was being out of round.
However I really believe that it's more than merely the imperfection of its roundness that allows the rifling to better stabilize it over a longer distance than if it were a more perfect ball being fired through a smooth bore.
It could be argued that there's really no such thing as a perfectly calm day without any crosswind.
It's the rotational stabilization of the round ball due to rifling that should help to negate the Magnus effect and the wind.
If someone really wanted to try to answer the OP, why not just shoot some very perfectly round hard steel or hard lead alloy balls and see how much better they shoot out of a smooth bore than "imperfect" pure lead round balls, to try to better maximize the theoretical accuracy of round balls without the use of any rifling?
If they shoot better without rifling then maybe there's a discovery to be made, or maybe there isn't.
 
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So then rifling would be required for best performance as current practices suggest and the question is very hypothetical.
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Certainly, there is no doubt that round balls benefit from rifled barrels, as empirical evidence and experience shows. So you're correct - my question is absolutely hypothetical. Specifically, whether there's a physical phenomena that explains the rifling benefit for a perfectly round and smooth projectile, or if the benefits only exist when the projectile *isn't* perfectly round and smooth.

I can give you the physical/scientific reasons why spinning a *conical* bullet stabilizes it and helps prevent it from pitching and yawing, but unlike a conical projectile, if a spherical bullet pitches or yaws, this doesn't change its frontal shape, frontal area, center of mass or center of pressure. You can't just handwave it away saying that imparting a spin stabilizes the bullet, because that doesn't explain *how* it actually stabilizes it. The arguments and conventional wisdom that work for conical bullets don't necessarily work for spherical bullets.

And the spinning doesn't negate the Magnus effect - it *causes* the Magnus effect. With a spinning bullet and a crosswind, the Magnus effect causes the bullet to fly high or low, depending on the direction of spin and the direction of the crosswind. If you were to remove the spin (or the crosswind), the Magnus effect would disappear completely.
 
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