THE MINIÉ BALL

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Martini-Henry. Lubricated minié balls.

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The percussion cap was a dramatic step forward, but within a short time after its development an even greater invention made the musket into a much more deadly weapon with greater range and accuracy, and it doomed the smoothbore musket forever.

The new development began in 1823 in India when a British officer, Captain John Norton, noticed something strange. The Indian natives used a tube for projecting darts at their enemies, and when they got ready to fire, they began by blowing into the barrel. He discovered that they were doing this to create a foam that would fill the barrel and effectively seal it, so that when the dart was shot, the force on it would be much greater.

In 1836 a London gunsmith improved on Norton’s idea by inserting a wooden plug in the base of the bullet so it would expand when shot. This helped, but the real advance came when a French army captain, Claude Minié, improved the design using a hollow cylindrical base. The bullet was now cone-shaped, similar to our modern bullets. So, even though it was called a Minié ball, it was not shaped like a ball. At first the Minié ball had a round cup in the base, and when the powder exploded the cup forced the lead outward to fill the barrel. What was particularly important about this was that the bullet was now fitting snugly into any rifled grooves that were in the barrel.4

Spiraling rifled grooves had been used for years, but for a snug fit, which was required, the bullet had to be slightly larger than the interior of the barrel, and it had to be pounded down to a position just above the powder, and this was a slow process. The Minié ball, on the other hand, could just be dropped into the barrel, and this was much faster. And as the Minié bullet caught the grooves as it exited it was forced into a spin, and as a result, it left the barrel with a very high spin rate.

To see why a spinning bullet was so revolutionary, we have to look at the physics of a spinning object. When an object of any type rotates, it rotates around an axis, and this axis of rotation acquires a special status. In the case of a bullet in flight (shot from a rifle) there are two motions we have to consider: its translational motion (that gives it its trajectory) and its rotational motion. It has both at the same time, in the same way a curving baseball does. A pitcher purposely gives a baseball a spin to curve its path so that it is more difficult for a batter to hit.

How do we deal with a spinning object? First of all, it’s easy to see that it spins about an imaginary line called its rotational axis, and we refer to its spin rate as its angular speed (or angular velocity, for a particular direction). Speed of rotation is usually measured as so many revolutions per minute (rpm). Scientists also use another unit, which is particularly convenient in physics. To define it we first have to define what is called the radian; it is 360°/2π, which is approximately 57°. The unit, radians per second, is commonly used in physics.

So what does it take to set an object in rotational motion—in other words, to make it spin? It obviously takes a force. This takes us back to the concept of inertia. Remember that according to Newton’s first law, an object in motion remains in uniform motion with a constant speed in a straight line unless acted upon by a force. In short, a body in motion has inertia, and it takes a force to overcome this inertia. Inertia is therefore a kind of “unwillingness” to change. In the same way, a spinning body has rotational inertia, and it prefers to maintain this inertia. In effect, it takes a force to change it. In the case above, however, we are dealing with a rotational motion, so the force is a rotational force, and we refer this force as torque. (You apply torque every time you turn a doorknob or open a jar.)

If we look at a spinning disk, however, it’s easy to see that the “linear speed” (e.g., feet per second) across the disk varies. The speed at a point near the edge is obviously greater than the speed at a point near the center. This means that for a spinning object the speed at various points throughout the object increases as the distance from the spin axis increases. Because of this, ordinary (or linear) force f, and rotational force, or torque, which we denote by τ, are related. This can be expressed as τ = f × r.

Getting back to rotational inertia, it’s easy to show that a spinning object prefers to maintain a spin in a particular direction. Assume you have a bicycle wheel with a handle on its axis so that you can hold on to it with your hands. If you set the wheel spinning, then try to twist it, you will find that it’s very difficult to turn. In short, the wheel wants to keep spinning in the same direction. This means that a bullet spinning around an axis along its elongated shape, and traveling in a certain direction, prefers to maintain this direction. Spin therefore “stabilizes” a bullet in flight. As it turns out, it also decreases the effect the air around it has on it (i.e., air resistance). Because of this, the Minié ball was much more accurate and had a greater range.

It’s important to note that applying torque to a nonrotating object gives it an angular acceleration, where units of angular acceleration are radians/sec2. And again, the relationship between linear and angular acceleration is given by the formula α = a/r, where α is angular acceleration and a is linear acceleration. Finally, in the same way that we have linear momentum, we also have angular momentum, and the conservation principle: the total angular momentum of an isolated system remains constant.

With a rifle that has four to eight spiral turns down the interior of its barrel, a Minié bullet will exit with a spin of up to twenty thousand revolutions per second, which gives it tremendous stability compared to the nonspinning spherical ball used in muskets.

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