Introduction
In response to a picture I posted online, a friend's adult daughter asked a question that made me realize that my photo collection of antique Somerset and Bedford County, Pennsylvania long rifles might be interesting to people who know little or nothing about antique muzzle loading firearms. This page was written for them, and is intended to answer the question "What is rifling?"
L. Dietle
A description of the problem rifling solves
With a smoothbore musket, the projectile is a lead ball that is slightly smaller that the bore of the barrel. When the musket is fired, the rapid combustion of the black powder propellent produces a rapidly expanding high pressure gas that blows the ball through, and out of, the barrel.
Since the ball is smaller than the bore, the ball may be eccentric with the bore at the instant it exits the muzzle. This produces an eccentric clearance between the ball and the bore that the rapidly expanding gas can easily pass through and deflect the flight of the ball. Furthermore, since the ball is smaller than the bore, it may bounce around within the bore upon firing, and exit the muzzle of the barrel at an angle.
Additionally, when the musket is fired, the ball may roll as it passes through the bore at high velocity. As a result, the ball may spin rapidly on an axis that is oriented roughly 90 degrees from the direction of flight. As a result, half of the forward portion of the ball is spinning toward the direction of flight, and half is spinning away. The forward spinning half has more air resistance than the rearward spinning half. The ball veers in the direction of least air resistance, causing inaccuracy. Because the axis of spinning can be horizontal, vertical, or any angle in-between, the ball may veer in any direction from the intended flight path.
As an example of this effect, when the top of a table tennis ball is spinning rapidly forward, the ball drops much faster in flight, compared to a non-spinning ball. As another example, if the bottom of a table tennis ball is spinning rapidly forward, the ball resists gravity and drops much less in flight, compared to a non-spinning ball.
What is rifling?
The following photo, which was was provided by Dr. Bill Paton, shows both types of rifling. It is a photo of the muzzle end of an 1800s double rifle that was made by the gunmaker Thomas Oldham of Bedford County, Pennsylvania. This double rifle was designed to fire a spherical lead ball from each barrel. Those projectiles were nearly as large as the bore of the barrel, and were used to hunt larger game such as deer. The barrel with straight rifling could also be used to fire small diameter lead shot for hunting small game, such as rabbits and grouse.
When such a rifle is used with lead balls, the ball is wrapped in a greased cloth patch that creates a friction fit with the bore of the rifle, grips the ball, and engages the rifling grooves. Accuracy is improved, compared to a musket, due to several factors. One factor with both spiral and straight rifling is the tight fit between the ball and the bore, which provides improved initial mechanical guidance and prevents eccentric gas blowby at the instant the ball leaves the barrel.
When the barrel with spiral rifling is fired, the ball spins on an axis that is substantially aligned with the direction of flight. Accuracy is improved, compared to a musket, due to gyroscopic stability and due to evenly distributed air resistance on the leading end of the ball. Although straight rifling does not provide gyroscopic stability, the absense of rotation does provide more or less even air resistance on the leading end of the ball in flight.
Straight rifling is more effective with lead shot, compared to spiral rifling, because spiral rifling tends to spray the shot out of the barrel in an ineffectively-large pattern.
Click here for information about how rifling was cut in the old days.
An mid-1800s theory about spiral rifling
This section is a summary of things I have read over the course of a lifetime that seem to ring true regarding the causes of musket inaccuracy.
In regard to muzzle loading firearms, the term "rifling" is used to describe spiral grooves cut into the bore of a barrel, and the term "straight rifling" is used to describe straight grooves cut into the bore of a barrel.
The following article, which is from the June 14, 1851 issue of the "Sunbury American" newspaper, attempts to describe why rifled firearms are more accurate than smoothbore firearms, but fails to mention gyroscopic stability.
