Rifling, by definition, sets modern rifles apart. It was a pivotal evolution. By applying a spin, projectiles conserved energy, achieving greater distances with increased accuracy.

Caveat: I’m not writing from a position of authority. I picked up a new hobby, and writing short articles is a way to document my learning, and encourage further study.

An iconic feature of James Bond movies arrives at the end of the opening song, when his figure walks across, then turns and fires on a would-be assassin. The viewer is looking down the rifled barrel of a firearm.

Rifling of a cannon (M75; 90mm; y.1891; Austro-Hungarian; exposed in Ljubljana, Slovenia).jpg
By Petar Milošević - Own work, CC BY-SA 4.0, Link

Barrels were originally smoothbore, simply a metal cylinder, without any rifling. Spherical bullets lacked sufficient engineering, and would bounce down the barrel when fired, and tumble through the air with haphazard accuracy. The history of golf also teaches us that a smooth, spherical object travels in odd ways when flying through the air.

Rifled barrels have helical grooves cut into the bore. The engineering precision must be incredibly fine, tolerances of only a few thousandths of an inch. But the idea is that when the bullet is pushed up the barrel, it engages the rifling and a spin is applied down the long axis of the projectile. This spinning stabilises the bullet in flight, using concepts similar to how a spinning top stabilises.

Polygonal vs normal rifling.svg
By Pbroks13 - Own work based on: Polygonal vs normal rifling.gif and Polygonal vs normal rifling.gif, Public Domain, Link

There are a few variations of rifling. The most common is shallow, square-cut grooves (diagram left). The grooves may also be angle-cut, making it a little easier to maintain a clean barrel. There is also polygonal rifling (diagram right), where the cross section of a barrel looks more like a round-edged polygon.

The number, size, and depth of grooves varies, usually depending on the engineering process used, but there are formulas to determine the optimum twist rate for a projectile. A common twist rate for a commercial rifle is 1 in 12, meaning for every 12 inches of barrel length, the rifling completes 1 full revolution. Problems may appear if the twist isn’t quite right. Too low and the bullet isn’t sufficiently stabilised, and may begin to tumble in flight, leaving a keyhole shape in the target (instead of a nice clean circle). Too high and excessive wear and friction may cause the bullet jacket to be ruptured as it travels down the barrel, and potentially fragment in flight.

There is an interesting twist alternative, called gain-twist. The rate of twist increases down the barrel. So the rate of twist is initially lower, making it easier for the bullet to pick up the rifling. Then the rate of twist slowly increases until the bullet is sufficiently stabilised. Gain-twist has several advantages, but it is harder to engineer, and therefore more expensive, making it rarer.