How spiral grooves in a gun barrel made bullets fly straight
A bullet leaving an M4 carbine spins more than 300,000 times a minute. That rotation comes from rifling, the spiral grooves cut inside a barrel, and it works like a gyroscope to keep the projectile nose-first. Old smoothbore muskets had none, so their loose lead balls rattled out in unpredictable directions.
Musket balls were made a loose fit so soldiers could ram them down the muzzle quickly, and they bounced along the bore on the way out. Straight grooves appeared in small arms by 1480, but only as soot channels for powder residue. Spiral grooving came later, with early European attempts credited to Gaspard Kollner of Vienna in 1498 and Augustus Kotter of Nuremberg in 1520. Archers and crossbowmen had already noticed that a spinning projectile flies faster and truer.
Every groove had to be engraved by hand, so rifles stayed expensive and rare until the mid-19th century. Wealthy hunters, who valued accuracy over rapid fire, were the main buyers. Armies disliked them because they were hard to clean and awkward to load, so the earliest rifled black-powder arms that suited soldiers were guns loaded from the breech, the Queen Anne pistol among them.
Gunsmiths describe rifling by its twist rate, the distance the grooves take to make one full turn. Short, fat projectiles need little spin: a round-ball muzzle-loader may use one turn in 48 inches or even slower. Long, slim modern bullets need far more, around one turn in 8 inches. In 1879 George Greenhill, a mathematics professor at the Royal Military Academy in Woolwich, devised a rule of thumb for lead-core bullets based on length and diameter that is still in use. A half-inch bullet 1.5 inches long, by his formula, wants one turn in 25 inches.
Getting it wrong has visible results. Too little twist lets a bullet yaw and tumble, punching elongated keyholes in a target. Too much wears the barrel faster and, at high velocity, can spin a jacketed bullet so hard that it tears itself apart in flight.
Source: Rifling
