Rifling
From Free net encyclopedia
Rifling refers to spiral grooves that have been formed into the barrel of a firearm. It is the means by which a firearm imparts a spin to a projectile to gyroscopically stabilize it to improve accuracy. Most rifling is created by either cutting with a machine tool, pressed by a tool called a "button" or forged into the barrel over a "mandrel". The grooves are the spaces that are cut out, and the resulting ridges are called 'lands'. These lands and grooves can vary in number, depth, shape, direction of twist ('right' or 'left'), and 'twist rate' (turns per unit of barrel length). The spin imparted by rifling significantly improves the stability of the projectile, improving both range and accuracy.
Typically in small firearms, the diameter of the bullet matches the diameter of the circle that encompasses the bottoms of the rifled grooves. When the cartridge is fired, the bullet is forced into the barrel and the rifling engages the bullet, deforming it somewhat. As the bullet is propelled down the barrel, it begins to spin. This rate of spin is dictated by a bullet's muzzle velocity and the twist rate of the rifling. For a given caliber, faster rates of twist are needed to stabilize longer (heavier) bullets.
Smooth-bore firearms muskets require bullets that are the same diameter or less than the diameter of the bore. The energy from the discharge upsets or obdurates the bullet and expands it to form a tight seal in the musket. Rifling allows tight-fitting over-bore bullets to be squeezed into the barrel. This allows for higher pressures and longer range for rifles.
A perfectly formed bullet would have the axial center of its rotating mass coincide with the axial center of its form. To the extent that these two axes differ, the flight path of the spinning bullet takes the form of an expanding cone due to gyroscopic forces. As the twist rate increases, so does the size of the "cone of inaccuracy". Therefore, increasing the twist rate beyond that required to stabilize the bullet is counterproductive.
History
The history of rifling a barrel is covered in depth in the Wikipedia article Rifle.
Recent developments
The grooves most commonly used in modern rifling have fairly sharp edges. More recently, polygonal rifling has become popular, as it seems to produce better accuracy due to the fact that it does not damage the bullet as badly as conventional rifling. Polygonal barrels also tend to have longer service lives because the reduction of the sharp edges of the land reduces flame erosion. Higher velocities may be generated due to a reduction of friction and an improvement of the gas seal between the bullet and barrel. A disadvantage of polygonal rifling is that if simple lead bullets are used, lead from the bullet tends to accumulate in the barrel (called leading) resulting in a dirty barrel, poor accuracy, and if the leading becomes severe, excessive chamber pressure which could cause a barrel or locking failure. Polygonal rifling is currently seen on most pistols from Heckler & Koch, GLOCK and Kahr Arms.
In Culture
The intro to the movies of James Bond usually contains the current actor of James to walk across the screen, while the audience views from the inside of a rifled barrel.de:Drall ja:ライフル ko:강선 pl:Lufa gwintowana zh:來福線