Quote:
Rifling is the helix-shaped pattern in the barrel of a gun or firearm, which imparts a spin to a projectile around its long axis. This spin serves to gyroscopically stabilize the projectile, improving its aerodynamic stability and accuracy.
Rifling is described by its twist rate, which indicates the distance the bullet must travel to complete one full revolution, such as "1 turn in 10 inches" (1:10 inches), or "1 turn in 30 cm" (1:30 cm). A shorter distance indicates a "faster" twist, meaning that for a given velocity the projectile will be rotating at a higher spin rate.
A combination of the weight, length and shape of a projectile determines the twist rate needed to stabilize it – barrels intended for short, large-diameter projectiles like spherical lead balls require a very low twist rate, such as 1 turn in 48 inches (122 cm). Barrels intended for long, small-diameter bullets, such as the ultra-low-drag, 80-grain 0.224 inch bullets (5.2 g, 5.56 mm), use twist rates of 1 turn in 8 inches (20 cm) or faster.
In some cases, rifling will have twist rates that increases down the length of the barrel, called a gain twist; a twist rate that decreases from breech to muzzle is undesirable, as it cannot reliably stabilize the bullet as it travels down the bore. Extremely long projectiles such as flechettes may require impractically high twist rates; these projectiles must be inherently stable, and are often fired from a smoothbore barrel.
OK....
First off...if you're going to quote someone or some place where data is written, you should include " " to indicate the thoughts and words are not your own. Instead they are information that you have "Googled" perhaps.
Your material failed to mention other variables such as altitude, barometric pressure, temperature that also "help" determine twist rate if you want to get into the physics of the issue. They are all important, but projectile length is the major one..
Many of the twist rate determination "tools" - including what you quoted - use the old Greenhill formula or a derivative of it to determine twist rate. The only problem with using it is that it was designed for artillery and is valid only to about 1800 FPS velocity partially due to the constant Greenhill used.
Here's the actual formula - Greenhill was an artillery officer....
Twist = 150 X D2/L
Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant
In essence, the current ways of determining twist are not very refined in most hunting/non-military circles.
In truth, the best way to determine required twist rate for an individual rifle is to shoot various bullets of different length through a rifle barrel. Some will work and some won't irregardless of what the math says should work, and generally, the longer the bullet, a faster twist rate is required to stabilize that bullet.. All the rest of the variables are just added "interference" at best in our practical hunting world.
-BCB