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Miller twist rule is a mathematical formula derived by American physical chemist and historian of science Donald G. Miller (1927-2012) to determine the rate of twist to apply to a given bullet to provide optimum stability using a rifled barrel. [1]
SAAMI recommends a six groove barrel with a rate of twist of one revolution in 9 in (230 mm), a bore diameter of .256 in (6.5 mm) and a groove diameter of .264 in (6.7 mm) with each groove having a width of 0.090 in (2.3 mm). The recommended maximum pressure for the cartridge (piezo) is 64,000 psi (4,400 bar).
The recommended optional twist rate is one revolution in 10 in (250 mm). Both SAAMI and Weatherby recommend a freebore of .378 in (9.6 mm). Early .257 Weatherby rifles has a twist rate of 1 revolution in 12 in (300 mm) but all current rifles are manufactured with a twist rate of 1 revolution in 10 in (250 mm).
Other variations of commercially loaded .243 ammunition are available with bullet weights ranging from 55 grains (3.6 g) up to 115 grains (6.8 g). Twist rate of the barrel can sometimes be a factor in deciding which bullets to use, 1:10 being the most popular as it is sufficient to stabilize up to 100 gr bullets.
With heavier bullets the .240 Wby. Mag. makes for a good deer hunting cartridge, but it does tend to require a long (>23 inches (580 mm).) barrel in order to achieve peak performance. Performance for 100 grain bullet from utilizing a factory 26" test barrel and a Nosler Partition bullet)
This allows gas to blow by, and loosens the hold of the bullet on the barrel. An increasing twist, however, will make the rifling become wider than the grooves in the bullet, maintaining the seal. When a rifled-barrel blank is selected for a gun, the higher-twist end is located at the muzzle.
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The .30-378 Weatherby Magnum is a .30 caliber, belted, bottle-necked rifle cartridge. [2] The cartridge was developed in response to a US Army military contract in 1959. While still unreleased to the public, the cartridge went on to set world records for accuracy including the first ten 10X in 1,000 yards (910 m) benchrest shootin