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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).
Before the barrel can release the bullet in a consistent manner, it must grip the bullet in a consistent manner. The part of the barrel between where the bullet exits the cartridge, and engages the rifling, is called the "throat", and the length of the throat is the freebore. In some firearms, the freebore is zero as the act of chambering the ...
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).
SAAMI recommends a barrel with a 6 groove contour and a twist rate of one revolution every 9 in (229 mm) with each groove having a width of .095 in (2.41 mm). The recommended bore diameter is .256 in (6.50 mm) and the groove diameter is .264 in (6.71 mm). SAAMI recommends that rifling commence at .221 in (5.61 mm) from the chamber mouth.
To obtain maximum accuracy, bullet weight and form are matched to the rifling twist rate of the barrel. Typically, 68 gr (4.41 g) bullets are used in barrels with twist rates of 1 in 13 inch (1 in 330 mm), while 1 in 15 inch (1 in 380 mm) barrel twists can accommodate lighter 58 or 60 gr (3.76 or 3.89 g) accurately.
SAAMI and the CIP recommend a 6-groove barrel with a twist rate of one revolution in 10 in (250 mm) of barrel length with a bore Ø of .330 in (8.4 mm) and a groove diameter of Ø of .338 in (8.6 mm), with each groove having a width of .110 in (2.8 mm). The SAAMI-recommended average pressure for the cartridge is 64,000 psi (4,400 bar).
Rifling of a 105 mm Royal Ordnance L7 tank gun Conventional rifling of a 90 mm M75 cannon (production year 1891, Austria-Hungary) Rifling in a GAU-8 autocannon. Rifling is the term for helical grooves machined into the internal surface of a firearms's barrel for imparting a spin to a projectile to improve its aerodynamic stability and accuracy.