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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]
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.
The bore twist is 1-turn-in-11.25 inches [1:285.75 mm] and the rifling is five radial lands and grooves (5-R) with a right-hand (RH) twist. Because of the odd number of lands, none of the lands are 180° apart, i.e. in direct opposition. This results in less bullet deformation, which (at least in theory) produces more consistent point of impact.
The rest of the barrel was maintained at the original thickness to enable the M203 grenade launcher to be attached. The barrel rifling was revised to a faster 1:7 (178 mm) twist rate to adequately stabilize the new 5.56×45mm NATO SS109/M855 ball and L110/M856 tracer ammunition.
An 18-inch (457 mm) (MOD 0/1) or 16-inch (406 mm) (MOD H) threaded-muzzle match-grade free floating stainless steel heavy barrel with a 1:7 (178 mm) rifling twist ratio is standard for the SPR. [11] [3] [13] The barrels are manufactured by Douglas Barrels with a unique contour that reduced weight but maintained rigidity for accuracy. [14]
The rifle is fitted with a stainless steel, fluted, 686 mm (27.0 in) barrel, which research has found to be the best compromise between muzzle velocity, weight, and length. The barrel has an unconventional 279 mm (1:11 in) right-hand twist rate, optimized for firing .338-calibre very-low-drag bullets up to 16.85 g (260 gr).
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).
As a result, the twist rate of the larger barrel will dictate which smaller bullets can be fired with sufficient stability out of a sabot. In this example, using 1:10 rifling in 7.62mm restricts saboting to 5.56mm bullets that require 1:10 twist or slower, and this requirement will tend to restrict saboting to the shorter (and lighter) 5.56mm ...