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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]
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).
Most factory Swift rifles come with a fairly slow twist-rate such as 1–12 or 1–14 inch, designed to stabilize the lighter bullets popular in varmint hunting. Custom Swifts can have faster twist-rates such as 1–9 inch allowing them to stabilize heavy bullets, including those with a construction suitable for larger game. [15] [16]
The standard bullet diameter for 6 mm caliber cartridges is .243 inches (6.2 mm), the same diameter used in the .243 Winchester and 6mm Remington cartridges. 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 ...
A 254 mm (1 in 10 in) twist rate is also commonly applied. [ 11 ] According to the official C.I.P. ( Commission internationale permanente pour l'épreuve des armes à feu portatives ) rulings, the .308 Winchester can handle up to 415.00 MPa (60,191 psi) P max piezo pressure.
Doppler radar measurement results for a lathe turned monolithic solid .50 BMG very-low-drag bullet (Lost River J40 13.0 millimetres (0.510 in), 50.1 grams (773 gr) monolithic solid bullet / twist rate 1:380 millimetres (15 in)) look like this:
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Rifles chambered for this cartridge bullet combination, with a cartridge overall length of 116 mm (4.6 in), have to be equipped with custom made 1,016 mm (40.0 in) long 279 mm (1:11 in) twist rate barrels to stabilize the .416 Barrett MSG projectiles and attain a projected 1,032 m/s (3,385 ft/s) muzzle velocity. [13]