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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]
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.
The cold hammer forged barrel features a 304.8 mm (1 in 12 inch) twist rate to adequately stabilize the military 7.62×51mm NATO ammunition of the era. This twist rate is slow for optimally stabilizing not at the time available 7.62×51mm NATO sniping rounds loaded with 11.34-gram (175 gr) Hollow Point Boat Tail projectiles.
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 stock is attached to the barrel, and the barrel to the action. The rifled barrel has a twist rate of 1 turn in 20", allowing heavy hunting bullets to stabilize. This is relatively rare for .44 Magnum firearms, which usually are rifled at a rate of 1 turn in 38 inches. The 1:20" twist enables the use of 325gr ammunition for hunting large ...
The C.I.P. recommends that commencement of rifling begin at 8.97 mm (0.353 in). Bore diameter is given as 12.55 mm (0.494 in) and groove diameter is 12.80 mm (0.504 in). The C.I.P. recommends a five groove barrel contour with each groove having an arc length of 5.33 mm (0.210 in) and a twist rate of one revolution in 406 mm (16.0 in).
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A gain twist has a slow initial twist in the barrel, progressively getting faster until the final twist rate is attained near the muzzle, resulting in less torque being imparted to the bullet during the highest stress phase of the interior ballistic cycle, and thus less wear in the throat of the barrel. Gain twist was phased out in the last ...