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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.
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.
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 overall case length after the shortening and re-sizing is 1.565". The cartridge was designed to use .308" diameter bullets in weights of 110 to 150 grains, with a standard twist rate of 1-12". [1] Wilson Combat states that heavier bullets can be used in a barrel with a 1-8" twist rate for subsonic loads. [4]
Barrels were supplied by Rock Creek Barrel Inc. Various twist rates were tried with 5R rifling. It was designed as a way to optimize shooting the 19.44 grams (300.0 grains) 8.59 millimetres (0.338 in) caliber Sierra HPBT MatchKing projectile from actions and magazines that lack the length to handle cartridges exceeding 91.44 mm (3.600 in) in ...
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A slight gain in the rifling twist, or a slight tapering down of the bore, will still ensure the tight fit of bullet to bore, so when selecting a barrel, the tighter or faster twist end should be chosen for the muzzle. [37] [38] [39] The twist of the rifling must match the intended ammunition for best accuracy.