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Civilian AR-15 rifles are commonly found with 1 in 12 inches (30 cm) or 54.8 calibers for older rifles and 1 in 9 inches (23 cm) or 41.1 calibers for most newer rifles, although some are made with 1 in 7 inches (18 cm) or 32 calibers twist rates, the same as used for the M16 rifle. Rifles, which generally fire longer, smaller diameter bullets ...
Thus the optimum rate of twist for this bullet should be approximately 12 inches per turn. The typical twist of .30-06 caliber rifle barrels is 10 inches per turn, accommodating heavier bullets than in this example. A different twist rate often helps explain why some bullets work better in certain rifles when fired under similar conditions.
The widespread availability of standard-size Mauser 98 rifles in central Europe and the fact that the .375 H&H Magnum cartridge and its necked-down version the .300 H&H Magnum with approximately 72.4 mm case length and 91.4 mm (3.60 in) overall length were too long to fit in standard-sized Mauser 98 bolt-action rifles made the shorter 9.3×64mm ...
In practical terms, this is only a problem for rifles chambered for common military calibers where a variety of different loads exist. For example, the M16A1 rifle is unable to accurately fire bullets that are heavier than 3.6 grams (55 gr) due to barrel twist that is too slow to stabilize heavier bullets. [41]
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 cartridge forces the bullet into the rifling. This is common in low-powered rimfire target rifles.
MV/ twist rate = rotational speed; Using metric units, the formula divides the number of millimeters in a meter (1000) by the barrel twist in millimeters (the length of travel along the barrel per full rotation). This number is then multiplied by the muzzle velocity in meters per second (m/s) and the number of seconds in a minute (60).
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Besides the rechambering and rebarreling with a 610 mm (24 in) long, 254 mm (1 in 10 inch) twist rate (5R) hammer-forged barrel the main reconfiguration changes compared to 7.62×51mm NATO chambered M24 rifles are: [18]