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Rifling
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<p>Rifling is the term for <a href="/facts/Helix/BqC4AKwL">helical</a> grooves <a href="/facts/Machining/Yrjfj6IV">machined</a> into the internal surface of a <a href="/facts/Firearm/GuHx4aGE">firearms</a>'s <a href="/facts/Gun_barrel/bPyBmQek">barrel</a> for imparting a <a href="/facts/Rotation/yChkz49x">spin</a> to a <a href="/facts/Projectile/kaDpG3N7">projectile</a> to improve its <a href="/facts/Aerodynamics/Cn6Lnl3H">aerodynamic</a> stability and accuracy. It is also the term (as a verb) for creating such grooves. The opposite of rifling is <a href="/facts/Smoothbore/T6z0unC3">smoothbore</a>. </p><p>Rifling is measured in <i>twist rate</i>, the distance the rifling takes to complete one full revolution, expressed as a ratio with 1 as its base (e.g., 1:10 inches (25.4 cm)). A shorter distance/lower ratio indicates a faster twist, generating a higher spin rate (and greater projectile stability). </p><p>The combination of length, weight, and shape of a projectile determines the twist rate needed to <a href="/facts/Gyroscope/UjXGsfMq">gyroscopically</a> stabilize it: barrels intended for short, large-diameter projectiles such as spherical lead balls require a very low twist rate, such as 1 turn in 48 inches (122 cm). Barrels intended for long, small-diameter projectiles, such as the ultra-low-drag 80-<a href="/facts/Grain_(mass)/kkDKk6na">grain</a> 0.223 inch bullets (5.2 g, 5.56 mm), use twist rates of 1 turn in 8 inches (20 cm) or faster. </p><p>Rifling which increases the twist rate from <a href="/facts/Breech-loading_weapon/bCzCKg7n">breech</a> to <a href="/facts/Muzzle_(firearms)/bPyBmQek">muzzle</a> is called a <i>gain</i> or <i>progressive</i> twist; a rate which decreases down the length of a barrel is undesirable because it cannot reliably stabilize the projectile as it travels down the bore. </p><p>An extremely long projectile, such as a <a href="/facts/Flechette/lPwq62Ko">flechette</a>, requires impractically high twist rates to stabilize; it is often stabilized aerodynamically instead. An aerodynamically stabilized projectile can be fired from a <a href="/facts/Smoothbore/T6z0unC3">smoothbore</a> barrel without a reduction in accuracy. </p>