Rat Man wrote on Jun 2
nd, 2010 at 8:38pm:
Does anyone know if a rifled spin gives you more distance than back spin? My guess would be yes, but I haven't measured one against the other yet.
As Brian says, with a spherical projectile, especially a golf ball, backspin will give better range. Golf balls are specifically designed that way with the dimples and backspin deflecting the air flow around the ball and giving lift. (look it up on Wiki for a detailed explanation of how it all works.)
As P-J says, golf balls will also curve in flight. If you impart side spin the same principles as apply with backspin will now deflect the ball sideways, making accuracy difficult. However this sidespin does not increase range, only backspin does.
But with an elongated projectile it is important to keep it orientated point forward as this greatly reduces air drag by presenting the smallest possible frontal area, effectively increasing sectional density. Rifle spin gives a sling projectile gyroscopic stability, just as rifling does a rifle bullet, and keeps it flying efficiently, ie. with as little air drag as possible. If an elongated projectile flies with backspin it just presents a larger area to the opposing air and loses velocity more quickly.
Quote: What I want to ask is, with rocks as ammo, is there a need for rifled spin? Does random spin affect rock flight as much as it can affect golf ball flight? Does rifled spin aid rock flight, or is it unnecessary?
Generally rifled or any spin does not affect a stone as much as a golf ball because stones are much denser (SG typically around 3 or higher) than golf balls which are only just denser than water (SG approx. 1.1) This means the side forces generated by spin are not as noticeable because of the greater mass, especially true of nice smooth rounded stones. However railway ballast stones are often very jagged and angular; these stones can have very unpredictable flight. Any stone which makes a loud whirring sound as it flies will give poor performance. An additional downside to irregular jagged stones is that they tend to hang up in the pouch, giving unpredictable release. No acident that in the DvG story David selects smooth river stones before battling Goliath.