Re: Is there danger in a faster twist?
Have you ever played with a toy gyroscope. If you spin one very fast and set it on its stand it will hold its position but it will be jittery from minor balance imperfections. If you tilt it, it will precess rapidly with a small radius on the upper axis. As it slows down it will stand up straight and run smoothly. As it further slows down it start to wobble and fall off it's stand. Bullets are a lot like that except that the external force on the spinning object is mostly atmospheric drag instead of gravity. The force of gravity is typically 1-2 percent of the drag force and for a horizontal shot they're perpendicular. To fly with least drag a bullet must fly nose first into the air it's passing through. If spun to fast it won't align into the air quickly. If spun too slow it will become very unstable and tumble. Air drag is not only a function of the shape of the bullet and it's velocity, but also of air density. Cold dry low altitude air is denser than warm moist high altitude air at at the same barometric pressure. There is no "best" twist rate for any particular cartridge unless you look at all the factors. Fortunately there is a fairly wide range of spin rate, maybe 30%, for a given bullet which works pretty well. Choosing too low of spin rate will limit how long (usually heavier) bullets you can shoot. Choosing too high of spin rate can cause the problems of bullet blow up and excess spin drift, and (maybe) limit accuracy.
When a bullet is fired it's ratio of forward velocity to rotational velocity is fixed by the barrel twist rate. Once free to fly that ratio is not maintained. Air drag slows both but not necessarily equally. Usually forward velocity reduces faster than rotational velocity. As a bullet approaches transonic velocities drag increases, so it's stability is reduced. The spin rate will be lower then when fired but how much slower is not predicted by any commercial ballistics software I'm aware of. The only way I know of to predict how accurate a particular rifle/cartridge/bullet will be past it's transonic range is with shooting tests. Ballistics programs aren't of much help.
Heres an interesting website:
http://www.nennstiel-ruprecht.de/bullfly/ Is's a description of how bullets really behave traveling though the atmosphere. . It may give the impression you can never again hit a target. Don't worry the effects are real, but usually small.
So why are specific twist rates selected?
1. Because they work well for most shooters. That's usually from experience of previous rifles and bullets in the hands of military, hunters, and target shooters.
2. Because shooters think a particular twist works well for a given bullet and the rifle maker's marketing has determined it will optimize sales. That's why you see more even number twist barrels than odd, more whole numbers than fractional, more fractional than decimal, and in the US, definitely more in inches than metric. In the case of the 9.35 twist for 338 LM, probably just to be exotic and mysterious. The 338 LM was designed before 300 grain VLD's existed. No one twist is optimum for all atmospheric conditions. I saw a post recently supposedly quoting a barrel maker that "8 inch twist is good for any 6.5mm" I'm skeptical.
3. Because a ballistics expert calculated that a particular twist is optimum at least for one application.
4. 50 BMG (also 1911 pistol) Because John Browning designed it. It works. Why change it? (same as #1 actually)
