While bullets may not "overstabilize" there are definite issues with bullets that are out out of balance spinning at higher-than-needed RPM, and there are issues with smaller bullets disintegrating. Inducing more torque than necessary on the firearm with an overly tight twist affects accuracy.
ratchet rifling question
- Thread starter madcowbreeder
- Start date
rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
But, this would be less useful, because it is not RPM that stabilizes a bullet. It is twist rate plus drag coefficient that determines stability.
In the velocity range 2000-3500fps, the drag coefficient is approximately constant, and the required twist rate is also approximately constant. If you want to look at it more exactly, there is actually a small decrease in drag coefficient as velocity increases, and as a result there is also a small decrease in the twist rate that is required. This is why a bullet/twist combination that is just-barely-not-stable at 2700fps can be made just-barely-stable by going to a hotter load, say 2900fps. It's not (directly) the added RPM that has made it stable, it is that the bullet is now seeing a lower drag coefficient at the higher speed.
That velocity range is about Mach 2 to Mach 3.5. In the "comfortable supersonic" area, which Mach 2+ is, it is pretty standard to see drag curves that are approximately flat but slightly decreasing with increasing velocity.
In the transonic area, which is Mach 0.8-1.2, lots of very complicated things are happening aerodynamically. Different bullet shapes behave differently, but in almost all cases the bullet's drag coefficient increases quite a lot in this area - it tends to be higher than the subsonic drag coefficient, and also higher than the high-supersonic drag coefficient. So in this transonic regime, a quicker twist rate is needed, because of the much higher drag coefficient. So something like a .300 Whisper, which likely is firing at a muzzle velocity of Mach 0.95 more or less, is smack in the middle of this very high drag coefficient area, so it will require a quicker twist rate than would be required to stabilize the same bullet at either a much higher or a much lower muzzle velocity.
If you were to fire a rifle bullet at Mach 0.6 (say about 650 fps) muzzle velocity, where its drag coefficient is comparable (actually somewhat less than) to its drag coefficient at Mach 3, would need a similar twist. So if a 1-14" twist is enough to stabilize a .308/155 target bullet at 3000fps, it would also be enough to stabilize a .308/155 target bullet at 650fps muzzle velocity, even though a 1-14" twist is probably *not* enough to stabilize a .308/155 at 1150fps muzzle velocity.
Dan, exactly. Litz, the exterior ballistics.com literature and many other articles explain this in intricate detail.
My point is that an RPM (for a given velocity range) is still far more useful. or stated another way, twist for a given velocity is much more useable information.
My point is that an RPM (for a given velocity range) is still far more useful. or stated another way, twist for a given velocity is much more useable information.
Mystic Precision
CGN Ultra frequent flyer
- Location
- Summerland, BC
The spin stability of the bullet actually INCREASES as it slows down.
when a bullet gets to the transonic range, what causes it to do funky thing and/or tumble is the change in AERODYNAMIC balance/forces/pressures.
All flying objects have a balance point for static as well as dynamic motion. When there is enough dynamic 'stability', it flies. Move/Change this balance point in the 'wrong' direction, the bullet becomes unstable and tumbles.
Some bullets have a design/balance so that the change in aerodynamic pressures from supersonic to subsonic flight, do not disturb the aerodynamic balance so the bullet just keeps flying on its merry way.
for a much better explanation, read Bryan Litz book...
Jerry
rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
The increase in drag coefficient, as the bullet slows from (say) M=1.6 to M=1.2, makes the bullet less stable.
But the rate at which the bullet is spinning (its RPM) actually decays very slowly. So the "effective twisting rate" (number of revolutions per unit of forward travel) of the bullet increases as the bullet slows down. This makes the bullet more stable.
The overall combination of these two changes may result in the bullet becoming more stable or less stable as it slows. Only if the stability is diminished enough (i.e. the stability coefficient becomes less than 1), will the bullet become unstable and start to tumble.
