Question on FPS vs Twist vs Shape??

[Will]

Is there any chart or information on twist rate vs boolit shape and FPS? Such as what is the beat twist and fps for 140gr 6.5mm vs a 240gr 45. I know some require a faster twist than others to stabilize but how do you know which other than trial and error?
Also if a boolit requires more rpm to stabilize can you accomplish this by increasing FPS?

[felix]

The "charts" are burried within computer programs. The more sophicated the program, the more different kinds of projectiles can be simulated. Some programs allow the entry of 15 different nose shapes. Yes, that is true, what you surmised...simulations are based upon trial and error, at least once past the theoretical calculations. You can increase stability only marginally by an increase in velocity; in other words, not worth the chance; therefore the answer is NO in real life except by chance. ... felix

[44man]

Other then testing, a slow twist needs more velocity when the boolit gets longer and heavier. Light boolits can be spun up by increasing velocity easier then a heavy boolit yet still shoot OK at a lower speed.
In general, the longer the boolit, the faster the twist must be.
That is why I love my BFR revolvers. The twist rates are faster then any others and my range of boolits is very large with the heavy ones showing supreme accuracy.
There is just no perfect formula to predict this. Greenhill falls flat on it's face showing twist rates way slower then what is needed.
This has been the downfall of a lot of guns like the Rem .244 which was vastly superior to the .243. WW made the .243's twist faster so a heavy deer bullet was accurate while Rem made the .244 for light varmint bullets.
It will also kill the Marlin .44 mag because of the stupid 1 in 38 twist.

[Larry Gibson]

Go to; www.nfa.ca/content/view/132/197

Click on; Bullet Stability Calculator (MS Excel)

It is an interesting program to work with. Failing that the Greenhill Formula still is a basic tool for calculating twists. You can GOOGLE "Ballistics Calculator and get several sites.

Larry Gibson

[44man]

I think where most formulas fall apart is they are based on the overall length of a bullet. That is the least fixed portion of any formula.
One should be developed for the actual engraved part of a bullet.

[runfiverun]

the green hill is definately length based.
and is pretty close if you have a bore rider or a loverin type of boolit.
but like 44 says if you have a swc or a full wc the bearing length can be vastly different.
most guys will say my gun just don't like that boolit when it isn't the weight it is the bearing surface.
i have shot 2 different 200 swc's side by side, one shot fine the other did not.
just different nose and bearing surfaces.

[Willbird]

I think where most formulas fall apart is they are based on the overall length of a bullet. That is the least fixed portion of any formula.
One should be developed for the actual engraved part of a bullet.


The length/dia ratio of the bullet is what determines stability, the SHAPE will have an impact on stability due to balance, but the length/dia ratio is still the most important factor. The Greenhill formula probably has enough "fudge factor" built in to allow any bullet shape.

Velocity is a consideration as well, if a bullet is stable in a 1/10 twist at 1000 fps, it should also be stable in a 1/20 twist at 2000 fps shouldn't it ??...IE a given bullet needs a certain RPM to be stable.

Bill

[Larry Gibson]

Willbird

"Velocity is a consideration as well, if a bullet is stable in a 1/10 twist at 1000 fps, it should also be stable in a 1/20 twist at 2000 fps shouldn't it ??...IE a given bullet needs a certain RPM to be stable."

That would seem logical but it doesn't work out that way. For example; we all know a 311921 is stable at 1000 fps out of a 10" twist and we'll get good accuracy. The same 311291 out of a 20" twist is horribly inaccurate and most like would be tumbling well before it reached the 100 yard target range. The reason is there is more to stability than just a certain RPM. Bullet stability varies directly as the square of the axial moment of inertia and inversely as the first power of the transverse moment. Confused? Don't feel bad, most are by that definition.

Let me put it another way. When we compute bullet stability other things enter in besides velocity and RPM or RPS. The formula (quite complicated) for bullet stability gives us a Stability Factor (SF). A SF factor smaller than "1" means the bullet will not be stabilized. A SF of 1+ means the bullet is marginally stable. A SF of 1.3 means the bullet is fully stabilized and a SF of 1.5 or higher means the bullet is maximally stabilized. In your example a 311291 at 1000 fps in a 10" has a SF of 2. The same 311291 out of the 20" twist at 2000 fps only has a SF of .6. Obviously the 311291 will not stabilize in a 20" twist at 2000 fps. I do know for a fact it is stabile at 2000 fps out of my 14' twist. Calculations show it is stable in a 16" twist at 2000 fps.

Larry Gibson

Additional; we do know that the .32 Special will sabilize a similar bullet at lower velocity than 2000 fos with it's 16" twist so the calculations of the formula are pretty close to reality. Understanding of course that a similar bullet in the .32 Special is shorter (due to the larger bore size) and that gives a slower twist a slight edge.

[44man]

Now your talking. I think the only way is to test with your gun and boolit. Nothing on paper will ever say what will work.