The one in 28 twist

[lemoyne]

What this means is that there is one full turn of the rifling in 28 inches.
What it does is stabilize the bullet so it flies strait. A top spinning to stand on a point is often used as an example.
How strong can this be. Well it is used to stabilize space vehicles.
I calculated the ideal length of bullet for some caliber to give an idea of what to look for.
The things you should under stand about the length of bullet to twist ratio.
First : accuracy at any distance past smooth bore capabilities depend on it.
Second : the results are lopsided if you spin a bullet to fast you will have much better results than if you spin it to slow. It will affect accuracy as in groups being slowly enlarged till you get to complete over stabilization in which case the bullet wobbles and wonders.
If a bullet turns to slow for the length it just does not stabilize and will not be accurate. Bullets going through the target sideways is a good example of this.
This is in answer to a beginner which is something we all were at one point. If it helps only one person it is worth the effort.
Here are some lengths for the velocity of 1600 to 1800 FPS to give a feel for what will work best. I use this method to answer because it might help some one else I realize all the regulars know this .
Caliber .400 ideal length .85 inches
.430 ideal length .99
.440 ideal length 1.03
.450 ideal length 1.05
.500 ideal length 1.30
If the bullet is slightly longer a little more powder may help
DO NOT EXCEED THE MANUFACTURES MAX LOAD
If the bullet is shorter it will usually work if it is to much shorter then slowly reduce the load until it is accurate.
For instance the ideal twist for a 50 caliber PRB is 1 in 75 but by reducing the load to 50 grains they will usually shoot well. So instead of over stabilizing at the same spin as an elongated bullet by dropping the velocity to 1100 FPS [estimated] you reduce the spin to where it stabilizes.
I hope this helps. Lee

[scoutdad57]

Do your Ideal lengths combine bullet and sabot? And what happens to the bullet after the sabot falls away? Now you have a bullet that was at one time traveling at a speed that is optimal for its length, now drastically shorter. Does it now begin to destabilize?

[lemoyne]

The sabot has no effect on the bullet length this has been checked by a number of people.

Greenhill Formula
The formula to determine the correct rate of rifling pitch for bullets of usual construction (lead alloy or jacketed) is known as the "Greenhill Formula".
Step One:
Determine the length of the bullet in calibers. (Bullets length in inches divided by the bullets nominal caliber).
Step Two:
Divide 150 (a constant) by the bullets length in calibers.
Step Three:
Multiply the result of step two by the bullets "nominal" caliber. This will give the desired rifling twist rate.
Example:
To find the correct twist for a .30 caliber 220 grain bullet 1.35 inches long.

1.35

150

33.33 x .30 = 9.999

.30

= 4.5 calibers

4.5

= 33.33

(or 1 in 10 inch twist)

This is a sample of the formula used . I have software that does the work.

[cayugad]

very good information there Lee.. thanks