I think I will say (without the math) that 1 in 10" twist in .243" bore can stabilize a 120 grain spitzer. Bullet type is important and the VLD BTHP type bullets will be much longer than say a flat base soft point spitzer.
I posted this a few years back so I'll redo it with some extra info.
The formula actually looks like this. Twist=CD^2/L * sq rt SG/10.9
C=A constant meant to represent velocity. Use 180 for velocity over 2800 fps.
D= bullet diameter in inches.
L=Length of bullet in inches.
SG=Specific gravity (10.9 for lea core bullets)
Barrel Twist Rates
There have been some questions about this subject lately and I have responded to the best of my ability specifically for each of them.
There is a post back on page three asking about the 450 Marlin and another on page one for the 458 WM twist.
In those two posts are formulas called the Greenhill Formula. Well, I make up my formulas as I go along because I understand math and what we are calculation here so the formulas are not exactly alike.
One is: Twist=150 X D/r (the length of the bullet in calibers)
150 is a constant.
D=diameter of the bullet.
The other is: Twist=150 X D squared/L (length of the bullet in inches)
These are the same thing and will give the same results with just simple check book balancing math.
A .458" diameter bullet is 1.374" long (500 grains) the twist will be 22.9" with either formula. I just recommended using the constant value of 100 for better impact stability and penetration.
That same bullet would be calculated at; 15.26" or 15.26" depending on which formula you choose.
I will add to this for those who care, a way to find the maximum bullet length that can be stabilized in a particular barrel.
Maximum Bullet Length (inches)=D x D x 150/Twist Rate
This way we can go backwards to find what bullet is optimum in our rifle.
A bullet needs a certain RPM or more specifically a rotational speed of enough surface feet per minute to stabilize.
To calculate this RPM= Velocity/Circumference of the bullet. OR MV* 12/Twist rate*60 = RPM
A bullet fired at 3050 fps from a 1 in 7 twist barrel will rotate at 313,714 RPM.