How do you pick twist rates for barrels? I assume trial and error but there must be some sort of calculation for which twist is best.

It is not trial and error.
Professor Sir Alfred George Greenhill of the British Royal Military Academy, who devised a formula for determining twist rate which multiplies the square of the bullet diameter by 150 and then divides the result by the length of the bullet, and looks like this: (C x D2) ÷ L = There are calculators on the net which will do it for you.
Bur most of us have the numbers in our heads for each caliber.

The industry has moved away from Greenhill's formula and uses the Miller twist rate formula.
Millers formula can also be used to solve for stability of a projectile for a certain twist rate (calibers per revolution) dependent on environmental conditions.

Bryan Litz's books go through the formula and it is an improved way to calculate twist rate and stability factor (SG) for modern projectile design.

The calculators on Berger's and JBM websites both use the Miller formula.

I don't use no calculator; I know them intuitively.

https://www.shilen.com/calibersAndTwists.html

This is the Shilen rifle barrel website that I have used to determine proper twist rate, and they are spot on. One caveat to this, if you plan on shooting mono-metal bullets such as Barnes, I would recommend a barrel with the next faster ROT; such as for a 264Win shooting 140 grain bullets, cup and core bullets are recommended a 1/8" Rot, but for a 140 grain mono-metal I would recommend 1/7" ROT. This is because the mono-metal bullets are longer for weight than C+C bullets. Most people that shoot mono-metal bullets opt for less bullet wieght so therefore increased ROT isn't necessary because the lighter mono-metal bullet is shorter and therefore will stabilize in standard ROT barrel.

Here's the link to Don MIller's White paper about his twist rate formula and the inputs and derivation.

A New Rule for Estimating Rifling Twist

quote:

Originally posted by dogcat:
How do you pick twist rates for barrels?

ask the bullet manufacturers. they usually state required twist for each bullet on their website.

Berger has one on their website for quick calculation of the twist required for different bullet designs.
https://bergerbullets.com/twist-rate-calculator/

I suck at math, reading is easy though. I believe that any formula had to have its root in trial and error at some point in time. Otherwise it wouldn't be science, it would be guesswork.