There is a formula for figuring twist needed when building a rifle. Does anyone have that formula?

If you’re talking about Greenhill’s formula, then this is it.

You need your bullet, a dial caliper, and a handheld calculator helps.

1. Divide the length of the bullet in inches by its caliber in inches. (ie. 30 caliber = .308â€)
2. Divide that number into 150 (Greenhill’s constant).
3. Multiple that number by the caliber in inches...and this is your answer.

This gives you a “ball park†figure for twist.

You can also log onto one of the barrel manufacturer's web sites. They will generally list the preferred twist rates based on bullet weight for caliber.

Geronimo

With all due respect to barrel manufacturers...bullet length, not weight, is the determining factor in rate of twist. Granted...a heavier bullet will normally be longer, but it is length, not weight, that you need to consider.

rick is perfectly right.. barnes bullets sometimes require quicker twists than the same weight, if it's "peaky"

http://www.realguns.com has one of the best, and registration is free

jeffe

Short, fat objects do not require as much spin to stabilize as do long, thinner objects.

Think of a kids top. They are made short and fat so they will stabilize with less spin. Bullets are the same way...the longer they are the faster they have to be spun to stabilize in flight.

The supposed “ideal†twist rate is that which is just fast enough stabilize the bullet...or so say the guys that are allot smarter than I am!

agreed,..the twist rate necessitated is based on bearing surface length,..which in many cases coincides with weight. But,..some bullets with different ogives and different bearing surface lengths can be only a few grains apart, which can often affect twist rate and one will stabilize very well while the other will not quite go to sleep.

I twist my tubes fast so I can use any bullet I want. My BR barrels are made for heavy bullets anyway,..so fast twists are the common practice.

HTH

Shilen's website has a good chart to start with as well.

quote:

Originally posted by Rick 0311:
If you’re talking about Greenhill’s formula, then this is it.

You need your bullet, a dial caliper, and a handheld calculator helps.

1. Divide the length of the bullet in inches by its caliber in inches. (ie. 30 caliber = .308â€)
2. Divide that number into 150 (Greenhill’s constant).
3. Multiple that number by the caliber in inches...and this is your answer.

This gives you a “ball park†figure for twist.

rick has given instructions on how to do the calculation- simply stated, the Greenhill formula is RTC = 150/LBC Where RTC is "rate of twist expressed in CALIBERS" and LBC is "length of bullet in CALIBERS".....

I used the term “ball park†for a good reason since Professor Greenhill died in 1927.

Bullet shape, velocity, and sectional density, can also have effects, so your milage may vary slightly.

Some in the high velocity crowd substitute 180 for Grenhill’s 150 constant.

Also...it is far easier to get a “true†rate of twist with cut rifling than it is with button rifling just because of the mechanics of the tools involved. That’s not to say one is “better†than the other, and the difference is probably so slight to not really make much practical difference, but its good to know that a button rifled barrel may have a plus or minus to the rate of twist...especially with the cheaper barrels.