Understanding Barrel Twist Rate Calculations

vancouverbc

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Is Greenhill's calculation still used for approx twist rate or is there something new being used? With his formula for optimal twist rate calculation for a 308 bullet I get the following:

CALIBERS = Length of 168gr SMK = 1.1835 inches divided by .3065 inch diameter = 3.861

Take 150 (some constant) and divide by CALIBERS (3.861 above) = 38.85 calibers

Thus, ideal twist rate = one turn in 38.85 calibers.

My bullets = 3.861 calibers, therefore 38.85 divided by 3.861 = 1:10

My Savage FCP-K is a 1 in 10 inch twist which, by this calculation, appears to be optimal for this bullet. Is this coincidence or do these calculations make sense?

If I was to use a longer bullet I assume then that a 1:9 then be more optimal? While shorter bullets want slower twist?

Does mass play a role at all in optimal twist rate? This calculation doesn't seem to think so but a lot of websites say it does. Obviously you could keep the same dimensions but change the mass/density of the bullet by a considerable amount.

I still have some homework to do, obviously, but thought it might be a starting point to discuss.
 
the greenhill formula is overly conservative. Other options today show twist rates coupled with out criteria.

For the 30cal, there is nothing on the market that needs a twist faster then 10 - assume of course, the barrel has the twist rate that is suggested on the barrel.

Some use slower for subsonic heavy bullets shooting but unsure if it really is necessary,

Jerry
 
vancouverbc said:
Is Greenhill's calculation still used for approx twist rate or is there something new being used? With his formula for optimal twist rate calculation for a 308 bullet I get the following:

CALIBERS = Length of 168gr SMK = 1.1835 inches divided by .3065 inch diameter = 3.861

Take 150 (some constant) and divide by CALIBERS (3.861 above) = 38.85 calibers
Click to expand...

So far so good. Arithmetic error follows:

Thus, ideal twist rate = one turn in 38.85 calibers.

My bullets = 3.861 calibers, therefore 38.85 divided by 3.861 = 1:10
Click to expand...

Minimum (not ideal) twist rate rate according to Greenhill's formula would be 38.85 calibers.

Your caliber is .308" (or .3065" if you prefer), so 38.85 calibers multiplied by .308 inches per caliber is 11.69".

Greenhill is telling you, you need a twist no slower than one turn in 11.7".

For what it's worth, Greenhill gives a pretty conservative number. In reality, you can shoot 168 Sierras out of a 1:13" or a 1:14" twist barrel, and they will stabilize (and shoot very accurately too, for what it's worth).

My Savage FCP-K is a 1 in 10 inch twist which, by this calculation, appears to be optimal for this bullet. Is this coincidence or do these calculations make sense?

If I was to use a longer bullet I assume then that a 1:9 then be more optimal? While shorter bullets want slower twist?

Does mass play a role at all in optimal twist rate? This calculation doesn't seem to think so but a lot of websites say it does. Obviously you could keep the same dimensions but change the mass/density of the bullet by a considerable amount.

I still have some homework to do, obviously, but thought it might be a starting point to discuss.
Click to expand...

Industry standard twist for .308 Win is 1-12". Savage makes most of their .308 Win rifles with the quicker 1-10" twist rate.

A .308 Win with a 1-12" twist rate will stabilize the Sierra 190 MatchKing, for what it's worth.

Your 1-10" twist Savage barrel will stabilize almost any .308" bullet you might care to try shooting.
 
Mystic Precision said:
the greenhill formula is overly conservative. Other options today show twist rates coupled with out criteria.

For the 30cal, there is nothing on the market that needs a twist faster then 10 - assume of course, the barrel has the twist rate that is suggested on the barrel.

Some use slower for subsonic heavy bullets shooting but unsure if it really is necessary,
Click to expand...

You mean "faster". And it is.

A 1:10 will generally stabilize subsonics up to about 180gr and a 200gr RN but the shape and length of the bullet does make a difference. Something like a 168gr BTHP is very difficult to stabilize. That is why the 300 Blackout specs a 1:8 twist.
 
Historical note:
Sir (Alfred)George Greenhill 29 November 1847 - 10 February 1927 was a British mathematician .
In 1879 Greenhill developed his formula to calculate optimal twist rate.

Nice to know that things developed with pencil and paper still work today...

Cheers
 
Last edited:
Greenhill formula is inddeed very conservative.
Use "170" as constant for BT bullets and "190" constant for FB bullets and you will be much closer to reality.
My 2c worth....GR8.
 
Longshot said:
Are there dies for that? I see heavy .30 hybrids out here once in a while and always wonder what I could put em through.
Click to expand...

Yup. Redding makes them. 30-338 lapua mag. Its a full length die. I would also recommend a bushing die for neck sizing. Just get a 338 LM and a few 30 cal bushings.
 
The Miller Stability Formula (see Litz, Applied Ballistics ... Chapter 10 and appendices) is more accurate and more recent - you can compensate for your local atmospheric conditions. A bullet/twist combination at sea level may not necessarily work a few thousand feet higher.
 
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