[MikeG]

One other thing, to Craig's point -

Stability in the air is one thing, stability in a denser medium (ie. the critter) is another. If it is marginally stable in air, it may not stay nose-forward, in the critter. With expanding bullets this is a whole 'nother can of worms, but generally speaking, the expanding bullets get shorter, quickly, and tend to keep going in the same direction.

With non-expanding (hard cast), you want the bullet to stay nose-forward, and not tumble. So, some 'extra' twist rate is good.

My .444 will never know the difference, if a heavy bullet tumbles in a 90 lb. whitetail. But on a bigger critter it might matter.

 

[Mecanik]

I think we all need to remember the great Colonel Greenhill and his twist rate formula. The classic formula is: T' = 150 / L' twist = 150 divided by the length of the bullet in calibers. The complete formula is available in several location around the net.
Example.. Measure the length of your bullet and divide it by the diameter. Divide 150 by that number and you have the minimum twist rate.
I saw a website that does all that for you: Barrel Twist Calculator

 

[CraigC]

Mike makes a good point. While I have nothing to support it, it stands to reason that if a bullet is marginally stable in the air it very well may not penetrate straight once it reaches the target.

I don't know where the Greenhill formula applies but it doesn't apply here. It errs well on the slow side.

The Berger calculator uses the Miller formula and seems to be more accurate.

 

[Groo]

Groo here
You all have been hitting the edges but not on target..
The twist rate is not important,,, it is the RPM!!!!!!!!!!!!!!!!
A given length of bullet needs a given RPM to be stable.
A 1/16in twist would be fine in a pistol with a short barrel as long as a specific RPM [caused by FPS]
can be gotten.
In a rifle the same RPM can be gotten at a much slower twist rate because the FPS his higher.[due to the much longer barrel]
Take a 44 at a 1/18 inch barrel twist at 900fps [240gr bullet 44 spec ] the rpm is 32000.
Take the same load in a rifle [1700fps 44 mag] and the rpm is 68000.
Change the rifle to a 1/32 twist and the RPM is 32000
The bullet will shoot fine in both .
So the key is to get the RPM's into the correct range.

 

[MikeG]

Groo, that is not entirely correct. Speed does matter.... to an extent. For a bullet that is marginal in stability due to twist rate, SOMETIMES increasing the speed helps. Sometimes. There is a theoretical advantage to increasing the speed, but as a practical matter, the increased drag on the bullet can somewhat cancel that out. So, it depends. You can play around with things like the JBM stability calculator and see that speed is a small part of it. There's also a further can of worms regarding bullets shape, and how it handles the trans-sonic zone (transition from super-sonic to sub-sonic).

 

[baddad457]

Groo, that is not entirely correct. Speed does matter.... to an extent. For a bullet that is marginal in stability due to twist rate, SOMETIMES increasing the speed helps. Sometimes. There is a theoretical advantage to increasing the speed, but as a practical matter, the increased drag on the bullet can somewhat cancel that out. So, it depends. You can play around with things like the JBM stability calculator and see that speed is a small part of it. There's also a further can of worms regarding bullets shape, and how it handles the trans-sonic zone (transition from super-sonic to sub-sonic).

I think you're missing his point here. the faster you drive the bullet down the barrel, the higher the rpms will be once it leaves the barrel. A heavier bullet due to it being slower needs a faster twist to obtain the desired rpm to stabilize it. He hit it spot on with that. All that aside, I've never had accuracy problems with any of my 44 mag rifles and carbines with heavy bullets (290-350 gr) due to the twist rate. My oldest is a B92, all the rest are Rossis made in the past twenty years.

 

[MikeG]

I am not missing the 'point' - the 'point' is not entirely correct. RPM is heavily influenced by velocity. But stability is not. RPM and stability are not the same thing. Example from the JBM stability calculator:

A 220gr. .308 bullet 1.5 inches long, at a velocity of 3,000 fps has a stability factor of 1.265 in a 1-12" twist barrel, which should be fine for most purposes.

A 220gr. .308 bullet 1.5 inches long, at a velocity of 1,500 fps has a stability factor of 1.004 in a 1-12" twist barrel, which is predicted to be BARELY stable with a stability factor of 1.004 - at exactly half the RPM. The RPM drops by a half, the stability by about 20%. They are not proportional. Twist has a far greater influence, than RPM.

 

[baddad457]

RPM affects the stability. They're not the same thing, but one affects the other. That was his point.

 

[ccargo]

Hey guys been awhile....read this post start to finish wow good learnings...I’ve got a win94 in 444...1/38 twist Ruger 96 44mag 1/20 a smith 629 6” with 1/20 i try to use only btb in my win444 it seems to be good with 250-290 lfn at about 2000-2250 tops...accy is ok i need more trigger time the ruger likes heavy bullets but have to use wfn to cycle or use 44spcl brass...my dillema is should I slow my 444 to maybe 1800 to see if accy gets better or go same speed with and save heavies for my nef444 1/20 twist... I fire lapped it but not the others...yet!! I reallt am quite reluctant to lapp the smith it loves cast and im nice to it...the win is tuff to shoot my brass are .060 shorter than normal to use lfn btb’s...again I need more time to get it right...marshalls advice on fire lapped nakes alot of sense to help in accommodating either twists in 44 cals...but to a point???? Again it seems more trigger time is needed to explore hope i made sense in my query...thanks guys...oh yea I’m fixing to go for a taylor/chiappa Alaskan in 45lc...$$$$ you think it’s worth playing with ??since I reload I should be able to find a good shooter load this would be my first 45lc...or I go with the Winchester short carbine 20” a lil cheaper but not a takedown??’....thanks again

 

[n55bz]

See https://arxiv.org/pdf/1309.5039. A very comprehensive paper on stability

 

[JBledsoe]

.


Years ago, some gun writer was sitting around trying to think of something half way intelligent to write about. Similar to those who started the Boxer Rebellion years ago. so he hit upon the idea of twist rates, and published his article. From that the argument has grown to what it is today, which is mostly just BS.

 

[HarryN]

There is another, possibly less appreciated aspect of rifle twist rates - lower density alloys.

I realize that most people don't have to deal with this problem, but in California, hunting with lead bullets is essentially not possible. We are more or less forced to accept copper and the resulting effects on costs and how the ballistics change for virtually every application.

For those in this situation, there is some very good technical info on the GS Custom bullets web site. While their bullets are of course somewhat different than other suppliers, there are similarities for me to gain some good insights.

I am in no way an expert at all, but my interpretation in general is:
- For whatever you are used to doing, plan on going up 1 - 2 calibers vs the lead bullets you are used to using
- For a given mass, the bullets will be longer than you are used to - so tighter twists are required
- These machined copper bullets are very precise, but also a lot more expensive than cast plinkers.
- Anyone who tells you that the ballistics are "close enough - just substitute these copper bullets for lead and you are good to go" - is full of it.