twist rates


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oz_lowrider
February 1, 2011, 09:25 PM
With my SAKO 223 why is it that a 1:8 twist will stabilise bullets from 55gn to 90gn when a 1:12 doesn't stabilise anything above 60gn. :confused:

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RNB65
February 1, 2011, 09:29 PM
The heavier bullets needs a faster twist rate to stabilize the bullet in flight. 1 in 12 twist rate is typically used for light varmint bullets 55gr or less.

nipprdog
February 1, 2011, 09:43 PM
With my SAKO 223 why is it that a 1:8 twist will stabilise bullets from 55gn to 90gn when a 1:12 doesn't stabilise anything above 60gn.

Hmmmmmmmmmmm, physics??

:D

But seriously, RN is right.

oz_lowrider
February 1, 2011, 09:43 PM
I understand that but it doesn't explain why the 1:8 stabilises all weight 223 bullets, even the light ones.

RNB65
February 1, 2011, 09:51 PM
1 in 9 and 1 in 8 are usually considered general purpose twist rates as they work well with a wide range of bullet weights. Folks who shoot 80-90gr bullets usually will go with an even faster 1 in 7 twist.

Do a Google search on .223 twist rates and you'll find lots of info.

redbullitt
February 1, 2011, 09:51 PM
Much easier to under spin (keyhole) them than over spin (shred) them.

If you over spinning them, the bullet comes apart. It is tougher to do that than simply not get them stable.

ie, I wouldnt run any superlight thin jacket varmint bullets at high speed from a fast twist.

mc223
February 1, 2011, 10:02 PM
I f you feel like doing some heavy duty reading on the subject of Spin Stabilization.

http://anarchangel.blogspot.com/2007/01/stabilization-mythology.html



Answered Here also
http://www.thehighroad.org/showthread.php?t=515479&highlight=optimum+twist+rate

Float Pilot
February 1, 2011, 10:28 PM
Actually it is the length of the bullet which needs a different twist rate.

Usually that applies to the weight as well. But not in all cases.
For instance a 70 grain flat base bullet might work fine in one rifle while the 70 boat-tail, which is longer, will not stabilize.

The same problem shows up in the new non-lead bullets since they are longer than usual lead bullets for the same weight.

BrocLuno
February 1, 2011, 10:45 PM
Physics is it. It's the acceleration curve and final exit speed plus the need to rotate a longer cylinder faster to stop oscillation. Generally heavier bullets are slower down the bore, so to get the same spin rate, the barrel has to have a faster twist. As an example (only), let's say all bullets need to spin at X rpm to stabilize. Short light bullets will get down the tube quickly at say 3000 FPS on exit, so out of a 24 inch barrel, they need a 1:12 twist. A heavier slower accelerating bullet will only exit at 2/3 the speed, so it needs 30% faster twist to get the same rotation rate, and because it's a longer cylinder, it may need even more? There are other factors, but that's the simple idea.

mc223
February 2, 2011, 12:21 AM
BrocLuno please educate yourself prior to such a wordy bunch of beep. a longer cylinder needs more what? This is almost all that was needed.

Study as a minimum, the imperfect Greenhill Formula. And Find a copy of WinGyro.
The JBM is also a very useful and accurate calculator.
http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi


If you are going to cite physics, provide a basis for you reasoning.

And to the OP : A 1 in 12 may very well stabilize a 90g projectile. There is no law that says you cant try it. There are barrels that might just do it. Although my better sense says not likely. I did however have a nearly shot out Bushmaster 1 in 9 barrel that shot the 80g Sierras to 400yd with 2MOA accuracy.

mshootnit
February 2, 2011, 12:44 AM
1/12 inch twist will stabilize 60 grain bullets, folks need to quit with the 55 grain limit for 1/12.

Is there really a 223 bullet that 1/8 won't stabilize? I mean seriously. Has anyone ever proven clearly that 1/7 is necessary because it is the absolute slowest twist needed to stabilize a certain bullet? To me 1/7 is fairly radical, in that it will cause a 45 grain bullet to fly apart at ordinary velocity. I certainly don't see it in a 20" rifle unless all you were ever going to shoot was 80 grain ammo, which to me takes up too much powder space in the round to begin with. I think these heavy 223 rounds are a stopgap measure to stretch the platform as far as it can go, and the radical twist is symptomatic of the need for a better platform for the job. All things being I deal I would like to see a 6mm very slightly upscaled platform (1/9 or 1/9.5 twist) that could push a 90 grain bullet to 3000 fps or so, but would also logistically not be too much (if any) more taxing to the individual soldier or marine. Then you would have something with great long range capability and without low velocities or radical barrel twist. Go ahead, I am up for a little education.;)

mc223
February 2, 2011, 01:05 AM
mshootnit I also shoot a 6mm WOA which is a 6.8 SPC necked down to 6mm. loaded with Berger 95 VLDs will achieve 3100 from a 26 inch, 1 in 8. Now that is a hoot to shoot and well under 1/2 MOA at 100. 107s at 2800.

It is an AR pattern upper made by John Holliger at White Oak Precision. The lower is a revamped Bushmaster.

mshootnit
February 2, 2011, 01:17 AM
that sounds like a great rifle. Wish i knew how to build one.

Magnuumpwr
February 2, 2011, 01:28 AM
Dang, not to try to jack this thread, but just found out why my savage 112fv fails to stabilize 40gr varmint bullets. Didn't realize my rifle has a 1/9 twist, just assumed it was 1/12. I'm ready, flame me for "assuming".

mc223
February 2, 2011, 01:45 AM
Buyer Beware

BrocLuno
February 2, 2011, 02:58 PM
OK, I'll bite - specifically what part of my reasoning is flawed? Which part of shape stability did I miss? How does gain rifling figure into the research of stability as both rotational acceleration and velocity acceleration come into play?

Before you bang me upside the head, finish the explanation. You want physics? Offer some yourself. Who's the audience here (MIT or day labor)? How do you break down complex concepts for short discussions? I may have not stated it as you would - but make real meaningful constructs to show where I was wrong. Citing others research and work is great, but it does not offer a short answer to the OP's original question.

TonyAngel
February 2, 2011, 07:34 PM
Magnuumpwr, you really shouldn't have problems stabilizing those 40gr bullets in your 1:9 twist barrel. Your barrel just may not like the bullet's you're using.

Not to throw a monkey wrench into things, but I'd like an explanation for this in street terms myself. All I've done so far is base my opinions on my experiences. Anyway, is it the length of the bullet or the bearing surface of the bullet that matters?

DRYHUMOR
February 2, 2011, 08:07 PM
Street Terms?

Condensed version.


It takes less twist to stabilize a given bullet at high velocity than at low velocity.

At the same velocity in the same caliber, longer (pointed) bullets require faster twist rates than shorter (round nose) bullets of the same weight.

Heavier bullets require faster twist rates than lighter bullets of the same shape.

BrocLuno
February 2, 2011, 10:23 PM
Street Terms?

Condensed version.


1.) It takes less twist to stabilize a given bullet at high velocity than at low velocity.

2.) At the same velocity in the same caliber, longer (pointed) bullets require faster twist rates than shorter (round nose) bullets of the same weight.

3.) Heavier bullets require faster twist rates than lighter bullets of the same shape.

Well said and much shorter than my ramble.

What I was trying to say equals your #1 statement. The need for rotational speed is roughly the same for stability in many cases so as the bullet moves down the bore faster, the twist rate can be less. It'll come out with the needed rotation.

#2 - right on - longer objects require higher rates of rotation - I was trying to state that too, but muddled it up.

#3 - right on :)

WNTFW
February 2, 2011, 10:29 PM
I don't see the problem with BrocLuno's post.

JDMorris
February 2, 2011, 10:55 PM
Same way you throw a spiral, you have to get the spin right with your wrist, a Junior league ball is easier to spiral than an NFL sized ball..
That's how I look at it but I might be off :scrutiny:

tactikel
February 2, 2011, 11:04 PM
Brocluno made some valid points. Remember each barrel is different. My .22-250 will shoot one load with 55 gr bullets in less than 0.5 MOA all other loads are 2-10 MOA (not stabilizing, and keyholing). All 50 gr loads shoot under 1 MOA (it has a 1:14 twist). Why slower twists? First less pressure (there is less friction pushing a 50 gr in a 1:14 barrel than a 1:7. Years ago bullets bullets were less robust (.22 hornet and .218 bee were hot cartridges) and powders for the most part were faster than todays choices. Over stabilizing a 40 gr bullet will give poor accuracy or may lead to bullet failure. A bullet traveling at 3500 fps from a 1:12 barrel is going a 210,000 rpm, from a 1:7 barrel the same bullet is a 360,000 rpm. If you are sure you are shooting only varmints a slower 12 or 14 twist barrel is fine, if you need heavier bullets for medium game a 9 or even 7 twist barrel may be more suited for your needs.

mc223
February 3, 2011, 12:40 AM
As an example (only), let's say all bullets need to spin at X rpm to stabilize. Short light bullets will get down the tube quickly at say 3000 FPS on exit, so out of a 24 inch barrel, they need a 1:12 twist. A heavier slower accelerating bullet will only exit at 2/3 the speed, so it needs 30% faster twist to get the same rotation rate, and because it's a longer cylinder, it may need even more? There are other factors, but that's the simple idea.

Nice effort

Brocluno made some valid points Name them.
tactikel Just more wordy .

Originally Posted by DRYHUMOR View Post
Street Terms?

Condensed version.


1.) It takes less twist to stabilize a given bullet at high velocity than at low velocity.

I had a real hard time with this due to the data pointing towards truth. Although the increase in velocity did indicate a slight increase in the stabilization factor for a short time. The stability factor did decline past that point of velocity increase.

2.) At the same velocity in the same caliber, longer (pointed) bullets require faster twist rates than shorter (round nose) bullets of the same weight.

The shape of the projectile has nothing to do with the twist rate. You are correct on the length part though.

3.) Heavier bullets require faster twist rates than lighter bullets of the same shape.

The Length not the weight





The original question was:why is it that a 1:8 twist will stabilize bullets from 55gn to 90gn when a 1:12 doesn't stabilize anything above 60gn

The predictions of the formulas in post #10 for the 1 in 8 have the broadest spread of projectiles by length when compared to the 1 in 12 predictions.


Long bullets need faster twist than a short bullet of the same diameter.





This is the second hit and run post from this op on the same subject reworded.

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