rifle twist rates

kentucky hareraiser · Posted: Wed Jan 06, 2010 11:27 am Post subject: rifle twist rates

i don't know much about this subject and i've done some reserch on my t/c barrels the 209x50 endeavor has a 1:28 twist the 30-06 has a 1:10 twist and th 22-250 has a 1:12 . with that beins said what grain of bullets would be best for each rifle .

Aloysius · Posted: Wed Jan 06, 2010 1:05 pm Post subject: Re: rifle twist rates

Some wise words on twist rates from Gerard (GS-bullets):

"With twist rates there are some rules that hold true:

1. If a particular twist will stabilise a particular bullet length, it will stabilise anything that is shorter but not necessarily a longer bullet.
2. Twist rates only have an influence on accuracy if there are serious concentricity anomalies in the core or jacket of a bullet. Turned mono metallic bullets are exempt from this problem.
3. For a hunting rifle, a tighter twist than the CIP specification hurts absolutely nothing. Slower twist than specified will usually detract from terminal performance.

There are more but these are the broad strokes."

so you see: they don't talk so much about bulletweight anymore, now they talk about bulletlenght...

fnuser · Posted: Wed Jan 06, 2010 3:25 pm Post subject: Re: rifle twist rates

This is usually where some one interjects the greenhill formula to make himself sound intelligent but what aloysius said does just as well for common purposes if you want to get a "feel" for where stuff is go to berger bullets and they list reccommended twist rates for their bullets in all calibers, having said that realize that theheir bullets will be a little longer due to their ogive and hollow point design so their twist rates will be shorter for a given weight back to aloy"s rule #1

chambered221 · Posted: Wed Jan 06, 2010 6:51 pm Post subject: Re: rifle twist rates

Here's a discussion we've recently had !!!
www.huntingnut.com/ind...im&start=0

In slimjims opening post there is a link to a website that has a wealth of information on it.
Somewhere in that thread is link to a stability factor calculator....... it could help with determining what bullets to try first for best accuracy.
(I got to find that calculator again Mad

and bookmark it this time)

slimjim · Posted: Thu Jan 07, 2010 5:35 am Post subject: Re: rifle twist rates

Here is the topic chambered221 was refering to.

Rifle Bullet Performance

It is a pretty long and detailed discussion. Page 4 is where the discussion gets heavy into understanding twist rate and its affect on bullet stability. Since the posts are not numbered in a topic on this forum, I included a link to a web page that will allow you to download an excel spread sheet where you can calculate your specific bullet stability factor for you rifle-bullet combination.

Calculating Bullet RPM — Spin Rates and Stability

You will notice that bullet speed, bullet weight, and even air temperture all have an affect. The recommended stability factor is between 1.4 and 2.0.

kentucky hareraiser · Posted: Thu Jan 07, 2010 1:04 pm Post subject: Re: rifle twist rates

very interesting read and links and i put the stability factor calculator in my favorites.. thank's everyone for the info..that's what i like about this crew . (ask a question) and as always there's always answers ,and most of the time ,they all point me in th same direction,with a few exceptions... thanks again

slimjim · Posted: Thu Jan 07, 2010 6:39 pm Post subject: Re: rifle twist rates

Glad we can help! Let us know what you decide to load even if it takes you 2 or 3 months to sort things out.

chambered221 · Posted: Thu Jan 07, 2010 7:00 pm Post subject: Re: rifle twist rates

The stability calculator was part of the jbm website !!! Bang Head

I bookmarked it this time !!!

www.jbmballistics.com/...ab-5.0.cgi

1895ss · Posted: Thu Jan 07, 2010 7:21 pm Post subject: Re: rifle twist rates

this info may be useful to some................

www.exteriorballistics..._Rifle.pdf

www.realguns.com/calc/...flingtwist

slimjim · Posted: Thu Jan 07, 2010 7:53 pm Post subject: Re: rifle twist rates

temperature can have a signficant impact, e.g., colder denser air requires a more stable bullet. I think I'll bookmark this topic also as it has lots of good references.

PaulS · Posted: Fri Jan 08, 2010 12:35 am Post subject: Re: rifle twist rates

Just remember that you can't have too fast a twist but you can have too slow a twist rate.

I should probably make that a bit less absolute so let me put it this way:
I fire pistol bullets that are .7" long from a 1:10" twist 358Win. at 2708fps and get .33" groups at 100 yards. The bullet only "requires" 1:28" according to all the math.
I don't know how fast the twist would have to be to "over-stabilize" a bullet but I would bet that the bullet would disintegrate from centrifugal forces before you would get any "over-stabilization".

slimjim · Posted: Fri Jan 08, 2010 8:03 am Post subject: Re: rifle twist rates

As I've been learning more about bullet stabilization and twist rate, I'm picking up that faster twist rates or over-stabilization can have a negative impact at longer ranges. First the bullet, if highly stabilized, may start to fly in a nose up attitude compared to its dropping trajectory. A stability factor less than 1.5 can minimize this effect. Second, crosswinds may affect bullet point of impact more because of higher lifting forces caused by the increased spin rate. Left crosswind may cause bullet to rise while right crosswind may cause it to drop more. I'm still trying to find out more about this second affect.

At close ranges, inside 400 to 500 yards, more stabilization does not seem to be an issue and can have its benefits.

Azar · Member Joined: Jan 04, 2010 Posts: 275 Location: Utah

The JBM site is an excellent resource, although it does require to take quite detailed measurements of the bullets. You can also download an excel spreadsheet called something like "Bullet stability and twist estimator" from here:

drop.io/unclenick

A fine gentleman by the nickname of Unclenick on The FiringLine Forums created it and put it in his online file repository. It is based on Don Miller's March 2005 Precision Shooting magazine article. The bullet gyroscopic stability factor "s" must equal 1 or higher for a bullet to be stable, with 1.5 being "optimal".

Savagenut · Posted: Thu Jan 14, 2010 1:38 am Post subject: Re: rifle twist rates

Azar wrote:

The JBM site is an excellent resource, although it does require to take quite detailed measurements of the bullets. You can also download an excel spreadsheet called something like "Bullet stability and twist estimator" from here:

drop.io/unclenick

A fine gentleman by the nickname of Unclenick on The FiringLine Forums created it and put it in his online file repository. It is based on Don Miller's March 2005 Precision Shooting magazine article. The bullet gyroscopic stability factor "s" must equal 1 or higher for a bullet to be stable, with 1.5 being "optimal".

Paul, I have a question for you. You stated that you get .33" groups at 100
yards with a bullet that is .7" long from a 358 winchester at 2708fps. Are you using sabboted bullets? .33" groups are less than the diameter of the bullet, how is that possible?

PaulS · Posted: Thu Jan 14, 2010 4:00 am Post subject: Re: rifle twist rates

You measure the outside diameter of the group and then subtract one bullet diameter to get the center to center distance. I am shooting groups that are .33" center to center of the two bullets that are the farthest apart at 100 yards. This is with Sierra PISTOL bullets - 158 grain JHC!
from outside to outside the group is just over 5/8".
I don't get that small a group using the 180 grain bullets.... they shoot .5" groups at the same range.