Twist rates why choose one over another

[B J Atkinson]

My barrel was a 1-13.75 twist just for reference.

Later, Frank
Bartlein Barrels

That's interesting Frank. In 2009, I used one of your 13.75 barrels at the WBC in South Africa. Stuart Elliott and I were fortunate enough to be part of the gold medal 2-Gun team and doubled up on my rifle - that barrel was superb! I have since managed to obtain another one in 13.75, and it is showing signs of being just as good. Bullets used were 67BT on an 825 jacket.

Brendan Atkinson

 

[Frank Green]

That's interesting Frank. In 2009, I used one of your 13.75 barrels at the WBC in South Africa. Stuart Elliott and I were fortunate enough to be part of the gold medal 2-Gun team and doubled up on my rifle - that barrel was superb! I have since managed to obtain another one in 13.75, and it is showing signs of being just as good. Bullets used were 67BT on an 825 jacket.

Brendan Atkinson

Hey Brendan! I'm primarily not a bench shooter so when the time came to do a barrel for a gun for myself I just split the difference between the 14 and 13.5 twist.

On a .830" bullet length at 3000fps. I come up with a 13.36 twist as a min. but at 3200fps. I come up with a 14.09 as a min. These are paper calculations.

You go with some of the twist rate calculators and at a default of 59 degrees F and at 1100ft. elevation both of the above twists and velocities says it's marginally stable. Temperature, elevations, actual velocity etc.... all play a part.

Just quick calculations for grins......

Thanks for using the sticks!

Later, Frank

 

[tim]

I actually decided to try one of Frank's at 13 1/4" just to see. I figured it would hang on there at 100 and maybe have a slight edge at 200. Shoots damned good at 100 with 65's & 66's haven't shot 68-70 grainers in it.

 

[JRB]

too much twist

Do you think that being over twist can cause bullet instability or lack of accuracy? and to what degree.
say if you had two equal barrels one in 10 twist and one in 14 for your 6 PPC.
would there be measurable loss of accuracy in the 10 twist barrel?
Jim

 

[Lee Martin]

Do you think that being over twist can cause bullet instability or lack of accuracy? Jim

Yes it can. In theory you want the bullet at the edge of being stabilized for precision shooting. Most 13 to 14 twist barrels yield stability factors between 1.10 and 1.25 depending on the bullet (length is as important as weight, if not more). Greenhill's model suggests 1.40 - 1.50, or higher, is the gyroscopic factor for complete stability. Anything less than 1.0 is deemed unstable. It seems dispersion becomes more of a factor when spun faster.

-Lee
www.singleactions.com

 

[Fergus Bailey]

Do you think that being over twist can cause bullet instability or lack of accuracy? and to what degree.
say if you had two equal barrels one in 10 twist and one in 14 for your 6 PPC.
would there be measurable loss of accuracy in the 10 twist barrel?
Jim

Depends on multiple factors. In precision accuracy, it is often not possible to give definitive "yes"/"no" answers in the precision shooting sports. And when you talk about "measurable loss of accuracy", I believe it depends on what equipment you are using.

Short range BR shooters are looking for any minute incremental advantage they can get, so they work very hard to spin their bullets just enough to stabalise them. And the equipment is so capable, you can see the difference if a barrel/bullet combination is not working. If you take the same approach with a factory rifle, there is every chance you would never tell the difference on the target.

For long rang shooting, there is some evidence that the relatively recent high BC bullets take some stabalisation, so a faster twist than was previously considered adequate may be a better option. Hence we are seeing more demand for twists like 1/8" 30 cal, or 1/8" 7mm barrels.

 

[JRB]

If your PPC shoots in the 1's with your favorite twist barrel
and you swapped out an "identical" barrel in say a 8 twist
what would you see your groups getting larger by?
.1? .2? or more?
Jim

 

[David Halblom]

that much

If your PPC shoots in the 1's with your favorite twist barrel
and you swapped out an "identical" barrel in say a 8 twist
what would you see your groups getting larger by?
.1? .2? or more?
Jim

or more. basically because of precession..
Over stabilization is as bad as under

 

[alinwa]

If your PPC shoots in the 1's with your favorite twist barrel
and you swapped out an "identical" barrel in say a 8 twist
what would you see your groups getting larger by?
.1? .2? or more?
Jim

I do this or have done this a lot. For instance I've got 6BR's, 6X47L's and have had .243AI's in all common twists from 14 to 7.5

I've blown up lots of bullets.

I've isolated some EXTREMELY well made bullets and some few that when spun up will show more dispersion.

Generally I'm hard-pressed to see the difference @ 100yds

But I will always tell anyone who cares to lissen, "if you're only gonna' have one twist, make it a TIGHT twist"

 

[JohnsonGunsmith]

fast twist and wind

Ok all due respect I have shot a lot of fast twist barrels lighter bullets. I have not seen this massive degrade in accuracy that is being mentioned. I only had a 12 twist 30caliber barrel and went ahead and chambered it up for my dad's first bench gun in 30 BR. We used 118 bullets in it and the main difference was this gun would beat the wind better than my 30BR WITH THE 17 twist. Saw no accuracy problems the gun shot great and I even used it when my dad couldn't go to a match. It was a great 200yd 30 I could out shoot my 17 twist every time at 200yds. Everyone told me I was nuts but the results speak for themselves. Done 12 twist PPCs no problem shot well I have went to faster twist now and am shooting better than I ever have this year. I am convinced that the faster twist stabilizes the bullet better and in windy conditions this allows the bullet to be less effected by the wind. Some may disagree and that's fine I need more Info and its early in the process but this is what I'm seeing and would love to hear what you guys think.

 

[Frank Green]

Ok all due respect I have shot a lot of fast twist barrels lighter bullets. I have not seen this massive degrade in accuracy that is being mentioned. I only had a 12 twist 30caliber barrel and went ahead and chambered it up for my dad's first bench gun in 30 BR. We used 118 bullets in it and the main difference was this gun would beat the wind better than my 30BR WITH THE 17 twist. Saw no accuracy problems the gun shot great and I even used it when my dad couldn't go to a match. It was a great 200yd 30 I could out shoot my 17 twist every time at 200yds. Everyone told me I was nuts but the results speak for themselves. Done 12 twist PPCs no problem shot well I have went to faster twist now and am shooting better than I ever have this year. I am convinced that the faster twist stabilizes the bullet better and in windy conditions this allows the bullet to be less effected by the wind. Some may disagree and that's fine I need more Info and its early in the process but this is what I'm seeing and would love to hear what you guys think.

I'll start with fast twist and wind drift but then go from there......first off I will say what looks like on paper isn't always in the real world is not always the same thing.

http://appliedballisticsllc.com/ballistics/

Change the twist rate and use any bullet you want. Changing to a faster twist rate and using the .30cal. bullet 175gr. at 2650fps. Going from a 1-12 twist to a 1-10 twist induces what is called spin drift. Does it make much of a difference? In my opinion no. According to the numbers a 10 twist barrel will have .04" more drop and .27" more drift at a 1k yards.

Can a bullet be over stabilized? I'm going to say yes but like was pointed out before there are a lot of factors and variables that each shooter has to contend with.

First question I ask is what type of bullets are you shooting? There are poor quality bullets, good bullets, great match bullets and excellent match bullets. This is one of the biggest variables.

Next question I ask is what is your accuracy expectation? Benchrest? Varmint hunting? Hi power match rifles shooting? Hunting? Blah, Blah, Blah!

For a benchrest shooter using choice components I'm going to say the difference in twist rates we are talking about I will say no for the most part as far as being over stabilized or how the bullet flys in the wind etc....

Now to go the other extreme and I've given this example before.....

Started shooting hi power service rifle matches in the late .80's. A couple of years into it I started playing with the AR15a2. Rack stock box Colt rifle! 1-7 twist barrel. That rifle with 69gr. Sierra handloads would shoot arount 3/4" moa at a 100 yards all day long. One time I ran out of my handloads at the range and had some ball ammo with me. The first 5 rounds fired 2 out of 5 went thru the target sideways. Those 55gr. bullets will fly in a 14 twist barrel and I was shooting them thru a 1-7 twist barrel. The first thing you would say is the bullets are over stabilized. I agree to an extent. Keep in mind these are poor quality bullets and this is the key point as well as the overall quality of the loaded ammo. Run out in the ammo etc....

I go back to the range a few days later with more 69gr. handloads, as well as I loaded up some 52gr. Hornady match bullets and some more ball ammo. The 69gr. Sierra's again shot 3/4" moa and sure enough the 55gr. ball ammo one out of 5 rounds went thru the target sideways. I then shot the 52gr. Hornady's. Guess what? The Hornady's held 3/4" moa and shot no better or no worse then the 69gr. bullets. None of them ever key holed either. So where is the over stabilization with the 52gr. bullets? Keep in mind the accuracy the gun gave for what it was etc....you couldn't ask for anymore.

I tell guys poor quality ammo/bullets a faster twist will amplify the run out of the bullet or the run out of the loaded ammo. The more the bullet enters the rifling with a wobble on it per say a faster twist will amplify the wobble and this is what makes the groups worse in my opinion.

An F-Class gun like a 7mm? I have plenty of customers shooting 1-9 twist barrels, 1-8.75, 1-8.5 even 1-8's and even have made barrels for a good friend and shooting buddy and his are 1-9.25 twist (his gun is a 7 short mag and pushing velocities of 3170fps and no I won't give out his load data). Everyone is shooting a 180gr. Berger/Sierra. Regardless of what the chamber is I don't see one being flat out better than the other. My .284win. is pushing 180's at 2900fps. My barrel is a 1-9 twist. Shot it in a mid range match a few weeks ago. At 500 yards I shot a 200-11x. It's a honest shooting rifle that will not let you down. You loose the match with that gun it's your fault not the guns fault.

I do error usually on the faster side than the slower side of the twist. To slow or ragged edge will do nothing for sure but cause problems.

Later, Frank
Bartlein Barrels