Can We Talk About Twist Rates?

[Monk Medic]

Hopefully I'm sharing this in the right space, somewhat new here.

I have a SIG SSG-3000 which I love, have been shooting it for 5 years, fits me well, shoots like a dream, has the 24" barrel.
The twist rate is 1:11, and is spot on accurate with the 175 gr Federal Match, largely one hole
at 100 yards, and repeatable data out to 1,000 yards.

SIG also made a short production run of the same rifle in the 18" barrel, but keep the twist
rate the same, 1:11 (also seen it given as 1:12), despite a nearly 25% reduction in barrel length.
I've noted that my accuracy on that barrel is considerably looser than
the 24" barrel. I've tried the 168 gr and 175 gr, seems to be about the same.
I've tired this on and off over the last year, to be sure it wasn't just a perception
thing, and it's not, the groups are noticeably "looser" and become more so at distance,
from what I assume to the subtending angle flight path.

Now, I understand that the propose of the rifling twist is to impart gyroscopic stability to the
round when it leaves the barrel, so it seems to me the 24" barrel has more "time" to act on
the round than the 18" barrel.

Question 1:
So my questions relates to twist rate, is there a formula or standard for twist rate
based on the variables of barrel length, and bullet weight?

If so, I'm assuming that the short barrels would need a "faster" twist rate to stabilize
the round, given that it has a shorter time in the barrel.

Question 2:
I see that Benchmark Barrels offers SSG barrels, they seem like good folks, have purchased
the factory mags for the SSG from them.

Is there another barrel manufacturer for the SSG?

If I were to spec a replacement 18" barrel, what would be the ideal twist rate for the 168 gr
projectile?

I've also considered going with an 16" barrel, at the cost of muzzle velocity, to make the rifle
easier to carry with me. Thoughts on this?

Thank you!

 

[BurnOut]

Well, in regard to question 1, it's not so much the time that the projectile spends in the barrel being acted upon by the rifling that results in a given level of gyroscopic stability, but it is instead the time that the projectile spends in the barrel being acted upon by the expanding gasses from the powder charge. In other words, muzzle velocity.

Gyroscopic stability for a given projectile is achieved by the projectile spinning at an appropriate (for the projectile and environmental conditions) RPM as it leaves the barrel... and if the twist rate of the barrel remains constant but the muzzle velocity increases, so then must the RPM at which the projectile leaves the barrel.

It's possible that you're right on the (lower) edge of projectile stability out of the longer barrel, and the muzzle velocity drop that you experience when you go to the shorter barrel is enough to drop the projectile out of stability... but I really have my doubts about that. What is more likely, IMO, is that the harmonics of the 18" barrel are different than the 24" barrel, and the load that shoots so well out of the long barrel simply doesn't work in the short barrel. I'd be willing to bet that if you did some load dev using the 18" barrel, you could find a load that results in the same kinds of groups that you get out of your 24" barrel.

Of course, it's also possible that there is a flaw of some sort present in the 18" barrel; maybe the chamber is different, or the crown has a ding in it, or any number of other things.

In regard to question 2, a 10-twist is the generic answer to, "Which 30-caliber twist rate will serve me best", though we're starting to see some heavy projectiles that require a 9-twist to perform their best... but I doubt that you'll be launching many 230+ gr projectiles out of that .308. If it was me, I'd go with a 10-twist and think no more about it.

 

[spife7980]

To question #1: sort of and yet not technically? The longer barrel doesnt twist more, the twist rate is constant and will be constant no matter how long the rifling is (not discussing gaintwist which is a new or old trend depending on your perspective). Now, the longer the barrel the faster the bullet will typically be going and thus the more RPMs, but you can load a long barrel to shoot slow and it will twist the same exact as a short barrel would with equal twist rate.

RPM= Muzzle velocity x 720 / twist rate

http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/

So yeah, if you were on the edge of stabilizing a bullet in a short barrel a faster twist would help.

I dont have any experience with the Sigs but I wouldnt assume that the 18" is magically worse than the 24, its just different harmonics. Im sure you could tune your reloads to shoot well in the 18 and they might not do well in the 24 at all. If you got another 24 you couldnt count on it performing like your current 24. Barrels are individuals.

In 30 cal I would go 10 twist, theres no reason not to. Benchmark are a known quality barrel manufacturer.

 

[V-Ref]

Take the average of 7+ measurements of the distance needed to rotate a cleaning rod a full rotation with a nylon brush. Most barrel length/twists you can use two rotations and divide by 2. Maybe not so with the 18" barrel above.

There's tolerances in all machining operations. Even the best send something other than the spec out the door from time to time.

 

[Monk Medic]

Alright, some excellent information there, thanks all.

I’d like to design my barrel around my round, 168 gr, and barrel length, 16”, and max target distance.
What I was hoping for was some sort of standardized formula that give me a twist rate based upon these factors.
Data to input would be - Ideal gyroscopic rotation rate / Projectile weight / Barrel length / Twist rate / Muzzle Velocity

From what I read, it seems like a 1:9 is the starting point for 168 gr projectile, which will assure adequate stability.
Agreement on that?

Any thoughts on the folks at Benchmark barrels?
They seem to be the only ones which advertise custom replacement barrels for the SIG SSG-3000.

Thanks!

 

[...]

Alright, some excellent information there, thanks all.

I’d like to design my barrel around my round, 168 gr, and barrel length, 16”, and max target distance.
What I was hoping for was some sort of standardized formula that give me a twist rate based upon these factors.
Data to input would be - Ideal gyroscopic rotation rate / Projectile weight / Barrel length / Twist rate / Muzzle Velocity

From what I read, it seems like a 1:9 is the starting point for 168 gr projectile, which will assure adequate stability.
Agreement on that?

Any thoughts on the folks at Benchmark barrels?
They seem to be the only ones which advertise custom replacement barrels for the SIG SSG-3000.

Thanks!

thats a little fast....but should work.

1:10
1:11.25
1:12
would also be sufficient.

we base twist rate primarily off projectile weight (more specifically projectile length)....barrel length and distance to target are irrelevant.

velocity also plays a small factor, but 308s are VERY resistant to velocity changes with changes in barrel length....so its not an issue we generally take into account for "standard" loadings.

if you start playing around with really heavy bullets or subsonic loads, your velocity will play a larger factor

 

[Monk Medic]

Outstanding sir!
I’m currently running at 1:11 right now with what appears to be some stability issues, so perhaps the 1:10 is the way to go.

 

[...]

Berger has a pretty good "stability calculator"
http://www.bergerbullets.com/twist-rate-calculator/

 

[...]

Outstanding sir!
I’m currently running at 1:11 right now with what appears to be some stability issues, so perhaps the 1:10 is the way to go.

now when you say you are having "stability issues".....what makes you think that?

im asking because ive never really heard of anyone having stability issues with a 1:11 twist barrel......hell, i rarely hear of people having issues with 1:12 twists barrels even shooting heavy 175 and 180gr bullets.

shots keyholing? erratic or unpredictable performance?

simply having larger groupings is not an indicator of bullet stability.

 

[Monk Medic]

Same model gun with the 24” barrel shots nearly one hole groups, the 18” barrel is still a tight group, but more like .5-.7 MOA.
The 24” vs the 18” is nearly a 25% reduction in barrel length, yet retains the same twist rate. I actually wanted to shorten it to 16”.
Group appears below, and yes it is consistent across multiple days and range visits.
My notes show that the shot at the left side of the diamond was the initial CCB.
May not be the rifling data, may be barrel or crow defect, though I can’t see it. See below.
Thanks!

 

[Monk Medic]

Berger has a pretty good "stability calculator"
http://www.bergerbullets.com/twist-rate-calculator/

Hey, that’s pretty slick, and right along the lines I was thinking!
Thank you sir!

 

[goosed]

Since you asked about formulas...

Berger's calculator uses the miller stability model. Miller's model (2005) is simply an adjustment to Greenhill's model (1870's) and gets you in the ball park with boat tails, but it falls well short with flat base bullets and even with boat tails it ain't perfect...

There are others as well:
Robert McCoy created a more complex model "Mcgyro" (1980's) that is available through JBM or Kolbe's, but requires far more inputs.
Prodas, I believe, had a model specifically for solid bullets though I am not sure on availability as Arrow tech, to my knowledge, no longer offers it.
Courtney developed models for plastic tip (2012), open tip match (2014) and there may be one for aluminum tip as well. Formula and white papers are readily available on google, but I'm not aware of any calculators currently using this model.

You didn't mention if group size is linear as distance increases, i.e. 1moa@100yds, 2moa@200yds, etc., if so this would point to a barrel that is simply less accurate vs a stability problem.

 

[Monk Medic]

Since you asked about formulas...

Berger's calculator uses the miller stability model. Miller's model (2005) is simply an adjustment to Greenhill's model (1870's) and gets you in the ball park with boat tails, but it falls well short with flat base bullets and even with boat tails it ain't perfect...

There are others as well:
Robert McCoy created a more complex model "Mcgyro" (1980's) that is available through JBM or Kolbe's, but requires far more inputs.
Prodas, I believe, had a model specifically for solid bullets though I am not sure on availability as Arrow tech, to my knowledge, no longer offers it.
Courtney developed models for plastic tip (2012), open tip match (2014) and there may be one for aluminum tip as well. Formula and white papers are readily available on google, but I'm not aware of any calculators currently using this model.

You didn't mention if group size is linear as distance increases, i.e. 1moa@100yds, 2moa@200yds, etc., if so this would point to a barrel that is simply less accurate vs a stability problem.

Good data, thank you!

Yes, the group progressively opens up as the range increases, which happens even with a tighter grouping.
I’m just not happy with the size of the group at 100 yards, and being that large at 100, it of course gets worse further out.
My purpose on checking on twist rate is if that I can going to do a 16” barrel, then I’d like to pick an effective twist rate for that length.

Thanks!

 

[Squibbler]

If I were going .308 again I'd go 1:8 no matter what length barrel, shooting 175's and 178's. Unless my goal was small groups at short distances then I'd use the minimal twist rate for the particular bullet I picked.

 

[SupressYourself]

Benchmark barrels are top-notch. I would put them in the same league as Bartlein, etc.
I have a 26in on my 6.5 Creed Desert Tech and it shoots one hole groups.

 

[101stinfantry]

Benchmarks are pretty damn good, I have one in 6.5CM on my SSG 3000. They are by no means cheap if you need them supply the barrel extension though.