Does Twist Rate Make a Difference in Velocity?

Found this on 6MM BR, now I know where and why I get the Violent FPS I get.

Yes. Our tests show that, with the 80-90gr bullets, a 1:10 or 1:12 barrel will give you as much as 80fps more velocity than a 1:8 barrel, shooting the exact same loads. Fast twist (1:8) barrels have more drag and friction, which can slow the bullet down. Ideally you want to use the slowest twist rate possible that will stabilize the bullet you choose to shoot. For a dedicated “point-blank” 100/200 yard Benchrest gun, you want a twist rate from 1:13 to 1:15. But if you want to shoot both light (60-80gr) and heavy (100gr+) bullets, stick with a 1:8.

Tanks Dean

Not only ,fast twist barrels will cause more spin drift , more gun torque , and magnify any bullet jacket/ core runout with larger groups.

But if you want to shoot at long range, you have to accept those facts because you need a fast twist barrel to stabilize the long low drag bullets required for the task.

More spin drift isn't an issue, since it is consistent and predictable. Nothing you can do about gun torque other than go with an offset, wide forearm. Good bullets will have negligible runout. The end result is that the difference in accuracy between a fast and slow twist barrel is more than offset by the decreased sensitivity to wind deflection that the long, high BC bullets bring to the table. For other than point blank benchrest rifles, it isn't worthy of consideration.

To help myself understand what twist does or how to select a twist I know that my 22LRs are 1:16 twist for the light little bullet that travels an avg of 1000-1500 fps. It doesn't require much twist for the tiny 22LR to stay on course. Then I open the cylinder on my 44mag revolver and look down the barrel. I don't know what the twist is in my revolver but I know that the big ol' 44mag needs much more twist to keep it going straight and it's quite obvious that the 44mag has more than a 1:16 twist. The 44mag flies about the same speed as a 22LR..
As far as velocity it's not hard to imagine that friction is required to generate twist in any bullet and friction slows things down.

Helotaxi ,i agree i was just stating what i remember about pros and cons of fast twist . I dont think its worth having fast twist if your only shooting range is gonna be 100-200 yds . Or shooting benchrest. Fast twist has its places . The weight of the projectile is not what determines twist it is the length, bu since our bullets have usually bee lead w wo a jacket ,twist rate was determined by weight. I hope this makes sense ,i ve not had my coffee yet.
No brag , just fact

With cast "Boolits" you have to keep the MV and the twist slower, Boolit concentricity is one of the issues, over spinning creates unstable Boolit.

Lilja barrels has a great page about twist, caliber, bullet weight, and bullet design:http://www.riflebarrels.com/products...wist_rates.htm

However the discussion has moved away from the OP

I think he stated that with all things being equal a faster twist rate increases MV?

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Originally Posted by Mach2

To help myself understand what twist does or how to select a twist I know that my 22LRs are 1:16 twist for the light little bullet that travels an avg of 1000-1500 fps. It doesn't require much twist for the tiny 22LR to stay on course. Then I open the cylinder on my 44mag revolver and look down the barrel. I don't know what the twist is in my revolver but I know that the big ol' 44mag needs much more twist to keep it going straight and it's quite obvious that the 44mag has more than a 1:16 twist. The 44mag flies about the same speed as a 22LR.

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I think that you'll find that the .44 mag typically uses an even slower twist than 1:16. Ruger ships theirs with a 1:20. Absolute bullet length isn't the issue, the issue is the ratio of bullet length to bullet caliber. A 1" long bullet in .224 caliber will require more spin to stabilize than a 1" long bullet in .30 cal.

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As far as velocity it's not hard to imagine that friction is required to generate twist in any bullet and friction slows things down.

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It's more a matter of conservation of angular momentum than friction. It requires energy to add spin to the bullet. The more spin you add the more energy it takes.

I would not be shooting a 224 in a 30 cal bore, now would i?
No brag ,just fact

Not my point. The point is that the bullet length, expressed in calibers, as well as muzzle velocity determine the rate of twist required.

Oh, sorry , agreed

In the OP the testing was done with; the same length/grain bullets, same powder type/weight, same barrel length, same cal, with the only difference being twist rate...and the MV was 80 fps slower with a 1-8 vs 1-10

Of course you can't say for certain that the rate of twist was the sole contributing factor to the difference in velocity. Two barrels from the same maker, of the same length and with the same rate of twist will fire the exact same loads at different velocities.

Most of the stuff on the accurateshooter site is put through the ringer so to speak, I'm surprised by 80fps MV difference, I would find 20 to even 40 fps within standard deviation, but 80? That's the nexus of the quandary.

As Helotaxi said earlier, the discussion has to go beyond point blank range, but it's more than just bullet length and includes many components-- Spin drift – affects windage in the direction of bullet spin, proportionally to drop velocity (which equals g * time-of-flight), and proportionally to spin speed, which is proportional to muzzle velocity and twist rate. So, for example, doubling the twist rate and holding all else equal will double spin drift windage...

The density of the bullets material will effect things greatly--make two bullets of the same caliber, the same length/shape and change the material density, and the longrange trajectory dramatically changes

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Originally Posted by FUBAR

The density of the bullets material will effect things greatly--make two bullets of the same caliber, the same length/shape and change the material density, and the longrange trajectory dramatically changes

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This is, of course all captured in the bullet's BC.

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Originally Posted by helotaxi

This is, of course all captured in the bullet's BC.

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No not always...We have been here before--weight of bullet is still the common rule for twist rate, this rule changes with long bullets that are designed/shaped with less dense material....

Here is what Lilja says about barrel twist selection (it's a note on their bullet weight barrel twist chart)

"This chart is based on jacketed lead-core bullets. With some monolithic and plastic-tipped bullets the above twists may be too slow. The reason for this is that copper and yellow-metal bullets are less dense than lead-core bullets and so with bullets of an equal weight the monolithic bullets are longer. And length has a lot of influence on twist required. The plastic tips on some bullets also add length without weight."

But you were talking length and weight with regard to long range ballistics in what I quoted, which isn't a product of barrel twist...

The materials outlined in the Lilja note are less dense and they can change the length & shape of the projectile ...my statement is in relation to bullet weight and barrel twist, and it's why a lighter weight bullet can be shot in a faster twist barrel.

However it still does not answer the OP..

A short bullet can be shot in a fast twist barrel as well. Read what you wrote that I quoted and point out where you were talking about rate of twist?

The only thing that the Lilja text explains is why a lightweight bullet might need a faster twist barrel because of its construction. No one that knows anything about the topic will dispute that.

In theory, a faster twist will reduce velocity; however differences in individual barrels make getting an accurate gauge on the real world effect essentially impossible.

I did not say a short bullet could not be shot in a fast twist barrel-- if that was interpreted then I miss-stated my position.

Relative to caliber, weight, and to the Lilja page for twist selection for example, Lilja does not recommend a 1-8 twist for 155 gr bullet for a 308, however many will jump up and down an point out that they can shoot a particular 155 grain bullets just fine.

The Lilja note on material density and design foretells this miraculous event in advance!

What the OP quoted was someone at the accurateshooter website stating that they had concluded that with all other things being equal ( same everything but twist rate ) a 1-12 barrel shot 80 fps faster MV than a 1-8 barrel

To me, and since he posted it, to the OP it merits discussion without summarily dismissing it...

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Originally Posted by helotaxi

A short bullet can be shot in a fast twist barrel as well. Read what you wrote that I quoted and point out where you were talking about rate .

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In another debate we went back and forth on density (weight ), BC, etc....

I will re-state: Material density of a projectile can directly impact trajectory--In exterior ballistic performance conceptual design theory, Drag Coefficient varies in direct proportion to sectional density. In other words, with all other things being equal, design a bullet out of a material 50% denser and drag coefficient increases by an equal 50%.

I agree that in current production Longrange supersonic target bullets in general, bullet density is directly correlated with BC.

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Originally Posted by FUBAR

I agree that in current production Longrange supersonic target bullets in general, bullet density is directly correlated with BC.

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It's not just "current production" bullets. There is no way to separate the two and BC captures everything that you need to know about the bullet to predict its flight path. Bullet length, expressed in calibers, tells you almost all you need to know about the bullet itself to determine what rate of twist is required to stabilize it.

The discussion prompted by the OP is the effect on velocity of barrel twist. The reason that twist should, in theory, affect velocity is simple conservation of angular momentum. You're accelerating the bullet with regard to rotation. That requires force and that is force that cannot be used to accelerate the bullet down the bore.

If one were so inclined they could model the force required and actually include that calculation as part of an interior ballistics model. That model could then tell you how much of any velocity difference noted between two barrels is due to a different rate of twist and how much is because of differences in the barrel itself. There are plenty of accounts of individual barrels varying by over 100fps, despite being identical on paper, to attribute the 80fps in the initial post entirely to differences in rate of twist. To actually test that with any degree of statistical significance would take a very large sample size of barrels and a very high round count.

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Originally Posted by helotaxi

It's not just "current production" bullets. There is no way to separate the two and BC captures everything that you need to know about the bullet to predict its flight path. Bullet length, expressed in calibers, tells you almost all you need to know about the bullet itself to determine what rate of twist is required ....

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No "BC does not capture all you need to know", exact muzzle velocity is assumed.

Take a modern benchrest .243 rifle, build one .700 BC bullet that is 117gr & build another .700 BC that is 234 gr and the external ballistics change proportionally e.g. material density makes a difference.

That's the reason I used the term "current modern supersonic target bullets", because no one markets a .243 .700 BC 234gr bullet. I was not inferring that the basic drag model rules used to approximate the "correct" drag function for the bullet shape changes.

In modern Drag Function Models the BC is relatively constant over the range of useful velocities, because no one would design and market that dense 700 BC 234gr .243 bullet for longrange target competition against the 117gr version.

However, if the 234gr bullet could be fired at the exact same velocity, using the same Drag Function Model and the same settings, it would render the same trajectory output. Just don't change the brand of Drag Function Models, "Ballistic" for iPad will render slightly different trajectories than JBM:cool:

Actually the BC changes, sometimes drastically, over the useful range of bullet velocities, but very few sources exist for finding the variable BCs. Sierra lists them. Litz lists them for the bullets that he tested in Applied Ballistics.

If two bullets have a BC that changes the same way, in other words the BC's are actually the same (at least over the applicable velocity range), and are fired at the same velocity, they will fly the same, differences in mass notwithstanding. Bullet mass by itself is irrelevant to the ballistics equation. All that you need to know about the bullet, not the load, the bullet, is captured in the BC. Velocity is data about the load and the rifle combination. Same BC, same velocity, same trajectory. We've been over this before. Check credible references.

ScCope Eye,
To actually answer the question you asked, yes twist rate will affect velocity. Personal experience with 2 different calibers, each one in a fast and a slow twist. In both cases the slower twist barrels were faster. People will and can (as you see) argue about it but first hand experience tells me what actually happens.

My degree is in mechanical engineering and having read texts, ballistic tables and now this ongoing discussion. It seems to me that the study of ballistics is not an exact science so to speak. The list of variables is large and open to interpretation. All points made so far are valid to a degree. So, I defer to Murphys laws which I assume you are all aware of. Please see my signature below for the interpretation. Not even discussed is the condition of the bore, no. of grooves, dia.tolerance in the bore size, cut rifled vs. other methods, lapped and on and on. I am simply trying to make a point here, not start a whole new set of threads.

El Lobo?

El Lobo

You are correct, it's all theory and approximation...use 3 different drag models, with all things being equal, they will produce 3 different trajectory(s)

And if you're using 3 different models, at least 2 of them will be wrong for the application anyway ;)

The real issue is that no one noticed the big mistake in my density theory...it was sort of a trick question.

Two 6mm bullets of equal shape & size ( was using the 117 DTAC for my example) one weighing the normal 117 gr and the other 234 gr, made out of something of with specific gravity greater than Tungsten, could never have the same BC because they have the same form factor.

The 234 gr version would have a BC of around 1.4 because of the double density, or twice that of the 117 gr DTAC, which has a .700 BC. It's the direct proportional correlation of density/weight to BC

BC by definition is the ratio of a bullets sectional density to its coefficient of form or form factor and it can be written as:

C = SD/i = w/id2

Where C = ballistic coefficient
SD = sectional density
i = form factor
w = weight of bullet, lbs.
d = diameter of the bullet, in. Sq



Form factor can be found in design by precise measurement of the projectile or by using a chart. In practice it's confirmed by doing a controlled environment/range firing of the bullet with two chronographs positioned in the trajectory and/or radars.

Now if we run the 6mm 234 gr DTAC with 1.4 BC through a Ballistic model? :cool: I just wanted to make sure everyone understood weight really does directly impact BC when all other things are equal.


If you want to read a good article on density, twist rates, and external ballistics Daniel Lilja has a great one:
http://www.riflebarrels.com/articles...al_bullets.htm

I was working under the assumption that the BC was held constant meaning, by definition, that for the bullet weight to increase the form factor number had to get worse. Happens in the real world all the time.

Howa's 1500 in .223 is sold in a 1:12 twist rate. Stevens 200 in .223 uses a 1:9 twist rate. Can anyone make a guess why the two companies use radically different twists for the same caliber?

Jamie I knew that it did I just posted they article since I thought it was an interesting read, I have been saying it for a long time and have my own personal data to back it up.

Tanks Dean

Your definition is ture only if the form factor increases, but with the correct design form factor normally decreases. By caliber the lowest form factor bullet will have a very high SD, and therefore weigh more.

Try reviewing the highest BC bullets by caliber, as the BC increases, so does the sectional density of the bullet, plus the form factor stays close to the same or is reduced. Form factor is completely related to bullet shape, but it takes material to design it.

That's how the heavy bullet does better in the wind rule got propagated and misunderstood, the rule is true when form factor is not disregarded.

With two pieces of information about a bullet, caliber & BC, one can extrapolate the bullets form factor number.

Again, I suggest reading Dan Lilja's article...

WOW ,no two barrels are EXACTLY the same , TOO MANY VARIABLES , it should slow the bullet down (fast twist ) BUT in real life poa to poi I think other more important factors should be trained for .
Excellent to see others as I am .
Gary macdonald

So while I have your attention , and not wanting to change this thread about velocity , did anyone read the new one about lead free primers and velocity loss and excessive barrel wear ? The thinking behind this one is the lead in the barrel (primer) lubricates and reduces friction much the same way leaded gasoline use to . I hope someone else read it . And no i didnt bring it up to raise blood pressure,just because by your responses you have a much greater grasp on these subjects than most .
Thanks Gary

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Originally Posted by Mach2

Howa's 1500 in .223 is sold in a 1:12 twist rate. Stevens 200 in .223 uses a 1:9 twist rate. Can anyone make a guess why the two companies use radically different twists for the same caliber?

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Howa sees their rifle as a varmint rifle and Savage/Stevens sees theirs as a more flexible rifle able to be used as a varmint rifle or for longer range work with the intermediate length/weight match bullets or able to shoot the longer monolithic bullets that some folks like for deer.

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Originally Posted by FUBAR

Your definition is ture only if the form factor increases, but with the correct design form factor normally decreases. By caliber the lowest form factor bullet will have a very high SD, and therefore weigh more.

Try reviewing the highest BC bullets by caliber, as the BC increases, so does the sectional density of the bullet, plus the form factor stays close to the same or is reduced. Form factor is completely related to bullet shape, but it takes material to design it.

That's how the heavy bullet does better in the wind rule got propagated and misunderstood, the rule is true when form factor is not disregarded.

With two pieces of information about a bullet, caliber & BC, one can extrapolate the bullets form factor number.

Again, I suggest reading Dan Lilja's article...

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I understand everything in the article. I also understand the relationship between form factor, bullet weight and caliber. My point was that just because bullet weight increases the form factor number doesn't automatically get better and the BC doesn't necessarily increase. In some cases it does. Hornady has done some really interesting design work in the last few years along with Berger in optimzining form factor to make bullets that have a very high BC but are not super heavy. The .224cal 53gn Vmax is one example and the .308cal 208gn AMax is another. In both cases the bullet ends up with a BC that is equal to or higher than heavier bullets in the same caliber. Their relatively light weight and short bearing surfaces mean that they can be shot much faster than a heavier bullet and their high BC means that they do a very good job of retaining that velocity and resisting wind drift. The little Vmax will almost mimic the trajectory and wind drift of the 69 SMK out past 500yds. The 208 Amax will vastly outperform pretty mcuh any other .30cal bullet at long range.

The idea that heavier bullets do better in the wind is completely born of the gross lack of understanding that most shooters have about external ballistics. Guys that have studied the long range shooting challenge have a better understanding than most, but even some of them don't get it.

I have two 6-284s one is a 1-8 and the other is a 1-10 because of the range of bullets that they will shoot and the type of shooting I do with each one....

I also have a 22-250 that has a 1-14 and a .243 with a 1-10 each work for what I do with them, and every barrel is a custom order, with 27 inches as the shortest of the barrels....

Best advice, decide what you are going to shoot with it, majority of the time, look at the barrel makers charts, study and understand the bullets available, and the subsequent ballistics for the sum of the components....