Does Twist Rate Make a Difference in Velocity?

[helotaxi]

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

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.

[FUBAR]

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 ....

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:

[helotaxi]

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.

[Jamie]

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.

[ellobo]

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?

[FUBAR]

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)

[helotaxi]

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

[FUBAR]

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/50calibre/material_50cal_bullets.htm

[helotaxi]

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.

[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?

[scope eye]

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

[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...

[Gmac5]

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

[Gmac5]

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

[helotaxi]

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?

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.

[helotaxi]

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...

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.

[FUBAR]

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....