Reply With QuoteNot only ,fast twist barrels will cause more spin drift , more gun torque , and magnify any bullet jacket/ core runout with larger groups.
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?
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.Originally Posted by Mach2
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.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.
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
This is, of course all captured in the bullet's BC.
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...
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.
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.
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
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?
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