Getting Technical With Barrel Twist Ratios - Pellet Stability & Env. Variables

Ok, I will throw one at the bbl. experts here. Has anyone tried a smooth twist with an increasing rate of twist from the beginning of the rifling to the end at the muzzle? Think of it as starting the rotation easy then spinning up to the 1 in 16? Would this momentum actually result in the rpm increasing even after leaving the muzzle? Just thinking that one could have the advantages of smooth twist and the effective spin rate provided by traditional bbls.

"Dan25"Has anyone tried a smooth twist with an increasing rate of twist from the beginning of the rifling to the end at the muzzle? Think of it as starting the rotation easy then spinning up to the 1 in 16? Would this momentum actually result in the rpm increasing even after leaving the muzzle? Just thinking that one could have the advantages of smooth twist and the effective spin rate provided by traditional bbls.

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Q. "
Has anyone tried a smooth twist with an increasing rate of twist from the beginning of the rifling to the end at the muzzle?"
As FX/Fredrik is the only one making ST barrels, he is the one to ask. He has tried many things. What you describe is generally known as "gain twist" and in full length rifled pb barrels it has never proven to have any advantage and introduces other problems.

Q. "Would this momentum actually result in the rpm increasing even after leaving the muzzle."
No it would not. With the exception of a very small extra push given by the air charge velocity still exceeding pellet velocity in a few inches of the "intermediate ballistic zone" , there is no force left to accelerate the pellet. As with any rotating object, when the forces producing the rotation are cut off there is no further acceleration. ... Think of a string pull child's top. While the string is being pulled there is possibility to accelerate the top. After the string leaves the top the top can only decelerate.

Q. "Just thinking that one could have the advantages of smooth twist and the effective spin rate provided by traditional bbls."
It would appear that the ST barrel configuration is already producing field accuracy equaling that of other standard manufactured barrels even though, as I have shown, the effective spin rates are only 1/5 to 1/3 that of the traditional barrels. ... Kind regards, Harry.

Cracking thread, thanks for the understanding fellas.

Protegimus

Admittedly, I haven't read this entire post. But, I have an issue with this statement:

an over-stabilized projectile is always more accurate than an under-stabilized one

While this is (semi) correct in most (not all) cases, it is not an absolute fact of life!

Having come from the powder-primer side of the fence, there are limits to over-twisting any given projectile. For example, the statement doesn't address any center of gravity issues with a given projectile, in this case a pellet. Nor does it address ogive ratios, or land-groove contact surface areas, which incidentally are related. If it were absolutely true, then higher velocities would result in higher group. That is NOT the case, as well all know. 

Hello.
Congratulations for a very interested thread, I have been interested for a very long time in this topic.
I´ve been doing simulations with the Don Miller's Twist Rule with the twist rate more comun in airguns and with the twist rate using by Lothar Walther and the results in theory are no good.
Standar twist 1:16 (more comun in airguns) using a Jsb 7.9 gr, long 0.236 at 800fps gives a Sg = 0.93
and a Lothar Walther with 1:17.7 twist using the same pellet and conditions gives a Sg = 0.76
This Sg values in theory are no good but all of us know the quality and accuracy of Lothar Walther barrels.
What is wrong?
I used a excell page from this link
http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/

What do you thing about these results?

Regards

Txebos, 
Good question. I have played with this formula for the past year and I wonder how much does the Diablo style pellets possibly change what that formula is based on which are bullets. Increasing twist rate from what I have read has not proved the best for diablo pellets. Using the formula with a TR of 13 places it at 1.47 but you can also shorten the length of the pellet to .195" and get SG of 1.47. It would be interesting to test results of compairing faster twist rate to shorter ammo...

Great topic!

Well, wouldn't it be nice if everything was as simple as expressed in the original post? Unfortunately, it isn't. 

I find it incredulous that the poster never mentioned RPM of the pellets in the original post. And, I'm just as mystified at the statement that more spin is better! That is an all-inclusive statement which isn't at all factual. Just as incredulous, is the fact that no mention of non-concentricity was been mentioned. 

Now to be sure, we are not talking about firearm components, which are at least 2 magnitudes closer to spec than most pellets. For example, a .22 caliber, 55 grain Nosler ballistic tip bullet, will be out of specification if the variation of weight is more than ± .1 grain! While (supposedly) one of the best pellets available (.177 cal, JSB, Match, Diabolo, 8.44 grain) pellet often varies more than the Nosler (8.42 to 8.46 grains). 

What's more, you often find pellets which aren't (to use a better term) straight—they're sort of bent with respect to the center line. In simple terms, there y're bent (yawed). Thus the more wobble they will have when spun faster! 

Part of the issue here is, firearms have dominated the firing of projectiles for a very long time. Yet, those of us devoted to airguns haven't realized that ballistics between firearms and airguns are mutually applicable. That is, as long as we look at ALL OF THE VARIABLES! 

The biggest variable of all, as alluded to above, are the pellets! Compared to firearm bullets, they're eighteenth century technology. And we need to understand that, before we apply pat rules!

AZ, would you say that the typical formula for proper barrel twist rates in regards to firearms is not a usable model for airguns?

Thank you very much for the very detailed reply!!

I don't wish to be argumentative, but this is another over-simplification:

BOTTOM LINE: RPM’s HAVE NOTHING TO DO WITH the stabilization rate (TR or Twist Rate or Ratio) that the barrel imposes to the projectile or how many revolutions the bullet did before reaching its destination.

At any given FPS the faster the twist the higher the RPM. This should be glaringly evident. If the pellet's (in question) center of gravity is slightly askew, the faster the twist, the greater the flight yaw around its axis. We often see this is slow-motion videos as a corkscrew-type flight. 

Pellets are VERY poor projectiles when compared to bullets of the same caliber. For example, ballistic coefficients are a faction of even a poor bullet with a lousy ogive design. Generally speaking, bullets of any given caliber with high BCs, will require a high twist rates to be stabilized. For example, if we build a .22 caliber powder rifle with say a 1:9 twist required to stabilize high BC bullets, shooting lighter bullets will cause them to tumble. Looking at this another way, the more square area any given projectile has with respect to the lands, the high the twist will need to be for proper stabilization. With respect to pellets, variations in the skirt, head diameters, and even the shape of the diabolo, will have an effect on the requisite twist needed. 

True, we're being a bit critical here, because the variation in FPS isn't very high, particularly with pellet rifles if accuracy is the goal. Depending on the caliber and type of airgun, average FPS almost always falls between 500 and 1,000 FPS (a factor of 2). In the powder arena, FPS may be from as low as 400 FPS to as high as 4,000 FPS (a factor of 10). The same basic scenario can be said about pellet weight as well. All of these factors have a direct influence on the requisite twist rate.

Lastly, except for a few special made airguns, the twist rates hover around 1:14 to 1:10. This fact doesn't give us much leeway. Which means of course, we still have to select the best pellet weight and design, based on the FPS and caliber for any specific gun. Obviously, the resulting twist rate is all but moot!

"Lastly, except for a few special made airguns, the twist rates hover around 1:14 to 1:10."

I have .22, .25 and .30 cal airguns and find this statement to be rather false. My barrels are 1:16, 1:18, 1:22, and 1:26. 

One discussion sticking point seems to be some "either or" concept of pellet stabilization as being the consequence of either rifling twist rate , or pellet spin rate (rpm).
To be clear, gyroscopic stability is the result of gyroscopic torque.
Gyroscopic torque = Angular momentum x Angular velocity.
Both on the right side of the equation, depend upon RPM.

So we cannot discuss the fraction of projectile stability that depends upon spin, without discussing RPM (angular velocity) of the pellet/bullet. We cannot afford to get the rifling twist rate and pellet spin per unit of distance travelled confused with the true spin rate of the projectile which is its spin rate per unit of time viz., RPM.

RPM is derived from rifling twist rate AND projectile velocity. AZ has given an example but I feel has failed to point out that the actual stability rating of the example with 36,000 RPM will not be the same as that with 18,000 RPM even though they both have the same number of spins per unit distance derived from the rifling twist rate.

Rifling/barrel twist rate and the spin imposed on the projectile per unit of travel, is not by itself the stabilizing influence. That twist rate must be matched to projectile velocity in order to produce the stabilizing gyroscopic torque which is designed to keep the projectile orientated in space; and to reduce the possibility of precession and in the case of bullets, from tumbling.

So, as I implied in introduction, stabilization of any given weight, diameter and length of pellet depends on the interaction of both twist rate and ACTUAL spin rate RPM.

One common misconception, which has also appeared here, is that the spin per unit of travel imposed by twist rate is constant throughout flight. It is not.
It is well known that radial velocity decays more slowly than linear velocity when compared to the initial states.
The outcome is that down range the number of rotations per unit of travel actually increases as though the "twist rate" has become faster whilst RPM is still decaying. This changes the relationship between the velocity and the effective "twist rate". So what may have begun as a well matched projectile to its gyro torque then may become less than optimal; and the stability may subsequently suffer. ... At that point, as Alan implied somewhere, the earlier stable projectile may become de- stabilized and in my son's terms (when he handed me the controls in an F111 simulator) it "Goes to God".
Here is some data from a test I did some years ago to establish what happened to a .25 cal JSB King pellet Revolution rate per unit distance (effective twist rate).
Result data points inserted were:
2 ft = 1 rev per 18.9 inches ie., 1:18.9 (assumed to equal rifling twist rate of the BSA 0.25 cal barrel).
So:
2 feet = 1: 18.9
225 feet/ 75 yards = 1:15.9
300 feet/ 100 yards = 1:14.9
450 feet/ 150 yards = 1:13.2
600 feet/ 200 yards = 1:12.8
Graph :

200 yd double target as example of data collection:

Example of two target angles rotation change from the two screens, Pellets were marked to show radial displacement through the screens.:

I hope this is found to be helpful within the context of the discussion.
Best regards, Harry.

I should offer AZ an apology for re-entering the thread after writing that I would leave it unless directly asked questions. But I did feel this last response justified in that it offered hard data and explanations that could add worthwhile content. .... Thank you, and best regards, Harry in OZ.

"TR = 150 x .03 divided by .21 tells me that my little pellet needs a barrel with a 22.4 TR"

Is this formula valid for all calibers?

"What TR should I use in my gun or what would you advice?I would suggest you to use a 1:18 TR for ALL calibers up to about a .25 caliber and 1:16 for any caliber from .30 caliber up…This is an infallible recipe that will make anyone VERY HAPPY but it is not exclusive, other TR’s might provide equal or even better results in other environments."

While most .25's have twist rates around 1:17-1:18, TR's as high as 1:22 work well but tend to only like longer heavier pellets. I have twist rates of 1:17, 1:18; and 1:22 in .25 caliber. I recently built a .30 caliber with a 1:26 twist barrel that is absolutely perfect for the 44gr pellets it shoots. Based on your above response and formulas, my guns should not shoot well as my barrels have the wrong TR. Of course this is not the case as these guns shoot very well.

My point is not to agrue, but show that there seems to be a rather large misunderstanding of proper twist rates in airguns. Common theme for firearms is a longer heavier bullet requires a faster twist. Example: a .223 AR-15 used for competition at Camp Perry shooting heavy match bullets tend to use the faster twist 1:7 and 1:8 barrels to shoot the heavy bullets for long range. For varmint hunting with the much lighter 45-50gr bullets a twist of 1:14-1:10 would be needed. So, the formula of heavier longer bullets need faster TR's is true in this instance and basically how it is for all firearms. So far, this formula is near opposite for airguns shooting diablo pellets based on my own first hand experience and seems somewhat backed up by what is working for others.

I had wanted to build a .25 as a bullet shooter. I spent alot of time trying to figure out what twist I needed. One person calculated 1:26 based on the length of bullet I wanted to shoot. Another said 1:14 while another claimed 1:22. In the end, I chose 1:22 which didn't work for bullets, but will shoot diablo pellets very well. But, it shoots longer pellets better than shorter pellets. My 1:18 barrel will shoot anything pretty damn good. So, This started to lead me to believe airgun barrel twist doesn't work like it does for firearms. Along comes my .30 cal. Based on traditional firearms knowledge, one would assume that a faster twist of 1:14 would be about right? No, 1:26 is what alot of manufacturers are using for .30 diablo pellets, yet a true .308 bullet shooter airgun uses a 1:14 or there abouts.

I am by far no experts, but experience is proving traditional firearms formulas and wisdom incorrect for airguns propelling diablo pellets. Maybe a point of interest is that pellets that take on more of a bullet shape and have little to no waist tend to perform more like bullets, and in my experience show a preference for a twist rate more to a bullets preference than a pellets. Either I am experiencing some weird phenomenon or conventional firearms TR wisdom needs thrown out the window in regards to airguns and diablo pellets?? I don't have the knowledge and understanding that some of you have. I only have first hand data and an attempt to understand it. That said, I would love for someone to "break the code" of airgun twist rates as I feel that the common "understanding" as posted by AZ and many others just doesn't hold true to airguns. Please understand that I am not trying to argue, but trying to understand why my experience with firearms TR's isn't working for airguns, and just get some real facts as to why??