Twist rates for shooting slugs

[TrinityMaker]

Hello fellow members. This is my first time posting on the GTA forum. Admittedly, I have trolled the forum for knowledge ever since I purchased my PCP about 3 years ago. So here's my question.... Forgive me in advance. The question will be long. I'm a writer, and leaving no stone un-turned (being as detailed as possible) is in my nature. So here goes:

I own an AirForce Condor SS. The gun has been fully modded to maximize it's performance. Now I understand the drawbacks of the AF platform. The guns are designed for power and hunting, not accuracy as in some other manufacturers. But I believe every PCP on the market has the potential for greatness. In nature, Tesla proved that everything has a resonant frequency. If you can find the proverbial resonant frequency of the rifle (pressure, harmonics, projectile, twist rate, etc....) then the gun can essentially become a tack driver. And that has been my quest.... And therein lies my issue. I've grown tired of trying to figure it our on my own. I need some expertise.

My Condor has a 24" LW barrel with a twist rate of 1-17.7. I've got 3 interchangeable barrels for the gun, all with the same 1:17.7 twist. I have one, 24", choked .22 caliber barrel. I have one, 24", choked .25 caliber barrel, And I have 1, un-choked .25 caliber barrel. For the life of me, I cannot get this gun to group tight at 50 yards. I know I'm being very efficient with air. I understand the components that go into tuning the gun, thus not wasting air and potentially destabilizing the projectile right out of the gate. I've heard that "pellets love a choke, bullets hate a choke." I've found this to be a myth. I've tried 10 different projectiles in the gun with different diameters and weights. I've even sought the advice of the legendary Doug Noble (who is brilliant BTW), who gave me feedback on slug diameters and what to use in the barrel. I've run the projectiles through the Berger Twist Rate calculator to find the ones that are predicted to be most stable based off of the BC, caliber, velocity, and twist rate of my barrel. To no avail. So here goes the question......

I found a guy who makes custom barrels for AF guns. He's in the Russian Federation. He said he can customize the barrel to my requested specifics. The following are my choices of twist rates for calibers that I own:

.22 caliber- 14, 16, 18, and 20 twists
.25 caliber- 13, 14, 18, and 20 twists

All barrels are unchoked. When I plug in the numbers on the twist rate calculator, I show that the faster the twist, the more stable the projectile becomes. Sometimes the stability is increased significantly. I heard that .22 like a 1:16 twist. Feedback from you guys regarding this? Do you think a 1:14 twist is too fast for a .22 air rifle slug? The twist rate calculator says it's even more stable than the 1:16. Does the faster twist negatively impact the performance of the projectile in .22 caliber?

But my main focus is on the .25 caliber barrels that he offers. I know that the TW calculator says my 1:17.7 twist is stable, but I've also been told that a faster twist would be ideal for that caliber. And when I plug in the fatser twist in the calculator, it predicts significant increases in stability. Do you guys think a 1:14 twist is too fast for the .25 air rifle? I can get the gun to produce 75-80FPE, so velocity isn't an issue. I also know that the .257 LOVES the 1:14 twist. So to me it just makes sense that the .25 would love it also. I know TJ makes a barrel liner in .25 with a 1:14 twist, but for the life of me I can't find a single review on here for someone shooting slugs from their air rifle with that barrel liner in it. Maybe you guys can give me some direction. Do you guys know anyone who owns that barrel liner and has has success using it for .25 slugs?

I know Bob from NOE is like a god when it comes to questions like this. I'm hoping that he will see this and chime in. But what do you guys think about those barrel choices with their faster twist rates? I recently looked up the twist rates for FX Air rifles slug liners. The info I got seemed confusing and counter-intuitive to me. The 500, 600, and 700mm barrels in .22 have a twist rate of 1:16, 1:15.5, and 1:15. The longer the barrel, the faster the twist. Well, the .25 slug liners in the same lengths have the following twist: 1:19, 1:20, 1:21. The longer the barrel, the slower the twist. When I plug the numbers into the TW calculator, the stability decreases with the slower twist, yet the FX guns (when tuned properly) are the most accurate slug shooting guns out there. Why would the twist slow for .25, but get faster for .22? Then get faster again for the calibers larger than .25?

So that's it guys. What say you? Thanks in advance for your replies. And thanks in advance for all the knowledge I've been able to attain up to this point from my "trolling".

~Scott~

[rsterne]

First of all, there is only one Twist calculator I have found that properly addresses what happens in the Subsonic and Transonic range (Mach 0.8-1.2)….

http://www.geoffrey-kolbe.com/barrel_twist.htm

If that calculator says it is stable (spinning fast enough), you can pretty much take it to the bank.... However, it may overestimate the additional spin required for rebated boattails…. ie if a given flat base bullet needs a 20" twist and the same bullet with a boattail needs a 16", a rebated boattail may actually work in an 18".... Incidently, a Stability Factor (SF) of less than 1.0 is unstable (the bullet will tumble)…. SF = 1.5 is considered ideal.... The Military use about SF = 2.0-2.5.... However an SF greater than 3.5 or 4.0 may be problematic (see below)…. Also note, there are NO twist calculators that work properly for Diabolo (waisted) pellets, as they are primarily drag stabilized....

Next, your statement that the faster the twist the greater the stability is only looking at the "Gyroscopic Stability".... If you spin a bullet too fast, you may find that while it remains gyroscopically stable, it may start to develop "Dynamic Instability".... All bullets wobble initially when they exit the muzzle.... If that wobble damps out as the bullet travels downrange, the bullet has dynamic stability.... If the wobble gets worse, it shows dynamic instability.... Pellets, for example, tend to develop dynamic instability at high velocities at longer range.... The cure is usually to reduce the velocity, or use a SLOWER twist barrel.... Bullets can show the same thing, and if you spin them fast enough that their SF is greater than about 4.0, you are inviting that to happen....

There is an additional problem with spinning a bullet faster than required.... Crosswinds cause what is called "Aerodynamic Jump".... That is a vertical deflection of the bullet path caused by the bullets reaction to the crosswind.... The longer the bullet, and the faster the spin rate, the greater the deflection.... This results in groups in variable wind being sloped (from 10 o'clock to 4 o'clock for a bullet in a RH twist) instead of just spread horizontally....

You can read about all these effects in my recent articles in HAM....

https://hardairmagazine.com/ham-columns/the-external-ballistics-of-slugs-in-airguns/

https://hardairmagazine.com/ham-columns/wind-drift-for-airguns-its-important-where-the-wind-is/

https://hardairmagazine.com/ham-columns/vertical-deflection-for-pellets-in-crosswind/

https://hardairmagazine.com/ham-columns/pellet-and-slug-stability/

The November article will deal with the effects of twist, some of which are mentioned above....

Hope that helps.... Incidently, I don't work for NOE, I just gave them some of my designs....

Bob

[TrinityMaker]

Thank you for the reply. Your answer is clear as mud (at no fault to you my friend.... trust me). That's a lot to take in. I tried plugging in the required information in the TW calculator you gave me. Once without the twist of the barrel, and the 2nd time, with the twist of the barrel. I still find the graphs somewhat perplexing. I needed to use one of your boat tail slugs in .25 caliber to get the most details in regards to the desired information for the program to run it's algorithm. Still somewhat confusing. It's not that I'm not capable of understanding it. I'm a pretty sharp guy. I just need to have it explained a bit more.

However, I did take away that the ideal SG is 1.5, and anything greater than 3.5 can be problematic. I'm just curious if switching from a .22 barrel with a twist of 1:17.7 to a barrel with a twist of 1:16 is going to increase my stability/accuracy. I purchased the MP-Molds boat tail air rifle molds at the beginning of this year. I purchased them in 5.52mm and 6.35mm. I saw Gregor hitting small targets at 200 yards consistently with his Huben. He said his barrel had a twist rate of 1:16. I'm also looking for the best twist and projectile in .25 caliber. I can always adjust my velocity. That's the easy part.

I was just curious if any other AF gun owners using small caliber rifles had tried these faster or slower barrels, and had success with tightening up their groups? If so, what projectile are you shooting?

Bob, I HIGHLY respect your information. I promise I'm not dismissing it. But at this time, it's a bit above my head. I need more time to play with the program and figure it out. But I'm short on some of the required information it's looking for to make it's calculations. Even looking at the NSA ammo, it only gives 3 of the 7 or 8 desired items to make an accurate calculation.