What is the Ideal Twist Rate of Slugs up to .25 Caliber?

[Ballisticboy]

Hey Ballisticboy,
Given the information you listed & your knowledge level:
> What twist rate range do you feel is ideal for .177 - .25 caliber hollow point slugs with MV of 900 to 1050 fps?

I asked for that really didn't I.

Based on a lead .22 slug of 25.9 grains mass and about 1.5 calibres long, fired at 950 ft/sec, for a stability factor of 1.5 (considered ideal for bullets) in the thread comparing lead and lead free slugs a twist rate of one turn in 21.7 inches is required for that design. Through simple scaling of this result, using the standard equation for stability factor used everywhere in the world, the small table below results for .177 and .25 slugs travelling at the same speed.

Calibre​	Mass​	Twist Rate​
0.22​	25.9​	21.7​
0.177​	13.5​	14.0​
0.25​	38.0​	28.0​

At higher speeds, slightly higher twist rates will be needed for the same stability factor, but, you can probably get away with the same twist rates at the expense of a little BC loss. Different designs will need different twist rates depending on the presence of boattails or much longer lengths to give more mass. The numbers in the table were chosen as being fairly typical.

Pellets of course require entirely different twist rates as they use a different stabilization method.

 

[Ballisticboy]

It would be a great start if FX would release Matt Dubber's notes when he was experimenting with rifeling and twist rates on slugs...but it still may be missing the atmospheric data.

Personally, I would not trust anything from that particular source.

 

[Rickrick]

There's so many videos, articles and personal experience on this subject I'm not sure what could be added to them.
I had to find out for myself which slug was best for my .25 acp barrel, I've spent more money and time than I want to admit but in the end it was worth it.

What helped me the most was all the slug makers, they are a great bunch of ppl and great wells of knowledge

The best we can do is continue to add our personal experiences to this forum.

My condor has a 24.75" 1:14 twist (.250 groove, .246 bore). The best slug for me is the Avs .253 48g with a cup base. It was giving me 1/2" to 5/8" 50 yd groups. Unfortunately I was also getting more 3/4" groups than I like.

I put a maddog stock on! Wow, that was the kicker. Now I get a consistent 1/2 to 5/8" 50 yd groups at around 965 fps.

My point is, it isn't always just the slug that improves accuracy.

 

[SpiralGroove]

I asked for that really didn't I.

Based on a lead .22 slug of 25.9 grains mass and about 1.5 calibres long, fired at 950 ft/sec, for a stability factor of 1.5 (considered ideal for bullets) in the thread comparing lead and lead free slugs a twist rate of one turn in 21.7 inches is required for that design. Through simple scaling of this result, using the standard equation for stability factor used everywhere in the world, the small table below results for .177 and .25 slugs travelling at the same speed.

Calibre​	Mass​	Twist Rate​
0.22​	25.9​	21.7​
0.177​	13.5​	14.0​
0.25​	38.0​	28.0​

At higher speeds, slightly higher twist rates will be needed for the same stability factor, but, you can probably get away with the same twist rates at the expense of a little BC loss. Different designs will need different twist rates depending on the presence of boattails or much longer lengths to give more mass. The numbers in the table were chosen as being fairly typical.

Pellets of course require entirely different twist rates as they use a different stabilization method.

I have LW Poly barrels which have a 1:17" twist rate in all the above calibers. This rate seems to fit between .177 & .22 calibers, but in your estimation is it too far out of the acceptable range for good accuracy for .25 caliber?

 

[Chukar60]

I have not been able to make sense out of any of the “published” information because my own experiences contradict what I knew from another gun.
Case in point. My wife and I both shoot M3’s with slug liners. Mine shoots Zan .218 23 gr in laser like fashion. Her gun likes .218 slugs better than .217 and won’t shoot the Zan 23’s. It does shoot NSA .218 24.8 gr as well as my gun shoots the Zans.
So you have two guns with the same liners, same length and they will not shoot the same slugs in anywhere near the same fashion, no matter how hard i tried to tune for them. Identical tunes with identical slug produced vastly different results
My Crown MKII in 25 likes several slugs but being bone stock does not drive them at high speeds. Just out of curiosity I bought some Air Impulse HBT slugs. To try them I backed the power wheel down three settings. They shot well but when I set the power wheel on max they shot considerably better. Logic would say that increased velocity would require some, probably a very minor increase in twist to improve stability. Nah. Standard liner and the faster I shot them the better they shot.
I think this slug game is still so new and basically in development that for now we are all stuck buying, testing and being lab rats.

 

[Ballisticboy]

I have LW Poly barrels which have a 1:17" twist rate in all the above calibers. This rate seems to fit between .177 & .22 calibers, but in your estimation is it too far out of the acceptable range for good accuracy for .25 caliber?

The most important aspect for small group sizes, which I assume is what you are after, is slug barrel interaction. Differing twist rates will only affect how the slug reacts to how well it has been launched, assuming there is sufficient twist to give a stability factor above 1. If the slug leaves the gun with large amounts of wobble, it won't matter what the twist rate was, you will still get big groups. Yes, the group will be smaller with the ideal twist rate, but it will still be unacceptable. The twist rate should be seen as fine-tuning the results after you have already got a slug and barrel which work. Changing twist rates will not work miracles and suddenly change a poor barrel/slug combination into a good one.

The only other factor which may come into play is that, if your slugs have small imperfections, for example the CG is not on the exact centreline, higher twist rates may lead to greater wobble when the slugs leave the barrel. In such cases the optimum twist rate may be less, but the change possible will be small and so may not give the results you want. Changing the slug would be the only option in such a case.

 

[SpiralGroove]

The most important aspect for small group sizes, which I assume is what you are after, is slug barrel interaction. Differing twist rates will only affect how the slug reacts to how well it has been launched, assuming there is sufficient twist to give a stability factor above 1. If the slug leaves the gun with large amounts of wobble, it won't matter what the twist rate was, you will still get big groups. Yes, the group will be smaller with the ideal twist rate, but it will still be unacceptable. The twist rate should be seen as fine-tuning the results after you have already got a slug and barrel which work. Changing twist rates will not work miracles and suddenly change a poor barrel/slug combination into a good one.

The only other factor which may come into play is that, if your slugs have small imperfections, for example the CG is not on the exact centreline, higher twist rates may lead to greater wobble when the slugs leave the barrel. In such cases the optimum twist rate may be less, but the change possible will be small and so may not give the results you want. Changing the slug would be the only option in such a case.

Thank You BB

 

[HogKiller]

I have not been able to make sense out of any of the “published” information because my own experiences contradict what I knew from another gun.
Case in point. My wife and I both shoot M3’s with slug liners. Mine shoots Zan .218 23 gr in laser like fashion. Her gun likes .218 slugs better than .217 and won’t shoot the Zan 23’s. It does shoot NSA .218 24.8 gr as well as my gun shoots the Zans.
So you have two guns with the same liners, same length and they will not shoot the same slugs in anywhere near the same fashion, no matter how hard i tried to tune for them. Identical tunes with identical slug produced vastly different results
My Crown MKII in 25 likes several slugs but being bone stock does not drive them at high speeds. Just out of curiosity I bought some Air Impulse HBT slugs. To try them I backed the power wheel down three settings. They shot well but when I set the power wheel on max they shot considerably better. Logic would say that increased velocity would require some, probably a very minor increase in twist to improve stability. Nah. Standard liner and the faster I shot them the better they shot.
I think this slug game is still so new and basically in development that for now we are all stuck buying, testing and being lab rats.

Please see my post about the Astrological sign of when your guns were made, it answers many questions as to HOW FRIGGIN' IRRATIONAL air guns are. You want some cheese?

 

[Vetmx]

I have not been able to make sense out of any of the “published” information because my own experiences contradict what I knew from another gun.
Case in point. My wife and I both shoot M3’s with slug liners. Mine shoots Zan .218 23 gr in laser like fashion. Her gun likes .218 slugs better than .217 and won’t shoot the Zan 23’s. It does shoot NSA .218 24.8 gr as well as my gun shoots the Zans.
So you have two guns with the same liners, same length and they will not shoot the same slugs in anywhere near the same fashion, no matter how hard i tried to tune for them. Identical tunes with identical slug produced vastly different results
My Crown MKII in 25 likes several slugs but being bone stock does not drive them at high speeds. Just out of curiosity I bought some Air Impulse HBT slugs. To try them I backed the power wheel down three settings. They shot well but when I set the power wheel on max they shot considerably better. Logic would say that increased velocity would require some, probably a very minor increase in twist to improve stability. Nah. Standard liner and the faster I shot them the better they shot.
I think this slug game is still so new and basically in development that for now we are all stuck buying, testing and being lab rats.

And this is exactly why so many of us found the whole “tune sharing” thing so ridiculous. We tried to get my buddies M3 Compact to shoot the same slug as my Compact. Mine was shooting exceptional at that time. I finally got his gun close but the settings and velocity were different. He said why is it still not as good as yours? I told him your scope is heavier than mine. He laughed because he doesn’t understand. There’s lots that guys don’t understand. These are not powder burners.

 

[Ballisticboy]

I have not been able to make sense out of any of the “published” information because my own experiences contradict what I knew from another gun.
Case in point. My wife and I both shoot M3’s with slug liners. Mine shoots Zan .218 23 gr in laser like fashion. Her gun likes .218 slugs better than .217 and won’t shoot the Zan 23’s. It does shoot NSA .218 24.8 gr as well as my gun shoots the Zans.
So you have two guns with the same liners, same length and they will not shoot the same slugs in anywhere near the same fashion, no matter how hard i tried to tune for them. Identical tunes with identical slug produced vastly different results
My Crown MKII in 25 likes several slugs but being bone stock does not drive them at high speeds. Just out of curiosity I bought some Air Impulse HBT slugs. To try them I backed the power wheel down three settings. They shot well but when I set the power wheel on max they shot considerably better. Logic would say that increased velocity would require some, probably a very minor increase in twist to improve stability. Nah. Standard liner and the faster I shot them the better they shot.
I think this slug game is still so new and basically in development that for now we are all stuck buying, testing and being lab rats.

Have you tried swapping the liners over to see if the guns now prefer the other slugs, i.e. yours likes the NSAs and your wife's like the Zans?

Increasing the speed of the slugs out of the Crown may improve group size due to reducing stability, not increasing it. It will all depend on how much stability there is to start with and how the slugs leave the barrel at the different speeds.

 

[Chukar60]

Deleted the post

 

[Altaros]

In general, you can start from gyroscopic stability and, for example, the ballistic program Appliet Balistic calculates this stability (must be above 1. Above 1.5 is better, but the basis is to be above 1).
But practice is important and that is the following.

For .25, LW tiwst 1:17.7 is completely sufficient up to a slug size of 14mm and a weight of 66gr. This is the current real data as it is the primary setting of my shooting ! ( probably also for 15mm and almost 70gr, but that I will must try first )
So, with the current range of .25 slugs on the market, if you have twist 1:17.7 and faster, you do not need to solve the twist at all. This is practice, not theory.

 

[Spinach]

In general, you can start from gyroscopic stability and, for example, the ballistic program Appliet Balistic calculates this stability (must be above 1. Above 1.5 is better, but the basis is to be above 1).
But practice is important and that is the following.

For .25, LW tiwst 1:17.7 is completely sufficient up to a slug size of 14mm and a weight of 66gr. This is the current real data as it is the primary setting of my shooting ! ( probably also for 15mm and almost 70gr, but that I will must try first )
So, with the current range of .25 slugs on the market, if you have twist 1:17.7 and faster, you do not need to solve the twist at all. This is practice, not theory.

So, what twist rate for your slugs in .22?
I'm not having much luck with 2 1:17.7. barrels. LW and CZ.
Thanks in anticipation.

 

[HogKiller]

In general, you can start from gyroscopic stability and, for example, the ballistic program Appliet Balistic calculates this stability (must be above 1. Above 1.5 is better, but the basis is to be above 1).
But practice is important and that is the following.

For .25, LW tiwst 1:17.7 is completely sufficient up to a slug size of 14mm and a weight of 66gr. This is the current real data as it is the primary setting of my shooting ! ( probably also for 15mm and almost 70gr, but that I will must try first )
So, with the current range of .25 slugs on the market, if you have twist 1:17.7 and faster, you do not need to solve the twist at all. This is practice, not theory.

TIME OUT, in the case of an FX you're talking about a Superior 'heavy' soda straw which has that twist rate. How in the H FX names their stuff is beyond me. So because I'm still fumbling around in this part of the Rabbit Hole,® does that twist rate apply to .22 or does the 1:24 'Superior' work? The FX/RMR .22 slugs weigh 22gr and the .25 FX/RMR slugs weigh 26gr.

 

[Altaros]

i went by the thread title and that is .25. So I was only talking about the .25 caliber barrel.
The situation is different with .22. A 40gr slug is also sufficient with a twist of 1:17.7
On the other hand, for my special 15.4mm long and 57gr, this twist is no longer enough and here a twist of around 1:12 would be needed (I used 1:9 and 1:8 )

There is also a fundamental difference between the gyroscopic stability of the slug provided by the twist barrel and accuracy. Stability or accuracy can be easily disturbed by not using the appropriate slug diameter for the given barrel bore diameter.

I will give a simple tutorial for slugs and FX barrel. Do not use anything other than Superior heavy or the newly made barrel that superior heavy suggests, i.e. twist 1:16 or 1:18, not 1:24, etc.

this applies to slugs over 30gr and more, we don't produce small slugs so I don't even deal with them.

 

[Ballisticboy]

The necessary twist rate is not just dependent on the length and mass of a slug or any other projectile. In fact, mass does not enter into it at all, the only reason people think mass is a factor is because heavy slugs are usually longer and thus require more twist. The important factors are the aerodynamic moments decided by the projectile shape and centre of gravity position, and the inertial moments of the slug, which are dependent on the shape, length and the internal structure. Thus, large hollow points and dished or hollow bases can be critical in deciding twist rate by both reducing the transverse inertia and by moving the CG position. Boattails, if they have been designed properly and are actually doing something, make the slug more aerodynamically unstable and thus requiring more twist rate. Many of the current boattailed slugs have boattails which are too large and too severe and thus do not do what they are intended to do as the air simply cannot follow the sides of the boattail. These slugs will only need the same twist rates as slugs with no boattail.

The twist rate calculators I have seen available on the internet are far too simplified to produce accurate values for slugs with hollow points and hollow bases and use very simplified aerodynamic calculations, particularly at speeds around 1100 ft/sec. This includes the Kolbe calculator used by many, which uses McGyro and cannot be relied on at speeds close to Mach 1. There are no quick and easy calculators if you want accurate estimates of required twist rates at the speeds mentioned here.

 

[JimD]

If you want to know what twist rate FX thinks is appropriate for slugs it isn't hard to google up what they are selling. The best source I found was from 2019, I don't know if the information is still current, but in 25 caliber the pellet liners were almost all 1:27 (there was one that is 1:20) and the three slug liners were 1:19, 1:20. and 1:21. More current information was available but complicated by the way they name their liners (may not be complicated but I don't understand it).

Looks like current twist rates for FX 25 caliber liners are 1:24 for pellets and 1:18 for slugs.

 

[Altaros]

I'm not talking about pellets, I haven't shot them in years. If you want an FX barrel that will shoot well practically all major slugs on the market, then I am sending the photo and text below for readability. it's a .22 and a .25 both have a 1:18 twist and are some of the last ones released by FX.
STX Superior heavy .22 TR 1:18
STX Slug .25 TR 1:18

STX Superior heavy .22 TR 1:18
STX Slug .25 TR 1:18

 

[Altaros]

I'm not talking about pellets, I haven't shot them in years. If you want an FX barrel that will shoot well practically all major slugs on the market, then I am sending the photo and text below for readability. it's a .22 and a .25 both have a 1:18 twist and are some of the last ones released by FX.
STX Superior heavy .22 TR 1:18
STX Slug .25 TR 1:18

View attachment 350541
STX Superior heavy .22 TR 1:18
STX Slug .25 TR 1:18

to Ballisticboy
yes, what you write is of course true. I've tried to explain this to people before too, but just like with firearms, it's easier to say weight because people don't understand it very well anyway .
On the other hand, I do not agree that there is a problem with the calculation. The ballistics mobile application from appliet ballistics calculates stability and I have not yet had a stable value above 1 and the bullet was unstable due to twist. Of course, this applies to fairly standard slugs, including our ATP with BT. If I made superspecies shapes, they could not predict this instability, but neither could the other mentioned calculation programs or equations, which enter much more data about the shape of the slug.

 

[Ballisticboy]

to Ballisticboy
yes, what you write is of course true. I've tried to explain this to people before too, but just like with firearms, it's easier to say weight because people don't understand it very well anyway .
On the other hand, I do not agree that there is a problem with the calculation. The ballistics mobile application from appliet ballistics calculates stability and I have not yet had a stable value above 1 and the bullet was unstable due to twist. Of course, this applies to fairly standard slugs, including our ATP with BT. If I made superspecies shapes, they could not predict this instability, but neither could the other mentioned calculation programs or equations, which enter much more data about the shape of the slug.

The errors which the readily available twist calculators give in all the applications I know of is that they overestimate the twist required for stability, not underestimate it. This is not a particularly serious problem as, once a slug is stable, small changes in twist rate are really only tweaking the group size, there are many more factors having a bigger effect. The JBM site has some calculators for the aerodynamics of bullets, but they will have errors for slugs with large meplats. It is also, as I said before, that many boattails on slugs are not working as they should and thus the twist calculated will appear to be correct.