Twist rate, how fast is too fast?

[Lenny Foffa]

Please allow a couple of stupid questions. Since I shoot an over bore cartridge from the 70"s . A 270 Weatherby Mag. I am trying to keep up. with the rest of you.
#1, How does one determine the RPM of the projectile to such a fine number like 300KRPM or 309KRPM ect?? Is there a specific math formula ? #2 Does the load you develop , (same Bullet in the same barrel) )based on expected Muz Vel change the RPM of the projectile? (Example a 2900 fps Muzz Vel. load vs a 3000fps velocity load ) and #3 What numerical value does SG represent? Thank you for your patience and response

 

[Snowbird]

It's going to depend entirely on the construction of the bullet. Cup and core will not take as much as solids.
I think cup n core start having issues about 300,000 RPM.

The faster you can spin a bullet and get away with it the better. Especially in terminal performance.

As I understand it, once you're over stabilized the projectile doesn't turn over and can fly point high.

 

[FEENIX]

Please allow a couple of stupid questions. Since I shoot an over bore cartridge from the 70"s . A 270 Weatherby Mag. I am trying to keep up. with the rest of you.
#1, How does one determine the RPM of the projectile Is there a specific math formula ?

Bullet RPM quick version formula: MV x (720/barrel twist)
= 3000 FPS x (720/8)
= 270, 000 RPM

#2 Does the load you develop , (same Bullet in the same barrel) )based on expected Muz Vel change the RPM of the projectile? (Example a 2900 fps Muzz Vel. load vs a 3000fps velocity load )

Yes, velocity will affect the RPM of the bullet
= 3000 FPS x (720/8) = 270, 000 RPM

= 2900 FPS x (720/8) = 261, 000 RPM

#3 What numerical value does SG represent? Thank you for your patience and response

SG (gyroscopic stability factor) is what it takes to optimize (>1.5) the stability of a projectile in flight.

https://bergerbullets.com/twist-rate-calculator/

 

[WildRose]

So I know ideally we want a stability factor (Sg) of just over 1.5, and generally over that isn't an issue until you get to a certain point and torque gets too high.

I know a lot depends on the bullet and probably how sharp the lands are, but at what point have you started seeing flyers or jackets being peeled off? For example a stability factor of 5? 6?

Type of rifling and depth of grooves can make a big difference.

Jacket type and thickness another big factor. If you have a really thin jacket and very tall lands with sharp edges you're going to have problems which will be magnified to some degree as the twist gets tighter.

All else being equal higher velocity is going to cause more and deeper engraving and the twist of the rifling can exert quite a stress on the jacket but may or may not be enough to cause most bullets to shed their jackets.

Gain twist barrels seem to be the worst about this because by the time a bullet gets out the end of the barrel the angle of attack on the bullet from the lands is constantly changing to an ever tighter twist.

I'm afraid for the most part it's an accumulation of tolerances where no one thing the factor but all combined they add up to a possible big problem.

 

[Lenny Foffa]

Mr. Feenix, I thank you for the education. !!! I intend to copy your answers, right now , and keep them on file. Really appreciate your time and response. Lenny

 

[Lenny Foffa]

Feenix, The Berger Twist Rate Calculator says that the Nosler 150 Grn. ABLR bullet , with a G1 BC of 592 , in 270 Caliber, with a Muzzle velocity of 3000 fps, results in a SG score of 1.69. Just a bit over the 1.5 Base line . Since I have a 26 inch , 10 twist barrel on the Weatherby Mark V , I must live within the limitations of my older rifle. I was very surprised to see the Berger 170 Grn. Bullet, in 270 with a G1 BC of 662 has a SG score of 1.87 at 2800fps. I did not think the older 10 twist would stabilize that very long , heavy bullet. Once again, Thank You.

 

[TXTrained]

My formula would not work where there is not enough energy to destroy bullets.
2 scenarios:
7tw, 6BR, 95gr VLDs at 3000fps, 309 KRPM
8tw, 243AI, 95gr VLDs at 3400fps, 306 KRPM

They both hit my KRPM Limit of 305 for 24cal, and you could find a few 243AIs killing bullets.
But you're unlikely to find a 6BR killing ANY bullets. It's just not enough heat and obturation energy.

So it is an important caveat to my formula that it only applies while at the correct twist for the bullet.
The correct twist in this scenario,, for this bullet, is 9tw.
You'd have to drive the bullet at 3850fps in 9tw to hit my 305 KRPM limit, and that is plenty of energy applied (for sure).
You would see failures right there.

Thanks. I will talk with the guys at Shilen.They helped me on myn223 AI and its a real Tack Driver.Thanks for taking time to put the numbers to it.I appreciate it.

 

[MagnumManiac]

As I understand it, once you're over stabilized the projectile doesn't turn over and can fly point high.

There is no such thing as "Over stabilised".
A bullet is either stable or unstable.
A bullet with a nose high attitude is not over stabilised. It is stable, just with a positive attack attitude, the same is true of a bullet with a nose down attitude, it is stable.
Bullets will change their attitude during flight, some stay the same and others start high nose up and then as range increases they continue towards Earth as nose down attitude….so it's not all Rice Crispys and porridge.

Cheers.

 

[FEENIX]

Feenix, The Berger Twist Rate Calculator says that the Nosler 150 Grn. ABLR bullet , with a G1 BC of 592 , in 270 Caliber, with a Muzzle velocity of 3000 fps, results in a SG score of 1.69. Just a bit over the 1.5 Base line . Since I have a 26 inch , 10 twist barrel on the Weatherby Mark V , I must live within the limitations of my older rifle.

You must be higher than my altitude to get an SG of 1.69, i.e., 5000'. Please double-check your inputs for your calculation. Using 150 ABLR's G1 of .592, OAL of 1.39" at my altitude of 3300', and MV of 3000 FPS:

You should be able the propel the 150 ABLR in the 3100+ FPS with your .270 WBTY without any issues.

I was very surprised to see the Berger 170 Grn. Bullet, in 270 with a G1 BC of 662 has a SG score of 1.87 at 2800fps. I did not think the older 10 twist would stabilize that very long , heavy bullet. Once again, Thank You.

Berger recommends the barrel twist rate for the 170 is 1:9" or faster. Using Berger 170's G1 of .662, OAL of 1.49" at my altitude of 3300', and MV of 2900 FPS:

My .270 AI has a 30" 1:8" if I use the same 170 Berger information:

NOTE: I should have used MV 2800 FPS instead of 2900 FPS. SG at 2800 FPS with the 170; 1:10" = 1.37, 1:8" = 2.15.

 

[Jeff Heeg osoh]

Type of rifling and depth of grooves can make a big difference.

Jacket type and thickness another big factor. If you have a really thin jacket and very tall lands with sharp edges you're going to have problems which will be magnified to some degree as the twist gets tighter.

All else being equal higher velocity is going to cause more and deeper engraving and the twist of the rifling can exert quite a stress on the jacket but may or may not be enough to cause most bullets to shed their jackets.

Gain twist barrels seem to be the worst about this because by the time a bullet gets out the end of the barrel the angle of attack on the bullet from the lands is constantly changing to an ever tighter twist.

I'm afraid for the most part it's an accumulation of tolerances where no one thing the factor but all combined they add up to a possible big problem.

Picture for you is a very aggressive gain twist with a bullet recovered shot into a snow drift. 16 to a 7.7 twist. To be honest with you its looks very good compared to what you mentioned.

 

[Greyfox]

This is an interesting thread. I'm in the process of doing some testing with twist rate with my 6.5x284's. I have traditionally run 140-143gr bullets(G7 BC .3-.32) in the 3000FPS range. Using 8TW barrels my SG's fall in the 1.5-1.8 range depending on the bullet and the altitude/temperature. Under these conditions, I have been able to consistently generate precision/ES in the .25MOA/ES<15FPS. 1000 yard accuracy is maintained within .5MOA and I get good terminal performace on game from 200-1000 yards. I have used this formula for over a dozen years and it has proven to be nuts-on for my LRH. When I load the 156 Berger EOL in my 8TW(which is Berger's minimum recommendation), I'm at marginal stability(SG147) at 300 ft Alt, and just over stable at 2700FT at SG 1.57, 1.7 at 4500FT). My precision/ES with the 156's, while not as good as good as the 140-143 bullets, is quite acceptable loosing about .25MOA accross the board with the 156's. But this may have nothing to do with twist rate. I(reluctantly) just mounted the same brand/chamber/dimension barrel in a 1.75TW on my rifle. This twist rate adds about .2-.3 SG across the board placing me in the full stability range with the 156's at 1.6-1.8SG, and 1.8-2.0SG with the 140-143's.
I will be most curious to observe the effects of this change. With the hopes of picking up stability with the 156's, I would also WANT to maintain my current performance with the 140 class bullets. Berger broadly says yes. I'm inclined to think that a .5 twist rate change shouldn't be a big deal. I have made much greater changes then this with my .223's with little effect using the lighter bullets. Perhaps my skepticism is a needless concern due to my OCD when it comes to fine precision, Combine this with an earlier competitive life where bullets(308) performed best at a particular spin rate window, where I observed this to actually be the case with the bullet designs of those days.
Any relevant experiences or insights would be appreciated.

 

[HARPERC]

You must be higher than my altitude to get an SG of 1.69, i.e., 5000'. Please double-check your inputs for your calculation. Using 150 ABLR's G1 of .592, OAL of 1.39" at my altitude of 3300', and MV of 3000 FPS:

I played with the inputs a bit last night, and it was 10,000' when I got numbers similar to the 1.69 SG.

I started to chime in, but got lazy, decided Feenix will fix, and went to bed.

Respectfully, made me laugh. Your predictable thoroughness is a comfort. You provided a much better answer than I would have. I SALUTE you Sir!

 

[FEENIX]

I played with the inputs a bit last night, and it was 10,000' when I got numbers similar to the 1.69 SG.

I started to chime in, but got lazy, decided Feenix will fix, and went to bed.

Respectfully, made me laugh. Your predictable thoroughness is a comfort. You provided a much better answer than I would have. I SALUTE you Sir!

LL! Thank you Sir.

 

[WildRose]

Picture for you is a very aggressive gain twist with a bullet recovered shot into a snow drift. 16 to a 7.7 twist. To be honest with you its looks very good compared to what you mentioned.View attachment 345266

It doesn't look like a thin jacketed cup and core bullet nor does it appear to have the real aggressive sharp edged lands either but difficult to tell in two dimensions.