Any twist rate experts?

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Any twist rate experts?

Post by mickb » 01 Jun 2019, 8:15 pm

Gents wondering if a 230 grain jacketed bullet out of a 5" 45 ACP with 1: 16" twist that stabilises nicely, will still stabilise if pushed 1500fps out of a rifle length barrel, same twist? Or will the high RPM for such a short bullet be over stabilising it by then and deteriorating groups?

I know there are a couple of formulas to calculate this, and also guys who know this stuff back to front( which I dont).
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Re: Any twist rate experts?

Post by Oldbloke » 01 Jun 2019, 9:37 pm

Certainly no expert here. But try this.

https://bergerbullets.com/twist-rate-calculator/
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Re: Any twist rate experts?

Post by bladeracer » 01 Jun 2019, 9:47 pm

mickb wrote:Gents wondering if a 230 grain jacketed bullet out of a 5" 45 ACP with 1: 16" twist that stabilises nicely, will still stabilise if pushed 1500fps out of a rifle length barrel, same twist? Or will the high RPM for such a short bullet be over stabilising it by then and deteriorating groups?

I know there are a couple of formulas to calculate this, and also guys who know this stuff back to front( which I dont).


Over-stabilising?
I've never had a bullet over-stabilise. I have had bullets disappear due to too high RPM from tight barrel twist rates.

Best way to answer your question definitively is to try some and see how they shoot.
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Re: Any twist rate experts?

Post by SCJ429 » 01 Jun 2019, 9:54 pm

You are shooting bullets very slowly out of a slow twist rate barrel, you will find the rpm will be relatively low.
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Re: Any twist rate experts?

Post by mickb » 02 Jun 2019, 2:56 am

Stubby bullets though too, 45 acp is a flying ashtray. Just reading up on it, you can overstabilise apparently if the RMP for short bullets is too high. While not throwing them of course or keyholing, the group sizes can widen or double. Its all difficult to prove and a lot of different opinions on it. maybe nothing to worry about.
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Re: Any twist rate experts?

Post by SCJ429 » 02 Jun 2019, 12:08 pm

You can have problems not spinning long bullets fast enough and they yaw too much and fail to hit the target. Shorter stubby bullets are more forgiving with low rpm.

Bullet runout will cause the bullet to destabilise if subjected to high rpm. The more concentric the bullet, the more rpm it can tolerate.

You should be OK with both of these because of the slow speed, low twist rate and short bullets. Fire these bullets at 4,000 fps through a 1:7 twist barrel and you may not be able to hit a barn door at 50 yards.
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Re: Any twist rate experts?

Post by Blr243 » 02 Jun 2019, 6:16 pm

Ask in2anity. After seeing some of his posts I have recently decided that he is the gun tech guru And I probably should always be nice to him in case I ever need his help !!!
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Re: Any twist rate experts?

Post by sungazer » 02 Jun 2019, 8:01 pm

the over stabalizing is more of a thing for long range target shooters if they are worried about extracting every last bit of BC from the projectile. What happens is when the bullet reaches the top of its arc, instead of the nose now tilting slightly downwards the bullet continues to fly horizontal. Slowly descending but in it horizontal gyro position. So it is now longer the extreme low drag it once had and this gets worse as the distance gets further. Thats the theory what happens in reality I have not seen in slow mo camera as yet. Certainly the first part of flight is no perfect stabilized flight path as we would imagine.
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Re: Any twist rate experts?

Post by straightshooter » 03 Jun 2019, 8:04 am

mickb wrote:Gents wondering if a 230 grain jacketed bullet out of a 5" 45 ACP with 1: 16" twist that stabilises nicely, will still stabilise if pushed 1500fps out of a rifle length barrel, same twist? Or will the high RPM for such a short bullet be over stabilising it by then and deteriorating groups?

I know there are a couple of formulas to calculate this, and also guys who know this stuff back to front( which I dont).


In my opinion just shoot them and don't think about it too much.

There is no such state for a firearm projectile, which is simply an unconstrained gyroscope, of being 'over stabilised'.
Gyroscopic Stability (GS) can be calculated and expressed as a number such that where GS<1 then unstable and where GS>1 then stable.
The sweet spot is where GS is around 1.5 for the entire flight of the projectile.
At perhaps GS>3 the projectile tends to remain in the exact same plane as it left the barrel which results in a diminished apparent BC at longer ranges due to the projectile not tipping to conform to the slope of the trajectory. Put simply the projectile does not travel point first for the whole of it's flight so is subjected to greater drag making it's BC appear worse than it could be.
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Re: Any twist rate experts?

Post by mickb » 04 Jun 2019, 10:34 am

Thanks fellas, it also strikes me that if it was a problem, the fast calibres the yanks use like 460 S&W would not work very well being they can use the same shorts bullets hundreds of fps faster. Main reason I was asking in advance was getting platform built with a certain twist so would rather not spend coin if it isn't going to be decent. I'll just jump in I think.
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Re: Any twist rate experts?

Post by brett1868 » 04 Jun 2019, 12:39 pm

mickb wrote:Thanks fellas, it also strikes me that if it was a problem, the fast calibres the yanks use like 460 S&W would not work very well being they can use the same shorts bullets hundreds of fps faster. Main reason I was asking in advance was getting platform built with a certain twist so would rather not spend coin if it isn't going to be decent. I'll just jump in I think.


The first Gen 460XVR's used "Gain Twist" rifling that allowed the projectile to accelerate down the barrel as twist increased before exiting the muzzle. Cool approach that seems to fall in and out of favour every few years.
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Re: Any twist rate experts?

Post by mickb » 12 Jun 2019, 10:33 pm

I remember that yes
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