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# The science to twist rates

#8
12-30-2010, 07:21 PM
 Silver Member Join Date: Sep 2008 Posts: 327
Re: The science to twist rates

To the OP, there are many bullet twist calculators that are public access online... input the bullet details and it tells you the required twist or if you designate a twist it will tell you the stability factor. Just google it, there are more than a few.

Now for the others, MikeCr, its gets a bit more complicated than that...

You know the don miller type stability calculators are SIMPLIFIED stability calculators because its too hard to know or measure certain physical properties required for the proper unadulterated stability equations. The REAL UNSIMPLIFIED stability equations use the following;

axial and transverse moments of inertia (which is one of the difficult things to measure or calculate)
Overturning moment related to normal force, center of gravity and angle of attack or yaw (again difficult to measure/calc without a wind tunnel)
Transverse radii of gyration (another difficult one)
Crosssectional area or frontal area
diameter or caliber (perhaps these combine for your "displacement" mikeCr?)
air density
spin rate
velocity
mass

The miller stability type formulas make approximations and assumptions which are fairly accurate for our typical bullets we use today so that we dont have to know the difficult things outlined above. However, its not perfect and some things can vary quite considerably.

For example, if your comparing a bullet with a plastic tip or a large void behind the meplat jacket where there is no lead core present, this can change the transverse inertia moment and transverse radii of gyration quite considerably and therefore the approximation given by the miller formula, underestimates the stability by an appreciable margin.

Something else to consider with regard to the spin/velocity ratio increasing as a bullet flys downrange. This is true, and commonly a bullet gets more stable the further it flys... However, again it doesnt always hold true. One thing thats overlooked is whilst this is happening, the pitching or overturning moment INCREASES with decreasing velocity due to the center of pressure moving further forward with decreasing mach number. This offsets some, or in extreme cases all, of the stability increase from the spin/velocity ratio increasing downrange. The overturning moment is highest in the transonic region, where its overturning moment is usually about DOUBLE that of when the same bullet is flying at mach 2.5 or a typical rifle muzzle velocity. This is why many bullets destabilize in the transonic region, despite having a much higher static stability factor than when it left the muzzle! However it can happen before the transonic region, if the stability factor is marginal and/or the aerodynamic design of the bullet is condusive to a rapid rise in pitching moment with decreasing velocity.
#9
12-30-2010, 10:24 PM
 Gold Member Join Date: Apr 2010 Location: NW Mt. Posts: 600
Re: The science to twist rates

I would guess that Berger wanted their new 87 grain 6m tested in a 1:10 twist
because even they can't be positive it will work in every gun according to the numbers.
It always boils down to real world experience when dealing with how a gun shoots doesn't
it?..
#10
12-30-2010, 11:04 PM
 Bronze Member Join Date: Dec 2009 Location: middle tn Posts: 72
Re: The science to twist rates

Its my understanding the twist is determined and most relative to the length of the bullet. generally the heavier the bullet the longer it will be. The longer the bullet the faster the spin to stabilize it. Barrel quality play a big part as mentioned also. The marginal quality barrel may appear to be the wrong twist when it may be the optimal twist for a particular bullet.
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#11
12-30-2010, 11:40 PM
 Silver Member Join Date: Sep 2008 Posts: 327
Re: The science to twist rates

Quote:
 Originally Posted by TnTom Its my understanding the twist is determined and most relative to the length of the bullet. generally the heavier the bullet the longer it will be. The longer the bullet the faster the spin to stabilize it. Barrel quality play a big part as mentioned also. The marginal quality barrel may appear to be the wrong twist when it may be the optimal twist for a particular bullet.
This is a simplified way of looking at the mechanics of it, which is perfectly fine for most purposes.

As you make a bullet longer in the same caliber, your increasing its tranverse inertia moment (nose to tail) alot, and only increasing the axial inertia moment a little, so it get more unstable the longer you make it. This is assuming a constant density construction that is.... Once you start altering the density by using plastic tips or hollow core meplats etc, then you can get away with making them longer WITHOUT increasing the transverse inertia moment. See my point? This has been one of the design improvements of modern bullets, using hollow and plastic tips etc... it allows designers to make the bullet a bit longer and more aerodynamically streamlined for less drag whilst still maintaining stability from common twist rate barrels.
#12
12-31-2010, 12:23 AM
 Bronze Member Join Date: Dec 2009 Location: middle tn Posts: 72
Re: The science to twist rates

I know that the length/weight/twist relationships seem to have become less straight forward in latter years but didn't quite understand why. You have shed some light on why. Thanks for that input. It never ceases to amaze me at the accuracies that are being accomplished. Bullets/guns that shoot in the .1's and even under really attests to the accomplishments in the industries. Thinking that distances out to and beyond 2000 yds in competition and in combat is simply hard to grasp.
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#13
12-31-2010, 12:43 PM
 Platinum Member Join Date: Jun 2010 Location: greenwood, IN Posts: 3,293
Re: The science to twist rates

benchrest shooters use a formula that figures the S/G and then plug that number into a formula that gives them a twist rate. Now the idea is relativly new to me, but it does work very well. I'll try to find the data and post it this weekend.
gary
#14
12-31-2010, 03:53 PM
 Bronze Member Join Date: Dec 2009 Location: middle tn Posts: 72
Re: The science to twist rates

what is the S/G relate to or what is it?

Tom
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