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texasmac
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Down Range Spin Rate Formula
Nov 20th, 2025 at 10:37pm
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I posted the following comment in a thread several months ago.

==============
Since the standard formula MV X 720/Twist Rate = RPM only applies at the muzzle, any down-range formula would need a decay factor which is very small.  According to a test done several years ago at the Aberdeen Proving Grounds, a bullets spin decay rate is approximately 2% per 100 yds.  So, a bullet leaving a 16-twist bore at 1100fps (49,500rpm) would have a spin rate of 44,744rpm at 500yds & 40,445rpm at 1000yds.

BTW, I've never found a published formula for calculating the down range spin rate based on measurable muzzle velocity & bore twist.  If you have a one, I'd sure be interested.

Wayne
========================

Since then I did some addition research but still could not come up with formula.  So, I created one as follows.  Knowing the spin rate can help in determining the stability of a bullet at various distances.  Keep in mind that since there are many factors that can effect bullet spin, this is a rough calculation.

Wayne
« Last Edit: Nov 22nd, 2025 at 12:23pm by texasmac »  
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JHand
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Re: Down Range Spin Rate Formula
Reply #1 - Nov 21st, 2025 at 10:59am
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Now you have me thinking and going down a rabbit hole... By any chance, is there an acceptable number for what rpm a bullet needs to be, to be stable? I thinkni have been unknowingly playing around with this. In silhouette,  I have always been great to pigs, but my loads fall apart after that. I crammed as much powder as I could in my 45-70 case which got my velocity to 1300fps on a 535gr bullet, and the long range suddenly worked.
  
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Re: Down Range Spin Rate Formula
Reply #2 - Nov 21st, 2025 at 12:42pm
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I think a bullet diameter factor needs to be added to any formula that uses time vs rpm decay.

A large diameter flywheel will hold it's rpm a lot longer than a small diameter one.

  
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texasmac
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Re: Down Range Spin Rate Formula
Reply #3 - Nov 21st, 2025 at 3:21pm
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JHand wrote on Nov 21st, 2025 at 10:59am:
Now you have me thinking and going down a rabbit hole... By any chance, is there an acceptable number for what rpm a bullet needs to be, to be stable? I thinkni have been unknowingly playing around with this. In silhouette,  I have always been great to pigs, but my loads fall apart after that. I crammed as much powder as I could in my 45-70 case which got my velocity to 1300fps on a 535gr bullet, and the long range suddenly worked.


If you search the Internet for external ballistic information there are articles that discuss & include bullet stability formulas.  They include the math to determine spin rate based on velocity & twist rate but also need the bullet weight & diameter.  The two most common are the Greenhill formula & the Miller Twist Rule.  The Miller Twist Rule is setup for a velocity of 2800fps & requires a correction factor for other velocities.  Here's an article that discusses both: (You need to Login or Register to view media files and links).

Using the Miller formula, for a bullet to be stable, the stability factor as the bullet leaves the muzzle should be around 1.4 or greater.  The stability actually increases down-range as the forward velocity drops much faster than the spin rate.

Wayne
« Last Edit: Nov 21st, 2025 at 4:22pm by texasmac »  
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frnkeore
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Re: Down Range Spin Rate Formula
Reply #4 - Nov 21st, 2025 at 6:09pm
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This is the best one I have found. It considers length, weight, bullet shape (and therefore CG) and velocity, including sub and transonic.

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texasmac
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Re: Down Range Spin Rate Formula
Reply #5 - Nov 21st, 2025 at 9:44pm
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frnkeore wrote on Nov 21st, 2025 at 6:09pm:
This is the best one I have found. It considers length, weight, bullet shape (and therefore CG) and velocity, including sub and transonic.

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Those formulas were created by Tom Myers (TMT Precision Software) which I've been using for many years.  Excellent stuff but it does not determine the down-range spin rate unless I've missed something.

Wayne
  
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Re: Down Range Spin Rate Formula
Reply #6 - Nov 22nd, 2025 at 4:07am
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First you have to have a accurate stability program, to base any other component of stability.

Then, as I said, I don't know how your formula can accurately determine down range decay, w/o having a flywheel or diameter component.   

Do you deny that a 22 cal bullet with the same or less SD as a 45 or 50 cal will loose rpm faster than the larger bullet?
  
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Re: Down Range Spin Rate Formula
Reply #7 - Nov 22nd, 2025 at 8:37am
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JHand wrote on Nov 21st, 2025 at 10:59am:
Now you have me thinking and going down a rabbit hole... By any chance, is there an acceptable number for what rpm a bullet needs to be, to be stable? I thinkni have been unknowingly playing around with this. In silhouette,  I have always been great to pigs, but my loads fall apart after that. I crammed as much powder as I could in my 45-70 case which got my velocity to 1300fps on a 535gr bullet, and the long range suddenly worked.


Don't forget about the instability in the transition down to subsonic.  Sharp mentioned it - smaller group - larger group - back to smaller group after the transition.
  
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Re: Down Range Spin Rate Formula
Reply #8 - Nov 22nd, 2025 at 10:36am
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I’m sure you’re familiar with Brian Litz who is pretty well known for being able to come up with an equation for everything Ballistic, pretty well gave up on this one. Too many variables. That might have changed in the last couple years after more manual testing, but In short he used foam panels and bullets marked with a sharpie to measure spin over a specified distance. 

Part of the problem with variables were things like different drags and rates of spin decay due simply to different rifling. From memory those variables alone had more than a 2% difference using the same bullet and velocity from different barrels

Modern advancements in long range shooting V1. Worth reading if you haven’t.
  
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texasmac
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Re: Down Range Spin Rate Formula
Reply #9 - Nov 22nd, 2025 at 12:35pm
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Dellet wrote on Nov 22nd, 2025 at 10:36am:
I’m sure you’re familiar with Brian Litz who is pretty well known for being able to come up with an equation for everything Ballistic, pretty well gave up on this one. Too many variables. That might have changed in the last couple years after more manual testing, but In short he used foam panels and bullets marked with a sharpie to measure spin over a specified distance. 

Part of the problem with variables were things like different drags and rates of spin decay due simply to different rifling. From memory those variables alone had more than a 2% difference using the same bullet and velocity from different barrels

Modern advancements in long range shooting V1. Worth reading if you haven’t.


I am familiar with Brian Litz's articles & can appreciate the complexity of attempting to accurately calculate down-range spin rate.  That’s why I noted it’s an approximation.  Having no insight into how the 2% decay rate was determined by the Aberdeen Proving Grounds, I can certainly appreciate your comments.

Wayne
  
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