I posted the following comment in a thread several months ago.

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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
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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

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.

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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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?

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.

Dellet wrote on Nov 22^nd, 2025 at 10:36am:


This is the best advice for information on bullet spin rate decay.  Read the Litz book recommended above for detailed information on this topic.

A bullet's spin rate (revolutions/second) is the lowest at the muzzle and increases as the bullet travels down range.  This is because the bullet's velocity decreases faster than does it's spin rate. 

texasmac wrote on Nov 28^th, 2025 at 11:57pm:


Quote:


It should read that, since velocity slows faster than spin, the bullet will spin more per inch of travel down range than at the muzzle.

The tricky part is that in general, a bullet that leaves the muzzle at 2000 fps will be spinning faster (rpm) down range at 1500 fps, than a bullet that leaves the muzzle at 1500 fps in the same barrel. 

It’s all interesting, but holes in target tell a better story. And it’s more fun than math.
  
Texasmac do you have a source for the test at Aberdeen?
I’ve found references to the test, but not the actual testing.