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#13548812 - 02/10/19 Re: Let's talk TRANSONIC! [Re: Sharpsman]  
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dodgefan Offline
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Bullet RPM = MV X 720/Twist Rate (in inches)

https://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/

The formula matched up pretty close to what I got scribbling math on the back of an envelope.

Last edited by dodgefan; 02/10/19.
300 BP

#13548975 - 02/10/19 Re: Let's talk TRANSONIC! [Re: Sharpsman]  
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jwp475 Offline
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A 338 cal 300 SMK at 2790 FPS has 200,880 rpms in a 10 twist barrel. The 9 twist barrel with the same velocity will have 223,200 rpm. This gives the 9 twist 22,320 more rpm’s.



I got banned on another web site for a debate that happened on this site. That's a first
#13549571 - 02/10/19 Re: Let's talk TRANSONIC! [Re: Sharpsman]  
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378Canuck Offline
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Thats a whole lot of spinning. So is the rpm a constant? or does it slow as gravity and friction works on it while it flies? Does a long flat bullet rotate for ever at the same speed? If it is not a constant how can you apply your math to come up with that rpm?


It is better to be judged by 12 than to be carried by 6.
#13550484 - 02/11/19 Re: Let's talk TRANSONIC! [Re: ChrisF]  
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Sharpsman Offline
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Originally Posted by ChrisF
Quote
Bob advised that I should have them made with a rebated boat-tail and when I asked him why?? that design he told me that everything he had fired through the spark graph range with a rebated boat-tail had shown a minimum of a 5% increase in performance over the standard boat-tail configuration! That doesn't sound like much but in the case of having to make a 'shoot off'....a 5% increase in X count wins the pot!!

Sharpsman, I've seen you recount this story several times on various sites. Can you help me understand this statement. What metric was McCoy referring to in that 5% increase in performance?
Do you have any samples of your 176 Delta's that you can share a photo of? I'm curious what they look like.

On a different note, I am pretty sure that your 176's did not drive Sierra to produce their 175's. It was actually PWS wanting an analog to the 173 FMJ for what ultimately became M118LR.




The "5% increase in performance" Bob was speaking to was basically less drag and better stability throughout the entire flight path! I think my son has possibly a thousand or so of my 176 so I'll check and see!


"There ain't no freedom....without gunpowder!"
#13550576 - 02/11/19 Re: Let's talk TRANSONIC! [Re: jwp475]  
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378Canuck Offline
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Originally Posted by jwp475

A 338 cal 300 SMK at 2790 FPS has 200,880 rpms in a 10 twist barrel. The 9 twist barrel with the same velocity will have 223,200 rpm. This gives the 9 twist 22,320 more rpm’s.

Still can't follow. How does fps become rpm? Wouldn't you multiply by 60 to get rpm?I can't believe rotations in seconds would remain a constant for a long distance shot.


It is better to be judged by 12 than to be carried by 6.
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#13550658 - 02/11/19 Re: Let's talk TRANSONIC! [Re: Sharpsman]  
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mathman Offline
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It doesn't remain constant, but the angular velocity (rpm) decays much slower than linear velocity (fps).


"In the real world, think of the 6.5 Creedmoor as the modernized/standardized/optimized version of the 6.5x55/.260." John Barsness 2019
#13550954 - 02/11/19 Re: Let's talk TRANSONIC! [Re: 378Canuck]  
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dodgefan Offline
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Originally Posted by 378Canuck
Originally Posted by jwp475

A 338 cal 300 SMK at 2790 FPS has 200,880 rpms in a 10 twist barrel. The 9 twist barrel with the same velocity will have 223,200 rpm. This gives the 9 twist 22,320 more rpm’s.

Still can't follow. How does fps become rpm? Wouldn't you multiply by 60 to get rpm?I can't believe rotations in seconds would remain a constant for a long distance shot.


This is how I did the math before finding that formula.

Went to JBM ballistic found Time of flight over 100 yards for a bullet at 3000 fps which was .103 seconds.

100 yards = 3600 inches which means a bullet fired from a 1 in 10 twist will rotate 360 times during it's travel over a 100 yards.

1 (second) divided by .103 (TOF over 100 yards) = 9.708

360 times 9.708 = 3495 rotations per second

3495 times 60 = 209700 RPM

* I rounded the numbers off.

After thinking about it a bit more I realized why my numbers don't line up with the real smart guys.


Last edited by dodgefan; 02/11/19.
#13551007 - 02/11/19 Re: Let's talk TRANSONIC! [Re: Sharpsman]  
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mathman Offline
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(feet / second) * (turns / foot) * (60 seconds / minute) = rpm Just look at which units cancel out when the fractions are multiplied.

The only trick fill in is the turns per foot. An 8" twist will give 12/8 = 1.5 turns per foot.

Example for 3000 fps in a 10" twist: 3000 * 12/10 * 60 = 216,000 rpm


"In the real world, think of the 6.5 Creedmoor as the modernized/standardized/optimized version of the 6.5x55/.260." John Barsness 2019
#13551012 - 02/11/19 Re: Let's talk TRANSONIC! [Re: Sharpsman]  
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ChrisF Offline
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Quote
Still can't follow. How does fps become rpm? Wouldn't you multiply by 60 to get rpm?I can't believe rotations in seconds would remain a constant for a long distance shot.

It might be easier to follow if we used simpler numbers;
If you are shooting a bullet out of a 12 twist barrel (1 turn in 12" which is 1 turn in 1 foot) at 3000 FEET per Second, the bullet at muzzle is making 1 revolution every 12 inches (1 foot). But that revolution is happening in 1/3000 of a second. In one second it will have made 3000 revolutions. In one minute it will have made 180000 revolutions (3000 revolutions x 60 seconds in a minute) and that is revolutions per minute (RPM).
As mathman has said, the spin slows down much more slowly than the forward motion.

#13551082 - 02/11/19 Re: Let's talk TRANSONIC! [Re: Sharpsman]  
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ChrisF Offline
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Quote
The "5% increase in performance" Bob was speaking to was basically less drag and better stability throughout the entire flight path! I think my son has possibly a thousand or so of my 176 so I'll check and see!


I understand that you might be taking some creative license with that statement, but just in case...a 5% reduction in drag, would have to be translated into a reduction in wind drift which would then be SWAG'd into an improvement in score...
There was a guy named Larry Medler that ran a webpage where he played with such "what if" with ballistics and score. His site is down, but I'll see if I can find it in the wayback machine.

On a different note, Bob McCoy published his research on the flight characteristics of different match bullets. It's in the public domain, and it's a good read if you're interested in such things (from a historical perspective since the studied bullets have been eclipsed for long range shooting). Google "The Aerodynamic Characteristics of 7.62 Match Bullets".

Bravo

#13551239 - 02/11/19 Re: Let's talk TRANSONIC! [Re: Sharpsman]  
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ChrisF Offline
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Last edited by ChrisF; 02/11/19.
#13552795 - 02/11/19 Re: Let's talk TRANSONIC! [Re: mathman]  
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Jordan Smith Offline
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Originally Posted by mathman
It doesn't remain constant, but the angular velocity (rpm) decays much slower than linear velocity (fps).

Rotational velocity? grin


"Others are happy to hit the rock, or maybe the dark spot on the rock. I wanna head shoot the mite clinging to the pubes on the left nut of the fly that's sitting on the black spot on the rock."
#13553279 - 02/11/19 Re: Let's talk TRANSONIC! [Re: Sharpsman]  
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Pharmseller Online content
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#13553972 - 02/12/19 Re: Let's talk TRANSONIC! [Re: Jordan Smith]  
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mathman Offline
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Originally Posted by Jordan Smith
Originally Posted by mathman
It doesn't remain constant, but the angular velocity (rpm) decays much slower than linear velocity (fps).

Rotational velocity? grin


Should I throw a 2pi factor in there to get to radians per minute? whistle


"In the real world, think of the 6.5 Creedmoor as the modernized/standardized/optimized version of the 6.5x55/.260." John Barsness 2019
#13554304 - 02/12/19 Re: Let's talk TRANSONIC! [Re: mathman]  
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Jordan Smith Offline
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Originally Posted by mathman
Originally Posted by Jordan Smith
Originally Posted by mathman
It doesn't remain constant, but the angular velocity (rpm) decays much slower than linear velocity (fps).

Rotational velocity? grin


Should I throw a 2pi factor in there to get to radians per minute? whistle

Nah, that’d just confuse the non-math/physics inclined folks even more. But you can’t blame a guy for taking advantage of an opportunity to poke mathman over a technicality- it doesn’t happen often! smile


"Others are happy to hit the rock, or maybe the dark spot on the rock. I wanna head shoot the mite clinging to the pubes on the left nut of the fly that's sitting on the black spot on the rock."
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