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RPM equal important as twist rate?

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tor47

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May 18, 2018
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A question for you knowledgeable out there: I see allot of discussin about 1/16 twist vs 1/18, and so on. Usually faster twist = better for slugs, seems to be what most agree on. But if a slug in a slower twist barrel are shot faster, so it spins with the same rpm as the faster twist barrel at slower speed, should not both slugs be stable then? For the sake of discussion, I asume we stay with speeds under 1000fps.
 
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tor47 said:
A question for you knowledgable out there: I see allot of discussin about 1/16 twist vs 1/18, and so on. Usually faster twist = better for slugs, seems to be what most agree on. But if a slug in a slower twist barrel are shot faster, so it spins with the same rpm as the faster twist barrel at slower speed, should not both slugs be stable then? For the sake of discussion, I asume we stay with speeds under 1000fps.
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Ooooh I wanna learn/know more! *posting to follow🤔🧐🤙
 
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tor47 said:
A question for you knowledgable out there: I see allot of discussin about 1/16 twist vs 1/18, and so on. Usually faster twist = better for slugs, seems to be what most agree on. But if a slug in a slower twist barrel are shot faster, so it spins with the same rpm as the faster twist barrel at slower speed, should not both slugs be stable then? For the sake of discussion, I asume we stay with speeds under 1000fps.
Click to expand...
I thiiiiiiiink it would be more unstable 🤷‍♂️?
 
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Speed is a speed and twist rate is - well, a twist rate.
Regardless of a speed, for now... lets shoot that projectile to a 100 meters.
1:10 - means 1 turn in 10"=25cm= turns 4 times in a meter length = the projectile turns 400 times in 100 meters.
1:20 - means 1 turn in 20"=50cm= in a 1 meter length turns 2 times *100 = the projectile turns 200 times in 100 meters.
and just follow that logic.
No matter if you shoot it 600 fps or 1000 fps or mps if you wish, you got a number of turns in 100 meters.
 
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bigHUN said:
Speed is a speed and twist rate is - well, a twist rate.
Regardless of a speed, for now... lets shoot that projectile to a 100 meters.
1:10 - means 1 turn in 10"=25cm= turns 4 times in a meter length = the projectile turns 400 times in 100 meters.
1:20 - means 1 turn in 20"=50cm= in a 1 meter length turns 2 times *100 = the projectile turns 200 times in 100 meters.
and just follow that logic.
No matter if you shoot it 600 fps or 1000 fps or mps if you wish, you got a number of turns in 100 meters.
Click to expand...
In your case does that mean a slug with the 1:20 rotate with the same rpm if shot with twice the speed than the 1:10?
With RPM, I mean revolutions pr minute, not speed of travel.
 
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tor47 said:
In your case does that mean a slug with the 1:20 rotate with the same rpm if shot with twice the speed than the 1:10?
With RPM, I mean revolutions pr minute, not speed of travel.
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Yes, correct, 1 revolution per distance of 20 inches for instance, and how many of 20 inches you have in 100 meters or yards. For a distance of 100 meters for example the projectile will turn 200 times between point A and point B no matter how fast will get there, in a 1 second or 2 seconds or 0.1 second.
 
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@bigHUN , you are trying to separate twist and speed, the number of rotations over a distance is completely irrelevant without velocity, RPM is the stabilizing force for a bullet.


Slugs = Bullets as far as ballistics are concerned. Twist rate means nothing without velocity, the two combined give you RPM. You can't talk twist rate without tying it to velocity, it makes no sense without the two tied together. Subsonic is more funky than supersonic, just about everyone here is shooting their air rifles in the transonic range, which is really funky. Generally, transonic is considered from mach 0.8 to mach 1.2, but even approaching either end on low or high side starts having huge affect on projectile stability, much more so on the subsonic side as you get closer to mach 0.8. The .8 - 1.2 mach is not a magic hard set of numbers, it is a fuzzy range.
Using ballistics calculator for stability factor isn’t pure science, but is a good starting point. One can find plenty of examples of projectiles at a certain velocity, out of a particular rifle that has a low stability factor yet is uncanningly accurate at any range, but it will have a lower ballistic coefficient than it could have if rpm was higher. So accuracy is not an indication of a projectile being stabilized properly. And unless going to the extreme of bullet deformation/destruction at “too high” rpm both accelerating in barrel and/or in flight, excessive rpm is generally not detrimental, airgunners cannot realistically ever approach to high spin.

I just pulled out my calipers and measured a worthless to me H&N 21 grain slug, my uragan HATES them.. I put all the measurements in a proper stability calculator at 59 degrees F and 29.92 inches of mercury atm pressure for a stability factor of 1.5. This is what it spit out, the lines I added for .8, 1, and 1.2 mach are not perfectly placed but are very close. Twist rate kind of hard to read, it goes from 20 to 40 in steps of 5.

pellet example.jpg
 
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Yes, correct, a number of revolution on a given distance is driven by a rifling twist rate no matter what time needed from A to B. You could have a speed of 500 or 1000 or 2000 fps or mps, but the projectile will turn 200 times within distance between A to B.
Simple math, but it is a theory only...because a thick air or thin air, or dust or moisture particles, all these will slow down both the speed and the revolution for a given time.
 
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bigHUN said:
Yes, correct, a number of revolution on a given distance is driven by a rifling twist rate no matter what time needed from A to B. You could have a speed of 500 or 1000 or 2000 fps or mps, but the projectile will turn 200 times within distance between A to B.
Simple math.
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You are missing the point. RPM stabilizes the bullet, FPS AND TWIST rate determines rpm. I don't know why you are caught up in how many times a bullet rotates on it's access over a given distance, that means nothing without the velocity which determines the RPM which determines stability.
 
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karl_h said:
You are missing the point. RPM stabilizes the bullet, FPS AND TWIST rate determines rpm. I don't know why you are caught up in how many times a bullet rotates on it's access over a given distance, that means nothing without the velocity which determines the RPM which determines stability.
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I don't argue with that. What is it exactly that you need again? give me your data and I will put it in a spreadsheet to calculate the RPM for you.
Which is more better? High RPM or low RPM? Well, that you can see on your POI.
 
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Wow... just do the math guys...
Let twist rate equal 1:18 or 0.66 revolutions per foot
Let MV = 800 fps

Revolutions per second = 800(fps)*0.66(revolutions per foot))
That becomes 800*0.667=533 RPS
Multiply that by 60 and you get RPM (60*533)=~32k RPM

Here is another example
Let twist rate equal 1:12 (1 turn per foot)
Let MV = 930 fps

Then 930/1 = revolutions per second
60*930 = RPM = 55.8k

stabilization.png
 
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Not a ballistician but will add, twist rate is only what the barrel says while the projectile is in the barrel. Twist rate increases as the projectile slows. The projectiles velocity slows much faster than the twist rate. There’s a great gta thread on this.

Edit: here’s the GTA thread.

High velocity accuracy - Airguns & Guns Forum

High velocity accuracy , Air guns, Airgun Forum
www.gatewaytoairguns.org
 
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tor47 said:
A question for you knowledgeable out there: I see allot of discussin about 1/16 twist vs 1/18, and so on. Usually faster twist = better for slugs, seems to be what most agree on. But if a slug in a slower twist barrel are shot faster, so it spins with the same rpm as the faster twist barrel at slower speed, should not both slugs be stable then? For the sake of discussion, I asume we stay with speeds under 1000fps.
Click to expand...

A projectile fired at well over supersonic velocities behaves more like you would expect. For a given twist rate, stability remains fairly constant over a large muzzle velocity range.

There is a disconnect in the transonic region. At which point, stability is not so simple.

Airguns are typically high subsonic (> mach 0.5?), so there are other stability issues in that range. For a given twist rate, a projectile fired at 1000fps will start with 2x the rpm as does a projectile fired at 500fps. However, in order to have a comparable stability factor when leaving the muzzle, the 1000fps projectile needs to be spining at more than 2x as fast. What happens with velocity vs rpm downrange is something else, but you still want sufficient stability from the get-go.

Pellets don't depend on rpm so much for stability, but slugs do.
 
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Airgunmann said:
So the faster threw the barrel the faster the rpm correct but changes at distance that's what I thought
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Changes at distance?

If the twist rate is sufficient to stabilize a given projectile at high subsonic velocities, the projectile make be spinning faster than necessary (over-stabilized?) as it slows down at far distances. That is usually preferable to leaving the barrel while under-stabilized from insufficient twist rate.
 
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