Once the bullet leaves the barrel, all that is keeping it stable is the bullet RPM.
What affects the flight of the bullet are environmentals.
Environmentals have no practical effect on the bullet RPM.
Bullet RPM range - worth knowing ?
- Thread starter big5ifty
- Start date
Once the bullet leaves the barrel, all that is keeping it stable is the bullet RPM.
What affects the flight of the bullet are environmentals.
Environmentals have no practical effect on the bullet RPM.
Consider a simple example of RPM and twist.
A 300 Blackout and a 30-378 Weatherby.
Both shooting 220 grain bullets.
If you go by twist alone, the bullet manufacturer says you need a 10 twist.
The reality is that the Blackout needs a 8 twist for that. Which produces 103 000 RPM at a very optimistic 1200 fps. [ this number goes down to 84 790 at the lowest velocity by Nosler ]
In other words, the manufacturer is saying 'spin my bullet at least 103 000 rpm or it won't fly properly'.
The Weatherby will produce 208 000 rpm with a 10 twist, and 174 000 rpm with a 12 twist. With a 14 twist, the Weatherby is still producing 149 000 rpm. To get down to 104 000 rpm, the Weatherby needs a 20 twist.
If a 220 grain bullet can come out of a Blackout at 103 000 RPM and be stable, why can't it come out of a Weatherby at 104 000 RPM with 2.4x the velocity, and be stable ?
A 300 Blackout and a 30-378 Weatherby.
Both shooting 220 grain bullets.
If you go by twist alone, the bullet manufacturer says you need a 10 twist.
The reality is that the Blackout needs a 8 twist for that. Which produces 103 000 RPM at a very optimistic 1200 fps. [ this number goes down to 84 790 at the lowest velocity by Nosler ]
In other words, the manufacturer is saying 'spin my bullet at least 103 000 rpm or it won't fly properly'.
The Weatherby will produce 208 000 rpm with a 10 twist, and 174 000 rpm with a 12 twist. With a 14 twist, the Weatherby is still producing 149 000 rpm. To get down to 104 000 rpm, the Weatherby needs a 20 twist.
If a 220 grain bullet can come out of a Blackout at 103 000 RPM and be stable, why can't it come out of a Weatherby at 104 000 RPM with 2.4x the velocity, and be stable ?
Not sure if this is a sidetrack, but I can remember reading something about testing done at Aberdeen back in the early 1900s, maybe prior to world war 1. Not sure if it was to help determine needed twist rates, or to verify effectiveness of twist with different bullets. Wish I could find it again, it was fascinating.
They used 03 Springfield rifles and fired them vertically, and at varying angles away from true vertical, and recovered the bullets to see if they maintained their "stability" upon returning to earth. This was done on tidal flats at a point in time when the water was shallow and flat, and the location of the returning bullet could be easily seen for recovery. They found that the bullets fired vertically had maintained their "point up" orientation throughout the flight, and that it was the base of the bullet that showed deformation from impact when landing. Bullets fired at a certain angle off vertical always hit point first, maintaining their stability too. But bullets in that zone between vertical and the angle where they'd turn over and maintain point first orientation, were often found with random impact markings, showing that they were tumbling as they fell.
I spend a lot of years working in pharma development, and did more than a fair bit of experimental design for determination of a specific data point. Imagine being on a team that had to run those experiments! The best instrument was the Mark 1 Eyeball, and everything was done real-life on actual scale.
They used 03 Springfield rifles and fired them vertically, and at varying angles away from true vertical, and recovered the bullets to see if they maintained their "stability" upon returning to earth. This was done on tidal flats at a point in time when the water was shallow and flat, and the location of the returning bullet could be easily seen for recovery. They found that the bullets fired vertically had maintained their "point up" orientation throughout the flight, and that it was the base of the bullet that showed deformation from impact when landing. Bullets fired at a certain angle off vertical always hit point first, maintaining their stability too. But bullets in that zone between vertical and the angle where they'd turn over and maintain point first orientation, were often found with random impact markings, showing that they were tumbling as they fell.
I spend a lot of years working in pharma development, and did more than a fair bit of experimental design for determination of a specific data point. Imagine being on a team that had to run those experiments! The best instrument was the Mark 1 Eyeball, and everything was done real-life on actual scale.
So maybe the next generation chronographs will have a Tachometer function
Your right, this just aimed at people who like “the easy button” and somewhat a Sarcastic remark LOL
jelenko
Gold $$ Contributor
And, calculated can be different than observed.
dcali
Bullet Maker
Because it just doesn't matter for 99% of the cases. Twist is enough, and it's carved in steel. It's one number that requires no math whatsoever that is directly pluggable into calcualtions to estimate stability.
Does twist tell you exactly what's required under every situation? No. Does RPM? No. (What happens to the required RPM when you go to 9,000' of altitude?). Is RPM even a legitimate way to test for stability? Again - no. It's just not that simple. There just isn't a benefit to trying to fudge some sort of RPM threshold that will be an approximation of stability.
It's true for 99% of the cases where the bullet is made by the manufacturer for a particular cartridge in production rifles made with a standard twist having a fixed range of muzzle velocity.
Would it not be good to know this ? We do know that bullets fly further in less air pressure. It's not unreasonable to assume that you need a faster RPM with higher air density, and less with lower.
It's not a test, it's simply that there is no stability if there is not enough RPM.
You can't separate velocity and RPM. RPM is a product of a constant - the barrel twist - and a variable - the muzzle velocity. It's always there.
If it's too low, the bullets don't fly right.
I want to know what is too low, and I don't care about recommended twist.
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dcali
Bullet Maker
Yes, but the best way to do that is with a stability calculator, which uses twist.
If it's not a test, then what good is it? You really can't separate out just one or two factors and expect to get good results in all cases, regardless of which ones you chose. Even the miller formula, which does exactly that, fails in many cases.
When you look at the physics, it's not exactly rpm that matters. It's the twist rate (calibers per turn) relative to the overturning force on the bullet - that's where velocity comes in. And the overturning force is not linear with velocity like rpm is.
Twist is at least something you can plug in to get at stability (with a calculator) for non-typical setups and conditions. With RPM you have to do the math to convert back to twist so you can calculate stability. So you might as well start with twist.
Which I won't need to use if the manufacturer can tell me the stable RPM range.
I don't know, maybe test fly a bullet with no RPM and see for yourself ?
Velocity is muzzle velocity. It has nothing to do with twist.
Twist rate matters, because it produces RPM together with muzzle velocity.
You cannot separate twist, velocity and RPM, because without one the rest are zero.
If you say twist is all that matters, then you're saying RPM is all that matters. And if so, what should it be for a given bullet ?
I read the whole string. Here is what I do.. used this: https://bergerbullets.com/twist-rate-calculator/
Be sure to use the LOWEST ALTITUDE YOU WILL BE SHOOTING AT. THE COLDEST TEMP AND LOWEST VELOCITY. Go for the twist rate that gives you the 1.5 SG. or as close as I can get with the twist rates offered by manufacturers.
Just my 2 cents
Be sure to use the LOWEST ALTITUDE YOU WILL BE SHOOTING AT. THE COLDEST TEMP AND LOWEST VELOCITY. Go for the twist rate that gives you the 1.5 SG. or as close as I can get with the twist rates offered by manufacturers.
Just my 2 cents
dcali
Bullet Maker
You're falling ot the trap of thinking stability = rpm. that's not true. Stability is impacted by twist and an overturning moment, which is not the same as velocity. the overturning moment is related to velocity, but it's not proprtional to it. So stability is *not* proportional to rpm. So to get to where you want to go, you have to just kind of fudge it, and you can do that with twist already.
But, think about it pragmatically. There's a box of bullets that says "recommended rpm xxx,xxx". I look at my rifle and say, well, it's got a 10 twist, and the velocity ought to be about 3000. Let me do a little quick math, and "yeah, thats probably not going to work".
"Oh, wait, I'm in Colorado and the elevation is 5,000 feet. Let me see if that helps." So i measure a bullet's length, and plug everything into a stability calculator (which requires twist, not rpm) to see if the lower air density gets me over the stability line.
Now do that with just twist. "the box says I need a 10 twist. I have that, but I'm concerned that I'm shooting subsonics." So I measure a bullet, and plug everything into a stability calculator to see if I'm still stable.
There's just no benefit to using rpm, because it's not enough information to get good estimates for weird cases, and for normal cases, twist is plenty good.
There are TWO attributes (types of stability): 1) Static (gyroscopic), which may be accurately calculated via math; 2) Dynamic, which thus far, has defied mathematical prediction. 
During my sole conversation with with Robert McCoy (MODERN EXTERIOR BALLISTICS), shortly prior to his death, Mr. McCoy stated that, my understanding that gyroscopic stability increases as velocity decays - the bullet always gains gyroscopic stability as it slows - was/is correct.
However, the only 'sure-fire' way to test DYNAMIC stability was/is, to shoot at distance - a distance which dictates the transition from super, to sub-sonic. RPM will NOT make up for a dynamically "poor" design.
Even a 'LOW' BC bullet, having both good gyroscopic AND dynamic stability will transition quite well.
So, bullets were designed and made, then, I went out and shot . Mr. McCoy was proven correct. Many (most?) would be surprised at the correctness of Mr. McCoy's advice. There are forms - relatively few - which are simply not dynamically stable: Mr. McCoy's example was the Sierra thirty Cal. 168 Gr. International HPBT, which, unon entering the transition (aka transonic) zone, was unpredictable . . . via a few of my [past] PALMA rifle customers, I've also been informed: "friends don't let friends shoot 168s" . . . In my experience, it's difficult to design/make a dynamically unstable bullet.
RG
P.S. Over possible time of flight, RPM decay is insignificant.
During my sole conversation with with Robert McCoy (MODERN EXTERIOR BALLISTICS), shortly prior to his death, Mr. McCoy stated that, my understanding that gyroscopic stability increases as velocity decays - the bullet always gains gyroscopic stability as it slows - was/is correct.
However, the only 'sure-fire' way to test DYNAMIC stability was/is, to shoot at distance - a distance which dictates the transition from super, to sub-sonic. RPM will NOT make up for a dynamically "poor" design.
Even a 'LOW' BC bullet, having both good gyroscopic AND dynamic stability will transition quite well.
So, bullets were designed and made, then, I went out and shot . Mr. McCoy was proven correct. Many (most?) would be surprised at the correctness of Mr. McCoy's advice. There are forms - relatively few - which are simply not dynamically stable: Mr. McCoy's example was the Sierra thirty Cal. 168 Gr. International HPBT, which, unon entering the transition (aka transonic) zone, was unpredictable . . . via a few of my [past] PALMA rifle customers, I've also been informed: "friends don't let friends shoot 168s" . . . In my experience, it's difficult to design/make a dynamically unstable bullet.
RGP.S. Over possible time of flight, RPM decay is insignificant.
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You just reasoned yourself wrong about RPM and twist.
The blackout may need a tighter twist than recommended for the bullet because you're releasing it with a high dwell -right at Mach1 (trans sonic). Get above or below that challenge zone, before barrel release, and stability follows required twist rate. It has nothing to do with RPMs.
And 10tw is the bullet requirement, not a cartridge requirement.
So the Weatherby barrel needs to be atleast 10tw for that bullet, regardless of RPMs.
Twist requirement (what works) is expressed in INCHES:TURN
That's not inches:second, or seconds:turn, there is no time in the requirement.
It's relative displacement per turn (to overcome that overturning displacement).
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I’m not a competitive shooter but I have figured out that this high speed/zipcode shooting really is rocket science. I can’t always listen fast enough so watching faster will have to do. I’m gonna rewrap my brain around spin speed and forward velocity affected by drag and gravity. Subsonic transition of forward speed is frequently discussed as undesirable but does spin speed decay also create issues? Never thunk about it, does this mean I’m more confused?
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RegionRat
Gold $$ Contributor
(He was a great guy. Memory Eternal )
Here is a rough plot of the stability factor versus range for three twist rates, followed by a clip I am pulling from the text.
McCoy 2nd Ed Ch9 pp 197
RegionRat
Gold $$ Contributor
STORM2, if and when there are "problems" going through sonic transition, they are not so much from the decay in gyroscopic forces as much as they are from the other forces that destabilize the axis growing and a lack of damping.
The load data for 220-grainers in 300 Blackout without pushing for max velocity is subsonic. That is what it was designed for, after all.
The required twist for 220 grain .308 is 10.
What is the twist on .300 Blackout barrels ?
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