RPM of a speeding bullet

I believe it has to do with the length of the bullet, there isn't a single ideal answer.

This is a good read on the subject and the calculations are really pretty simple. A few months ago we had a thread on this subject.

This thread

This thread with a focus on the 223 Remington cartridge.

Ron

If I had to guess from a true physics point of view, I'd guess it actually comes down to how far the center of drag force is in front of the center of mass. Given any deflection, that would create a moment to start tumbling and that is what the gyroscopic force serves to combat. Without the spin stabilizing it, an object naturally wants yo fly with the center of mass in front of the center of drag such as in an arrow or dart where the fletching keeps the drag in thr back and the center of mass closer to the nose. I'm guessing bullet length is just sort of a proxy/estimate for that distance. If you ever saw the movie The Fastest Indian, that talk a little bit about that in there. He added weights to the front of the vehicle to move the center of mass forward so that it wouldn't suffer from all the aerodynamic buffeting.

I should warn you I'm just guessing and am not a aero engineer of any variety.

IMO and I'm no expert, but I believe its almost impossible to over stabilize a bullet. However, that is unless we are talking about very light weight, thin jacketed bullets shot at very high speeds. When in doubt about what barrel twist to go with, go faster. Just my $.02

ZWCoffindaffer said:

I was familiar with the formula for calculating I. It's is very simple once you know it. I was looking more for what the ideal rpms are for stabilizing a bullet with out over or under stabilizing it

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My read has been it depends on the bullet. A Google of "bullet stabilization chart" will get some interesting results. The one thing everyone agrees on is we want just enough spin to stabilize our bullet, the minimum spin will be a function of the bullet design. I really don't know of any sort of one size fits all formula?

So to answer your initial question:

I'm not sure if anyone has a good answer to this question but, is there an ideal rpm range to hit for optimal stabilization of a bullet?

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No, there is no ideal RPM or spin rate range because it depends on bullet weight, caliber and design. The given seems to be don't over stabilize. I realize this goes against CarJunkieLS1 but is my read and my take on over stabilizing. Not saying I am right on that.

Just My Take....
Ron

It is a complex issue. The bullets length is the determining factor. Longer bullets need to spin faster to be stable in flight. Within the same caliber heavier is USUALLY longer, but not always. The bullet shape is also a factor. RN bullets of the same weight are shorter than pointed bullets.

Here is a good website that will allow you to plug in factors and calculate the ideal twist.

http://www.bergerbullets.com/twist-rate-calculator/

Even then it is not an exact science. Often bullets perform better or worse than you'd expect.

I learned a lot in this recent tread: http://www.thehighroad.org/showthread.php?t=794201

I was familiar with the formula for calculating I. It's is very simple once you know it. I was looking more for what the ideal rpms are for stabilizing a bullet with out over or under stabilizing it

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There's really no such thing as "under stabilized" or "over stabilized". It's either stabilized, or it isn't. The misnomer of "over stabilization" has nothing to do with stabilization, but rather that twist rate is too fast for the velocity achieved, ergo a bullet is spinning so fast that centrifugal force may (or does) exceed the bullet's integrity, causing it to deform or even disintegrate in flight. Was a fairly common phenomenon BITD when people tried to push light weight and lightly constructed .22 Hornet type bullets out of cartridges like the .22-250 or .220 swift, or used lightweight .25-20 and .25-35 JFPs in the .25-06. Today, metallurgical improvements, manufacturing technology, and manufacturers that cater to the super speed crowd have largely mitigated this by offering lightweight pills that can handle being driven to such speeds and the associated RPM & centrifugal force.

MachIVshooter said:

There's really no such thing as "under stabilized" or "over stabilized". It's either stabilized, or it isn't. The misnomer of "over stabilization" has nothing to do with stabilization, but rather that twist rate is too fast for the velocity achieved, ergo a bullet is spinning so fast that centrifugal force may (or does) exceed the bullet's integrity, causing it to deform or even disintegrate in flight. Was a fairly common phenomenon BITD when people tried to push light weight and lightly constructed .22 Hornet type bullets out of cartridges like the .22-250 or .220 swift, or used lightweight .25-20 and .25-35 JFPs in the .25-06. Today, metallurgical improvements, manufacturing technology, and manufacturers that cater to the super speed crowd have largely mitigated this by offering lightweight pills that can handle being driven to such speeds and the associated RPM & centrifugal force.

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Tractability (aka "over-stabilized").....

Not a problem in low angle fire. A problem in high angle fire with extremely fast spun bullets.

If one were to listen to the 'nothing but mil spec AR crowd'
240,000 isn't enough if your bullet weighs more n about 55 grains.
You need closer to 300,000

In reality, bullet length and barrel quality are far more important.

Can't say that any RPM could be used as a minimum or maximum.
As mentioned, far harder ime to over stabilize than under stabilize.

The RPM of a bullet is related to barrel twist rate (and velocity, etc). An indirect way to learn more about the RPMs would be to see what twist rates for a given caliber (and bullet weight ranges) folks prefer.

Mythbusters did a show five or so years ago where they were shooting hand gun bullets (9 mm I think) onto ice. They could fairly easily see the bullets and capture the bullets spinning. It was pretty cool to see and hear them spinning at high velocity on ice (frozen lake).

MachIVshooter said:

There's really no such thing as "under stabilized" or "over stabilized". It's either stabilized, or it isn't. The misnomer of "over stabilization" has nothing to do with stabilization, but rather that twist rate is too fast for the velocity achieved, ergo a bullet is spinning so fast that centrifugal force may (or does) exceed the bullet's integrity, causing it to deform or even disintegrate in flight. Was a fairly common phenomenon BITD when people tried to push light weight and lightly constructed .22 Hornet type bullets out of cartridges like the .22-250 or .220 swift, or used lightweight .25-20 and .25-35 JFPs in the .25-06. Today, metallurgical improvements, manufacturing technology, and manufacturers that cater to the super speed crowd have largely mitigated this by offering lightweight pills that can handle being driven to such speeds and the associated RPM & centrifugal force.

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I like that. A bullet n flight either is or is not stabilized when we get right down to it. Makes for simple looking at it that way too.

Ron

Yeah, I always though the term "over-stablized" was just a misnomer for "spun too fast".
The only way something can be TOO stable, is if it's somehow desired that it be right at the edge of losing stability.

Not the Mythbuster's but this video seems to answer the questions

"Spinning Bullets on Ice - Demonstration and RPM Calculation"

https://www.youtube.com/watch?v=_57GbWN1G1w


"Uploaded on Jan 3, 2011

There seems to be some debate over whether or not this is some sort of internet trick. This video shows that seeing bullets spinning on ice is really not uncommon. The first bullet spinning is a 147 grain 9mm that was fired from a Beretta 92. The rest of the spinning bullets are 230 grain 45 ACP, fired from my Glock 36 compact. This video also gives the mathematical formula for calculating your bullet's RPM."
.

I can't say if this truly happens or is largely theoretical, but at least in theory, there are consequences of a bullet being overstabilized beyond the bullet coming apart. In theory if the bullet is spinning too fast, the bullet may not actually arc with the curved trajectory. In other words instead of coming down nose first like an artillery shell, it might fall more like a landing plane with its nose oriented higher than the actual trajectory. Here is a link describing the phenomenon with an illustration:
http://www.tuffsteel.com/Ballistics/bullfly/fig15.htm

MachIVshooter said:

There's really no such thing as "under stabilized" or "over stabilized". It's either stabilized, or it isn't. The misnomer of "over stabilization" has nothing to do with stabilization, but rather that twist rate is too fast for the velocity achieved, ergo a bullet is spinning so fast that centrifugal force may (or does) exceed the bullet's integrity, causing it to deform or even disintegrate in flight. Was a fairly common phenomenon BITD when people tried to push light weight and lightly constructed .22 Hornet type bullets out of cartridges like the .22-250 or .220 swift, or used lightweight .25-20 and .25-35 JFPs in the .25-06. Today, metallurgical improvements, manufacturing technology, and manufacturers that cater to the super speed crowd have largely mitigated this by offering lightweight pills that can handle being driven to such speeds and the associated RPM & centrifugal force.

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I do remember it well when my friend and I would shoot light 40-45gr. 22Hornet pills out of my 22-250. It was very easy to see a vapor trail from the bullets as they went out to hammer groundhogs. Problem was, sometimes on 250yd and longer shots, the bullets never even reached the targets before they just vaporized. When they did reach though, they were bombs!!

The formula's velocity in fps multiplied by 720 then divided by twist in inches.

Example; 3,000 fps X 720 = 2,160,000. 2,160,000 / 12 = 180,000 rpm. The "720" is a constant that converts twist in inches with fps in feet per second for rev's per minute.

There's sites using the Miller or JBM formulas for calculating optimum twist rates for a given bullet at its muzzle velocity.

www.bergerbullets.com/twist-rate-calculator

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

You'll need data on the bullet, too. Good on line place for bullet lengths and other stuff:

http://www.accurateshooter.com/ballistics/bullet-database-with-2900-projectiles/

'Tis my opinion that the slowest spin rate that stabilizes the bullet in the environment you'll use it at gives best accuracy. Any faster and the more unbalanced the bullets are, the more they'll jump off the muzzle axis upon exit from the barrel. All of which is why benchrest winners oft times will add a tenth of a grain of powder for each 10 degrees F lower than their best load was first developed with. Add that if temperatures are higher. Faster twists don't matter too much but too slow of a twist gets accuracy in trouble quickly.

An incident in the late 1960's with Sierra's 30 caliber 168-gr. HPMK bullet is interesting. One production lot of them was shipped to both the USN Small Arms Marksmanship Unit and the US Army Marksmanship Unit. The bullets shot by USN team members in 1:12 twist Garand barrels leaving about 2600 fps often shed their jackets in the first 100 yards of travel. I've watched them going down range viewed in a spotting scope splay jacket parts in all directions. The AMU international teams shot them in reduced loads in .308 Win. free rifles (about 2200 fps) with 1:12 twist barrels for 300 metre matches without problems. Both units reported better than normal accuracy with those 168's when they reached the targets intact. Both teams were offered replacement bullets for both units; the Navy got replacements but the Army kept theirs promising not to shoot them in their M14NM's which they already knew shot 'em in their 1:12 twist barrels to fast to keep them intact.

To the first order it is a function of length and density (but for a given caliber both will be linked to weight).

You are actually asking about something that has been extensively studied and discussed and here are lots of calculators on line.

Mike

CarJunkieLS1 said:

IMO and I'm no expert, but I believe its almost impossible to over stabilize a bullet. However, that is unless we are talking about very light weight, thin jacketed bullets shot at very high speeds. When in doubt about what barrel twist to go with, go faster. Just my $.02

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There are two effects of overstabilization.
1. If the bullet is non-uniform, faster spin can increase the impact of this. This was a big deal in the early days of jacketed bullets when the jackets were not very concentric with the core.
2. Spin is gyroscopic stabilization intended to resist small pertibations on the orientation of the bullet. The problem with too much spin, at the point the bullet trajectory becomes "rainbow like", say 400-600+ yards an overstabalized bullet will resist the steering forces of drag and fail to turn into the direction of flight and will try to keep the orientation it had leaving the muzzle.

With modern bullets, 1 is usually small and 2 does not come into play at shorter ranges, but it still is an issue. Not a huge issue but compared to things like primer pocket uniformity, flash hole uniformity, neck/bullet concentrically, etc. is is not small at all. Anyone machining their primer pockets or turning case necks would be a fool not to be concerned with overstabilization.

Mike

On a related note. There are some who claim that a faster twist on a hunting bullet is beneficial to taking game animals quicker. For example a 308 with a 1:10 barrel will be more effective on game than a 1:12 barrel with the same bullets even though either barrel could be perfectly fine as far as being stable. The claim is that more bullet RPM's at impact creates more damage.

This is a concept that I've only read about within the last year or so. I haven't seen any data to prove or disprove the idea. But it seems plausible.

Arizona_Mike said:

If the bullet is non-uniform, faster spin can increase the impact of this. This was a big deal in the early days of jacketed bullets when the jackets were not very concentric with the core.

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Back in the Cold War the Russians found the 1:12.6" twist on the SVD designated marksman rifle had trouble stabilizing AP-incendiary bullets so they changed it to a faster 1:9.45" twist. However, the faster twist increased dispersion by a stated 25% when using standard sniper cartridges, their steel core construction apparently caused some problems with consistency.