RPM question>>>>>>

[NSP64]

I have read the debates over RPM thresh holds and was wondering if anyone has figured out if there is a MINIMUM RPM to stabilize Boolits?

[StarMetal]

I have read the debates over RPM thresh holds and was wondering if anyone has figured out if there is a MINIMUM RPM to stabilize Boolits?

If you mean if there is a chart per caliber I doubt it. There is definitely a minimum rpm or the bullet will tumble. Also a bullet may start out just over the minimum rpm but as it travels along it will lose that and start to tumble.

[hyoder]

Look up the Greenhill formula.

[jimb16]

It depends on the length of the bullet. Generally speaking, the longer the bullet, the faster the rotation needed to stabilize.

[jandbn]

Greenhill - http://kwk.us/twist.html

Miller (newer version of Greenhill) - http://www.jbmballistics.com/cgi-bin/jbmstab-5.0.cgi

Stability tables - http://www.montanabulletworks.com/page11.html

Calculating Bullet RPM — Spin Rates and Stability article - http://accurateshooter.wordpress.com/2008/06/03/

[303Guy]

... wondering if anyone has figured out if there is a MINIMUM RPM to stabilize Boolits?You know how there are rules and exceptions to the rule. Well, A 1-in-16 twist 22cal is not supposed to stabilize a 55gr spitzer, but a hornet will stabilize a 60gr spitzer with a 1-in-16 twist! And shoot it accurately.

[ihmsakiwi]

You know how there are rules and exceptions to the rule. Well, A 1-in-16 twist 22cal is not supposed to stabilize a 55gr spitzer, but a hornet will stabilize a 60gr spitzer with a 1-in-16 twist! And shoot it accurately.

NOW DON'T TELL ME THAT!!
I have read and re-read all i can find on .22 Hornet and cast as I have a virgin 12" Hornet pistol with a 1 in 16 twist and I have just put my money down on a 47-48gr boolit.
Oh well we shall see what we shall see!! Peter

[303Guy]

ihmsakiwi, I don't think the 12" barrel will behave like my mine. I think it's to do with muzzle blast. "Required Twist" has a 'stability factor'. That may have to do with the destabilizing effect of the muzzle blast - not sure. If I could cast such small boolits I would give my 63gr mold a go.:roll:

[HORNET]

Most on-line versions of the Greenhill Equation are actually the simplified version, which has distinct limitations.
A good article on the actual (full) Greenhill Equation can be found on http://www.fulton-armory.com under "info you need", then Fulton ARmory FAQ, then AR-15 FAQ, then AR-15 Barrel Twists. It's pretty tricky stuff, which is the reason that somebody solved it for everything but the Diameter and Length and made a constant (150, which is specifically for the 220 RN in the .30-40 Krag at standard Krag velocity). Ever since, people have been mis-applying it to everything else and complaining that it isn't correct.:groner:

[Larry Gibson]

You know how there are rules and exceptions to the rule. Well, A 1-in-16 twist 22cal is not supposed to stabilize a 55gr spitzer, but a hornet will stabilize a 60gr spitzer with a 1-in-16 twist! And shoot it accurately.

Obviously you have a magic 16" twist 22 Hornet if it shoots any regular cast or jacketed 60 gr bullet with accuracy. I have been shooting jacketed and cast bullets in Hornets with twists of 12 to 16" for many, many years. I have never gotten any 60 gr bullet to shoot acurately in any 14 or 16" twist Hornet and I have shot a lot of 60 gr cast ut of several of them. As a matter of fact I have to shoot the 60 gr jacketed bullets at maxvelocities out of a 14" twist 22-250 to get any decent accuracy. Theoretically if one could shoot a 60 gr bullet fast enough out f a 22 Hornet it would stabilize for good accuracy. However don't you think the Hornet isn't quite up to the 22-250 in the elocity department. Now one can get 1" or so groups at 50 yards with 60 gr bullets in some 16" twist Hornets but at 100 yard the groups are improved cylinder ith perhaps some tumbling evident in the bulets Same holds true with many longer 55 gr bullets both jacketed and cast. A 50 gr jacketed bullet is about max weight for reliable accuracy in a 16" twist 22 Hornet for stability.

Now if you have a short cylindrical cast bullet of your .303 type that is within the shorter length of most 50- gr bullet lengths then you may get some accuracy. The point is, that bullet then is the exception as it then fits withi the calcuable stability range for the 16" twist.

Larry Gibson

[StarMetal]

Obviously you have a magic 16" twist 22 Hornet if it shoots any regular cast or jacketed 60 gr bullet with accuracy. I have been shooting jacketed and cast bullets in Hornets with twists of 12 to 16" for many, many years. I have never gotten any 60 gr bullet to shoot acurately in any 14 or 16" twist Hornet and I have shot a lot of 60 gr cast ut of several of them. As a matter of fact I have to shoot the 60 gr jacketed bullets at maxvelocities out of a 14" twist 22-250 to get any decent accuracy. Theoretically if one could shoot a 60 gr bullet fast enough out f a 22 Hornet it would stabilize for good accuracy. However don't you think the Hornet isn't quite up to the 22-250 in the elocity department. Now one can get 1" or so groups at 50 yards with 60 gr bullets in some 16" twist Hornets but at 100 yard the groups are improved cylinder ith perhaps some tumbling evident in the bulets Same holds true with many longer 55 gr bullets both jacketed and cast. A 50 gr jacketed bullet is about max weight for reliable accuracy in a 16" twist 22 Hornet for stability.

Now if you have a short cylindrical cast bullet of your .303 type that is within the shorter length of most 50- gr bullet lengths then you may get some accuracy. The point is, that bullet then is the exception as it then fits withi the calcuable stability range for the 16" twist.

Larry Gibson

I'll have to disagree. The best bullet to date in my CZ Hornet with 16 twist is the 55 grains NEI Loverin. We're looking at 1/2 inch so far at 100 yards. Remember me shooting the 72 grains NEI bore rider in the same Hornet and so far the furthest I've shot it was 75 yards and not keyholing. Now I won't claim 1/2 groups with it, nor will I claim improves cylinder either. Haven't tried it a 100 yards.

All this is kind of like the Sierra 63 grain semi point. Supposely it would shoot in 22-250's with the fast twist. Well I have friends that shot them and the keyholed in their 22-250's while other's shot them perfectly fine.

I always say try it before you condemn it, don't go on condemning someone's rifle that you haven't shot nor seen shoot.

[Marlin Junky]

RPM's again, hmmm...

Just out of curiosity, what IS the RPM limit supposed to be and assuming there is some leeway (pun intended) or window associated to it, what does that "tolerance" depend on?

MJ

[Larry Gibson]

There ya go, Joe is the exception to the rule as every cast bullet he shoots in every rifle seems to shoot accurately with most of them always shooting 1/2" groups. I bow to the master.......

BTW; the Sierra 63 grain semi point is the one heavier bullet that is supposed to shoot well in 14" twist 22-250s because of it's shorter length.

Larry Gibson

[StarMetal]

There ya go, Joe is the exception to the rule as every cast bullet he shoots in every rifle seems to shoot accurately with most of them always shooting 1/2" groups. I bow to the master.......

BTW; the Sierra 63 grain semi point is the one heavier bullet that is supposed to shoot well in 14" twist 22-250s because of it's shorter length.

Larry Gibson


.....and the 63 semi point doesn't shoot well in all 14 twist 22-250.

Larry a 55 grain bullet isn't too heavy or long for a 22 Hornet, they even have 55 grain jacketed loads for the Hornet. Why it shoots the 72 grain cast without tumbling is beyond me. I knew that bullet was too long and heavy for the Hornet, but I tried it and was surprised it didn't go 100 percent sideways. Another one you'll remember because you had me measure the twist was shooting those 150 grains RCBS SWC for the 357 mag that I step by step sized down to .309 and they shot out of my M1 Carbine.

Maybe you didn't do a lot of crazy experiments in you life Larry. I did, I was that crazy science nerd you see on TV movies when I was growing up. I experimented with match tip, black powder, rockets, the b words. I took everything apart I could get my hands on. I repaired everyone's bicycle in my part of town when I was a kid. My best friend and I made gas engine karts, we flew gas engine model airplanes, we made rockets, we even made a water cooled .049 inboard engine hydroplane (his dad use to build and race hydroplanes). We made cannons, Carl and I were casting bullet in junior high school. And that's not nearly one quarter of the stuff I've done as kid and adolescence. Maybe you just chased poosey around when you were young, I dunno, but it's apparent you don't have nearly the firearm experimentation that I do.

[Larry Gibson]

MJ

Not wanting to waste your time but the RPM threshold, if that's what you refer to, is not a limit. It is a threshold, as the name implies, that may be pushed or crossed and even may be lowered. For regular cast bullets (the current popular designs with bore riding noses) in CF rifles using the common reloading methods the RPM threshold is most often between 120,000 and 145,000 RPM.

The "leeway" is dependent on the consistency of ignition, acceleration rate, (burning rate of the powder), quality of the cast bullet, alloy used (ductile yet strong enough to resist set back, sloughing or uneven obturation), fit of the bullet to the throat/leade, square ness of the base of the bullet on barrel exit and of course the concentricity of the bullet (center of gravity rotating around the center of form).

All of the "leeway" is what occurs during the internal ballistic phase when the bullet is still within the barrel and on exit; the less deformation of the bullet during acceleration the better the balance of the bullet. Given a square base and a good barrel crown the launch should induce minimal if any yaw. Thus the well balance bullet is less affected by the centrifugal force applied by the RPM during flight (the external ballistic phase). We must understand that no fired bullets, not even jacketed bullets, are perfect on launch: this why rifles shoot groups instead of the bullets all going into one hole. However, the better balanced the bullet is during flight the more accurate it will be.

The RPM threshold may be lowered simply by producing an unbalance bullet or one with an uneven base on launch.

The RPM threshold may be pushed by using a bullet that fits the throat/leade and is minimally unbalanced during acceleration and launch. This bullet will have minimal yaw and he center of gravity will rotate around the center of form as close as possible. Such a bullets flight is then less affected by the centrifugal forces of the RPM. With such we can increase RPM (i.e. increase velocity within a given twist barrel) until we induce imbalances in the bullet or the already induced imbalances allow the increased centrifugal force (increasing RPM increases the centrifugal force) to cause some amount of inaccuracy (it may be a small or large amount depending on th degree of imbalance and the amount of centrifugal force).

Let's take a typical .30 cal bullet for example; the 311291 shot in a 10" twist 30-06. Cast of an alloy of 16 - 18 BHN (Lyman's #2) we can drive it with the slower pistol powders up to 1700 or so fps and get the best accuracy. If we try to shot it faster with that powder accuracy will start to fall off. (Now someone, please don't post that you shoot it at 1750 fps with 2400 with great accuracy. This is an example and there are variables as I've mentioned). Thus with that load the RPM threshold is around 1700 fps or so.

We can switch to a medium powder like 4895 thereby reducing acceleration and lessoning the deformation of the bullet until we get into the 1900 fps range. The accuracy level between the 2 loads is the same but by using different loading methods we have increased the RPM threshold of the 4895 load.

We can then switch to a slower powder like 4350 or 4831 and up the same level of accuracy to 2100 fps or so. Thus, gain, we do so by lowering the acceleration rate and lessoning the adverse imbalances until a higher velocity/RPM rate has been achieved.

How do we know when we exceed the RPM threshold for our specific load? Simple; accuracy deteriorates. It will deteriorate faster with faster acceleration. At some point it will really go bad. If pushing the RPM threshold and you are testing at 100 yards you should also shoot at 200 yards if you think you've a god load. Taking the above 30-06 for example; if we get best accuracy at 1900 fps of say 1.5 moa for 5 shots with a 4895 load and we push the velocity to 2100 fps for hunting load. At 100 yards our accuracy for 5 shots is 2.5 moa. Is that a good load for hunting out to 200 yards? Maybe yes and maybe no; if we shoot a 5 shot group at 200 yards we would expect the group to open in a linear fashion giving a 5 - 6" group. That may or may not be acceptable hunting accuracy but that is not the point. If the RPM threshold is exceeded the 200 yard group size will not be linear in nature from the 100 yard group size. If the 200 yard group size is 6+" or larger then we have exceeded the RPM threshold. The size of the non linear group at 200 yards tells us if we are exceeding the RPM threshold and whether by a lot or a little.

Thus we see, the RPM threshold is not a “limit” as many seem to erroneously believe.

Larry Gibson

[Larry Gibson]

Starmetal

No need for the explanation as I said; I bow to your obvious superior knowledge and experience.

Larry Gibson

BTW; the Sierra 63 gr semi point shoots quite nicely in my 14" twist 22-250 out to 300 yards (farthest I've shot it). The slightly longer Hornady 60 gr HP doesn't. Could it be the dfference in bullet length? Greenhill and other twist/stabilization frmulas say it does.

[Pat I.]

MJ

For regular cast bullets (the current popular designs with bore riding noses) in CF rifles using the common reloading methods the RPM threshold is most often between 120,000 and 145,000 RPM.
Larry Gibson

Larry I'd pretty much agree with this but from my experience and studies I'd apply it to 30 calibers only. I've found that smaller calibers raise the threshold and larger calibers lower it. I've gotten my best accuracy out of an 8 twist 6.5 at 1750 fps which translates to 157,000 RPM.

[felix]

It's not the RPM, but the surface linear speed. ... felix

[Pat I.]

Could be. All I know is that it seems you can spin a small bullet faster than a big one.

[Larry Gibson]

Larry I'd pretty much agree with this but from my experience and studies I'd apply it to 30 calibers only. I've found that smaller calibers raise the threshold and larger calibers lower it. I've gotten my best accuracy out of an 8 twist 6.5 at 1750 fps which translates to 157,000 RPM.

That's a generally held assumption and there is much to support it. Given an equal defect (in equal % to bullet weight) or imbalance in the cast bullet the farther away from the center of spin the greater the adverse affect on accuracy seems to be. It is compounded by the distance the center of spin is away from the center of gravity. The smaller the bullet the less these distances are. That's why larger diamter bullets seem to be more affected. However, the twist rate for most larger diameter bullets is slower so it appears the RPM threshold is less. I've not come to a specific conclusion on either yet.

The fact is the RPM threshold is just that; a threshold. It is not a specific limit as many here still want to erroneously believe. The specific point where the RPM threshold is for a specific cartidge and load is dependent on several things as I mentioned. Regardless of what some may want to think; when we push a cast bullet to a certain point of velocity the centrifugal force of the RPM will overcome the stability of the bullet to some degree and some amount of inaccuracy will result. The faster we push the velocity the the higher the RPM and the greater the inaccuracy will become as the centrifugal force is increased via the increased RPM.

Larry Gibson

[Larry Gibson]

It's not the RPM, but the surface linear speed. ... felix

Correct; the larger the diameter of the bullet (given and equal RPM and equal defect or imbalance) the higher the surface speed of the bullet. This is why the defect has a greater affect from the centrifugal force. Also if the defect is on the out side of the bullet the adverse affect will be greater than if the defect is inside the bullet (air bubble for instance) and closer to the center of spin and center of gravity.

Larry Gibson

[Marlin Junky]

could be. All i know is that it seems you can spin a small bullet faster than a big one.

lol

And I can spin a hard one faster than a soft one. :lol:

mj

[StarMetal]

Correct; the larger the diameter of the bullet (given and equal RPM and equal defect or imbalance) the higher the surface speed of the bullet. This is why the defect has a greater affect from the centrifugal force. Also if the defect is on the out side of the bullet the adverse affect will be greater than if the defect is inside the bullet (air bubble for instance) and closer to the center of spin and center of gravity.

Larry Gibson

This is only if you cast defective boolits. [smilie=1:

[303Guy]

Theoretically if one could shoot a 60 gr bullet fast enough out f a 22 Hornet it would stabilize for good accuracy. However don't you think the Hornet isn't quite up to the 22-250 in the velocity department.Larry, this is as much of a mystery to me as it is to you. One point though, which might answer some questions but lead to still more, is the powder I used - Lil'Gun.

It's not quite true that driving a bullet faster makes it more stable by spinning it faster. What happens is that the faster a bullet is driven, the higher the spin needs to be for stability. This relationship is not linear so can indeed result in stability at higher velocities/RPM but at some velocities the reverse can also be true. The 'threshold' for increase in stability is above 3500fps or something.

It is possible that the impression is given that driving a specific bullet faster improves its stability when what is actually happening is the muzzle velocity/muzzle blast relationship changes as powder charge is increased and velocity increases, meaning the destabilizing influence of the muzzle blast can be reduced relative to the spin stability.

With the hornet, I am talking accuracy way beyond 200m (220yds). This rifle has an effective small critter point blank range of 170m (185yds). It has a suppressor fitted which does strip away muzzle blast from the bullet.