RPM theshold discussion

[Larry Gibson]

DrB

Here's the thread. Please address the examples and points I gave you to consider in your thread, answer my questions and give the parameters of that "test" you mentioned to prove or disprove the RPM threshold.

Larry Gibson

[Char-Gar]

This ought to be interesting.

[Larry Gibson]

This ought to be interesting.

That it should be unless all they want to do is argue it is a "limit" or request a hard and fast RPM figure to say it is a "limit'..Do I have to mention the RPM threshold is not a "limit" BTW?

Larry Gibson

[Wayne Smith]

Larry, how many times have you mentioned that already? I think I can remember at least six!

[Larry Gibson]

Larry, how many times have you mentioned that already? I think I can remember at least six!

Umteen bajillion probably.........

Larry Gibson

[303Guy]

OK. So for me, I want to know what a reasonable velocity is going to be for my 303 Brit. A threshold is going to mean a zone into which things start to get difficult. If you tell me the thrshold is 2200fps then I would expect to not have spin rate issues below that velocity but above that I do need to take it into account. That means that 2200 fps isn't a threashold but rather, from about 2200 fps to about 2400 fps is. The threashold is going to be a 'grey area'. It could also mean that above that threshold things could get better because one set of influencing factors give way to another. Maybe those would include boolit distortion as it exits the muzzle of after it has left the muzzle. Maybe even friction erosion of the boolit in flight plays a role but that's independant of spin rate.

[geargnasher]

Larry, how about a cut'n'paste of your condensed thesis, say, in the OP, so the rest of the world that hasn't been reading about this for years knows what you're talking about?

Gear

[Larry Gibson]

303 guy

It is not a "grey area". However, I can not tell you what it will be, your rifle with the bullet and componants you use will. That is just as i can not tell you the exact velocity your rifle will be the most accurate at. Sounds like a non answer but it is not. Let me explain; assuming your 303 Brit has a 10" twist and you use a 311291 or some similar cast bullet, you cast them of WWs and WQ, they are sized correctly, you use an adequate lube and a GC, the cases are properly fire formed to the chamber and you are using a medium burning powder such as 4895. With such your best accuracy will be in the 120,000 - 140,000 RPM or 1650 fps to 1950 fps. If the bullet fits really well and you use a slower burning powder such a AA4350 or one similar that give consistent ignition then you may push it maintaining that best accuracy up through 2100 fps/ 151,000 RPM. However, if you use one of your custom solid type cast bullets that leave little to no room for unwanted obturation, setback, sloughing, etc. during accelleration with a slow burning powder then you may very well get equal accuracy upwards of 2300 - 2400 fps or 170,000 RPM give or take.

Somewhere in there at a certain point (RPM) with either bullet and componants the centrifugal force will act upon any imbalance in that bullet and the bullet will begin to helically spiral losing accuracy in a non linear fashion as range increases. The rifle and the load/componants will tell you when it has reached the RPM threshold because you will begin to get those uncalled pesky little flyers. Exactly where that will be (either in velocity or RPM) I can not tell you. I will tell you though that if you are using a regular cast bullet of proper alloy that is well cast to get or maintain "best" accuracy at higher than 1950 fps/140,000 RPM will take attention to detail on your part in the casting, sizing, loading process. If i recall correctly you are getting some decent accuracy out of the 303 at higher than 1950 fps. I also recall that you do take care and pay attention to detail which is why you are successful in pushing the RPM threshold higher with your loads and componants.

Larry Gibson

[Larry Gibson]

Gear

Have you any idea how many times I have posted all that in the last 8 years on this forum? Here's what I posted on DrB's thread;

I have described the RPM threshold many times. I have already posted the basis of it in this thread but will post it here again;

“The bullet is unbalanced or becomes unbalance due to obturation in the bore during acceleration. The unbalanced bullet is forced to conform while in the barrel and its center of mass is revolving around its geometric center. When the bullet is free of the barrel's constraint, it will move in the direction that its mass center had at the point of release. After exiting the muzzle, the geometric center will begin to revolve about the center of mass and it will depart at an angle to the bore (line of departure). At 54,000 RPM to 250,000 RPM, depending on velocity and twist, the centrifugal force can be tremendous. It will result in an outward or radial acceleration from the intended flight path (line of departure) and will try to get the bullet to rotate in a constantly growing helix.”

Generally this means that with a regular cast bullet common today, cast of an alloy with a BHN of 16 – 22, GC’d properly, with a good lube, sized correctly, driven with a medium burning powder in a case with a medium bore ratio capacity will give its best accuracy up through 120,000 – 140,000 RPM. Where, exactly, in there depends on the combination of variables and conditions. Above that the RPM threshold is when the centrifugal force becomes strong enough that the bullet begins to follow a helical path outwardly from the line of departure or intended path. The “diameter of the path” is dependant on the centrifugal force (the higher the RPM over the threshold the higher the centrifugal force and the larger the diameter of the helical spiral will be) and the range at which measured. It is not a constant diameter but increases as the range increases. This is the why/where the non linear expansion of the groups comes from and why it is more pronounced at longer ranges past 100 yards.

The RPM threshold may be lowered in a given cartridge with a given bullet simply by using a faster burning powder. You may very well get 1 ½ moa accuracy in an ’06 with a 10" twist using a 311291 cast bullet with 4895 powder at 1900 fps. But if you switch to Unique powder will you get 1 ½ moa at 1900 fps? No, you won’t because the acceleration (the time/pressure curve) is much quicker with Unique and the bullet sustains more obturation, setback, sloughing, etc. during this acceleration. This creates a much more unbalanced bullet of which the centrifugal force will act on sooner and to a greater degree to begin that helical spiral. Conversely the RPM threshold can be raised by using a slower burning powder than 4895 and we can get 1 ½ moa upwards of 2000+ fps. That of course has an increased RPM. what we have done there is to lengthen the time/pressure curve to lesson the obturation, setback, sloughing , etc. and thus having a less unbalanced bullet on exit from the barrel. Thus there are many variables and conditions that affect exactly at what RPM the threshold occurs so we can not give an exact RPM that it will occur at.

The point being is that at some level of RPM the helical spiral will begin if the bullets center of spin does not coincide with the center of gravity and center of form. This is the RPM point at which threshold for that particular cartridge and components will begin. Most every ballistics book/manual has a description of the helical spiral. I use that ballistic definition. The “forward progress per revolution” or the diameter of the helical spiral is dependant on the degree of unbalance and the amount of centrifugal force as already explained. By “per revolution” if you are thinking of the revolution of the bullet (spin) then you misunderstand. The revolution of the helical spiral around the line of departure (intended flight path) is not the same as the revolutions (spin rate) of the bullet to maintain point on stability. The bullets may very well maintain complete rotational stability while following the helical spiral caused by increased centrifugal force acting upon the unbalanced bullet.

If, perhaps, you’re not still understanding my explanation of "helical spiral" then consider the simple example of a balanced tire vs an unbalanced tire as speed/RPM is increased. What occurs to the balanced vs unbalanced tire is the same as what happens to the cast bullet (actually any unbalanced bullet but it's more pronounced and obvious with cast bullets). Consider that you are looking a a cross section of the bullet as you consider the tire and are looking down along the line of departure.

Larry Gibson

[btroj]

So if I understand this correctly a bullet that is cast with no voids from a very round mould, size correctly, and well balanced will have a better potential to be shot at high velocity with good accuracy than one that is somewhat our of round or sized such that it becomes slightly unbalanced.
Interesting concept Larry. I do like the explanation of why a bullet can be accurate at a certain velocity with one powder yet with a different powder accuracy at that velocity may be very poor.
I rarely shoot cast over 1800 fps but will need to look at RPM as a variable along with pressure, velocity, and others.

Interesting.

Brad

[geargnasher]

Larry, I know that every time this comes up it seems that you get swarmed with a bunch of people simply hell-bent to discredit you, and a couple interested in polite conversation. I'm not out to put you on the defensive, so please don't get that way, but the more I shoot cast the less I subscribe to your theory. I observe similar trends with RPM and accuracy limitations, but I simply don't believe that RPM has much to do with it, both because I've noticed corollary trends involving other factors, the physics doesn't make sense, and because I've noticed some things that just don't support the boolit behavior you postulate is responsible for large groups exceeding the particular "RPM Threshold" of the load.

First, and someone brought this up on DrB's Lotak thread, why is it that even an average reloader can take a boolit, say the classic 311291 in a .30-06, find that cast from Lyman #2 and checked it can be fired with 1.5 moa accuracy up to about 1800 fps from a ten-twist bolt-action sporter, but if the same boolit, sans check, is sized to .301" and half-arsed patched with some cheap notebook paper it can be fired with the same accuracy at, say, 2500 fps with a powder appropriate for the velocity/pressure/boolit weight? This happens predictably, often with no special loading techniques or case prep, and often with bore-rider designs where the nose portion, the portion most often blamed for HV unbalance is left bare and un-supported by the patch! WHY? A patch supporting the driving bands at the land contact points doesn't explain much, because that part is supported by the barrel anyway, right? The paper doesn't have any power to protect the nose from slump, the lube grooves from collapsing (boolit accordion), or anything else related to balance that I can think of. If you twist a tail and clip it long enough to make a wad behind the base accuracy is improved, I think mainly due to acting as a wad and sabot to the boolit's base and making it exit the muzzle straight. The way the patch seems to work is by literally acting as a flexible gasket which prevents gas-cutting and leading to a degree large enough to allow the boolit to be fired 25-30% faster with the same accuracy. So I don't think RPM is ultimately what makes a PP boolit reach an accuracy limit. In many cases the peak pressure of the load is the limiting factor to velocity, not the projectile, and accuracy can be maintained in many cases up to that pressure limit.

Second, and I don't think this has ever occured to anyone, if lack of balance from some boolit defect (incited at launch or when cast or loaded) is truly making a boolit follow a helical path of any significance (significant being enough to cause your group dispersion observations partly responsible for your theory's existence), then the groups would show a certain trend to being rings on the target. They would be doughnuts. I do not observe this in my HV groups when they start to fall apart, have you?

Last, following my second query, are some thoughts and questions based on with my observations of boolits being actually fired. I've had several opportunities recently to observe long-range shooting of cast boolits that were, in one case at least, well above the RPM threshold for the gun/boolit, and were indeed making fairly large groups at long range. The interesting thing for me, sitting directly behind and inches above the shooter with a 50-power spotting scope, was I could call a 300 yard shot right out of the muzzle after I'd had a chance to note trends of flight and get used to seeing the boolit in the air. If it were going high and right, it was doing so right out of the muzzle, NOT smacking the paper high and right just because it encountered the target at that point in it's corkscrew path. I never saw any significant "helix" pattern to the flight, and I could clearly see it's path, arc, turbulence of the air around it, and impact with the target. Out further, at 400 and 500 yards it's harder to see the whole path of the boolit, and at my local range there is a valley with some wind currents between 300 and 400 yard berms, but I could clearly see the boolit ride the wind currents. They tended to zig and zag, really neat to watch the boolit's flight path as it encountered temperature layers of the air, currents, and gravity/loss of velocity. It's also possible to see the subsonic transition with jacketed bullets, brief blur of the copper streak and almost a "puff" of mirage from the shockwave. I didn't note this with cast, not certain that the ones I saw were going subsonic at the ranges being observed. Now, the persistence of human vision is a measureable constant, and the frequency of the helix, if anywhere near the rpm (and it would practically have to be, or an order above or below that frequency) of the boolit's rotation, would exceed that persistence by many, many times, and the boolit would appear simply as a grey blur the size of the helix spiral diameter. I can clearly see the boolit as its actual size diameter-wise (but a streak a few feet long) right until it punched the target, so I don't believe that any of the boolits or bullets I've ever observed at long distance followed a helical path anywhere near the size of the groups on the target. No doubt the physics dictate that the boolit/bullet will rotate about it's center of mass once out of the barrel and follow a "corkscrew" path, but I think that the helix would be measured in fractions of an inch, maybe only a few percent of boolit cross-section, rather than inches or feet. Comments?

I just think that there is something else is going on here, not just RPM. I find most fliers caused by lube or powder-fouling purging from the barrel, or just plain old bad boolits once in a while. I find regular ol' lousy groups caused by something going awry at launch, most commonly in my case poor boolit fit and excessive chamber neck clearance. In fact, assuming decent powder/boolit/cartridge combinations, I find excessive chamber neck clearance or eccentric static boolit body/bore alignment to be my biggest detriment to achieving accuracy with regular, unpatched boolits. Lube has it's limitations, some earlier than others in the velocity world. Here I gotta poke a little fun and say facetiously that your whole theory of RPM threshold is really just a mistaken observation of the accurate velocity limits of Javalina Alox! (I know you've done your homework on lubes, but so have I). Alloy, of course, is a factor. I'm beginning to think that barrel harmonics has more to do with HV limits with cast than has previously been thought, at least my me.

Gear

[303Guy]

Thanks Larry.

What I meant by 'grey area' was that it is not a sharp cut-off point but rather an increasing degree of boolit instabilty as velocity increases.

What I find useful in the RPM thrshold hypothesis is the difference of normally expected accurate velocity versus rifling twist rate.

What this tells me is that to maximise performance from a given twist barrel, I need to select an appropriate boolit weight. The faster the twist, the heavier the boolit and lower the velocity.

[crabo]

Trying to understand all of this makes my head hurt, but there is an interesting article in July Rifle Magazine by John Haviland on the 220 Swift. He quotes Lester Womack in P.O. Ackley's Volume 1 Handbook for Shooters and Reloaders about the 220 Swift.

It is Womack's opinion that the 220 Swift was far superior to all of the cartridges used in a burro culling operation that weighed up to 600 pounds.

Womack contends that the reason for the Swift's "phenomenal sucess" was the bullet's fast rotational spin. "Velocity falls off rapidly due to air resistance, BUT, the bullet loses very little of it's rotational spin during its entire flight... As a bullet enters an animal. at any range, and begins to upset, the centrifugal forces cause it to go to pieces- with devastating effect"

Now if I am following this discussion correctly, we are talking about accuracy vs wound channels, and cast boolets and not bullets, but I thought I would throw this tidbit in the mix.

[geargnasher]

Crabo, I've heard reports of .22 caliber j-words being fired at such velocity as to disintegrate upon muzzle exit.

Gear

[303Guy]

I've often wondered about the influence of spin in wound effects. Then I started using my hornet with its 1 in 16 twist which made me think spin had no influence on penetration or wound effects. I had a mini-14 with a 1 in 10 twist and shot bullets to the same velocity. However, if a boolit driven to its RPM threshold it may well play a major part in wound effects.

[geargnasher]

The boolit will only be making one rotation per X number of inches, even at half the muzzle velocity in a 9" twist it's only going to make one revolution in 4-1/2", not exactly a tornado. The action of boolits buzz-sawing their way through a target, in my opinion, is highly overestimated.

Gear

[303Guy]

The action of boolits buzz-sawing their way through a target, in my opinion, is highly overestimated.

Yes, that's a total myth. But a boolit that is spinning fast enough to be only just staying together might just blow up with the additional forces of impact. I don't know, it's just a question.

(I can see Larry is going to be kept busy with this thread! )

[lwknight]

303 , the rpm will be reduced my mass expassion instantly. I'm too tired to articulate this idea thoroughly but its like a gyro-pendrulem effect. An object spinning in its own energy will slow down as the object expands because a longer radius off the axis would have more energy and since its rotational energy is limited to what you gave it in the barrel it has to slow the rate of spin.

Conversly , things spin faster if they get squeezed tighter like the things in the toilet whirl faster as it nears the vortex

[crabo]

Crabo, I've heard reports of .22 caliber j-words being fired at such velocity as to disintegrate upon muzzle exit.

Gear

I think it has something to do with the jacket being designed for a slower velocity while being used in a higher velocity platform.

[303Guy]

303 , the rpm will be reduced my mass expassion instantly. I'm too tired to articulate this idea thoroughly but its like a gyro-pendrulem effect.

Got it! A light just went on. Thanks. That explains quite a bit.