Originally Posted by
Larry Gibson
Gear
I’m going to suggest you research rate of spin as it relates to bullets. You will find that the rate of spin of bullet deteriorates very little as range increases and velocity drops. That is a fact. No kidding. I learned that in high-school physics. It was never a point of contention for me. A thrown football, regardless of the quarterback, comes no where near the RPM we are talking about. Not a good analogy at all. It's a perfect analogy. A boolit isn't as big as a football, nor carries most of it's mass on the outside like a football does, but they are rotating aerodynamic bodies in flight, and subject to the same forces. Ever see the slow-motion shot from behind the Qback as he zings a Hail Mary pass? Notice the helical path the football makes due to the mass of the laces? Notice how small the helix is?As I explained to 44man, a bullet “going to sleep” is a different aspect of ballistics and not the same as the RPM threshold, apples and oranges. I never said it was, where are you coming up with that? The only mention had to do with a thrown football, after a couple of revs it stabilizes from the launch embalance of the passer's hand.
BTW; the bullet will make more turns in the donkey but the RPM of the bullet remains the same. Glad we agree on something. Spin a top on a small stool. Now quickly lift the stool 6 feet. Does the RPM of the top increase? Now low the stool very quickly. Does the RPM decrease? It does neither because the RPM of the top will remain relatively constant whether the top is moving in either direction. You correctly answer your own argument here with “a rate of 4,000 rotations in 3,000 feet at the muzzle is now traveling at a rate of 4,000 rotations in 2,000 feet”. The bullet is making the same 4000 revolutions (that’s the same RPM) regardless of the distance. You really do need to read up on this in a good ballistics book or manual. Larry, what's the matter with you? You're the one arguing that I'm mistaken about it, now you use my own and correct example and tell ME that I need to study ballistics more? I NEVER argued that a boolit maintains virtually the same RPM from muzzle to target, I was trying to point out that a boolit won't make the same number of turns through a given distance downrange as it would at the muzzle, and you and Tatume both told me I was wrong.
You’re reading more into the tire analogy than is there. Jack the car up and spin the tires free of road surface and the same thing I described will happen.
An equation? So why then can’t you or DrB come up with an equation that will tell us the exact velocity at which a .308W with a 150 gr bullet will get the best accuracy? You can't. Now you're getting my drift. You can't either. Know why? Because it ain't the RPM that's causing the problem. If it were, it would be a very simple problem to solve, and DrB and I both are familiar with the math it takes to calculate the effect of dymamic embalance of a rotating object and the effect upon the orbit of the projectile about an imaginary, or inertially neutral, geometric center.Once again you, like DrB, want an exact RPM figure or a way to figure exactly at what RPM the threshold will be reached and will happen. As with an equation to tell us exactly at what fps accuracy will occur, it doesn’t work that way. Then what merit does you theory even have as far as a loading consideration goes? It is no more useful than Richard Lee's theory of ONLY considering alloy strength when loading cast boolits. Don't get me wrong, his method has worked and yielded satisfactory accuracy every time I've followed it, but it is very limiting performance-wise and isn't the gospel on what CAN work, only on what almost always DOES work. I find it VERY interesting that any given rifle load I've looked up or tested using Lee's load data and my chronograph seems to have the same accuracy limits as your RPM theory. You say it's the rotational embalance, he says it's the strength limit of the alloy. Two theories, both explain the same threshold limit, neither one really accounts for what's going on or how to overcome it, although at least his theory follows a fairly simple math formula that holds water in practice.I have already explained that the reason is the variables. That's right. You have to consider the big picture, don't you? Lots more to it than RPM, much of it not well understood.
But once again; we can assume with a regular cast bullet with a BHN of 15 -18 in a medium capacity case using a medium burning powder that the bullet will reach the RPM threshold between 120,000 – 140,000 RPM. That is based on my 45+ years of shooting such and 45+ years of observing the same with other shooters. Many here like felix also agree. No equation, no way to determine the exact RPM when that set of components in that particular rifle under the conditions tested comes up to the threshold other than to test it. Fair enough. So what CAUSES this limit? Is it the one thing, RPM, or is it a combination of the multitude of variables?
Again; the helix is not my theory. The helix is a ballistic fact. Yes it is fact. Watch a football game. If you would research it in a ballistics book or manual you will find that out. I never discounted that, only the AMOUNT you claim it affects group dispersion above a certain RPM point. I suggest Understanding Modern Ballistics by Robert A. Rinker for those just beginning a study of ballistics. A better understanding of ballistics might enhance the credibility of your statements. You have yet to refute or even answer any of the questions I have yet posed, so how is my lack of education involving ballistics (which is nothing more than applied physics, of which I have a formal education, my degree is in mechanical engineering, we touched on applications of physics a time or two) relevant to the questions I've asked? You understand it better than I do. Are not my questions legitimate?
In the above post to you I explained that the helical spiral may take a long distance to even make a significant arc around the flight path. I also explained that it may not spiral but may simply go off on a tangent (the “blown group” you refer to. BS. orbiting bodies don't just "go off on a tangent line" unless something seriously changes, like a rock thrown from a spinning sling or gravel from a tire tread, which is a little different from boolits anyway because of the perpendicular direction of force to the rotating axis. A spinning boolit has a gyroscopic effect acting on it, and while it will shed a pp or grease from the grooves by force, it won't suddenly depart from it's flight path unless encountering some force far greater than it's stabilizing inertial forces, like deflecting off of water or steel, or air. Besides, a boolit can go sideways through a target and still hit in the group, and do so with regularity. I disagree very much with the significance of air pressure acting on the boolit to make shots go severely wide. The air brings up another point so often misunderstood. A puff of wind doesn't push a boolit off course. The wind moves the entire column of media through which the boolit travels, taking the boolit with it. Like a fly buzzing around in you car while you're going down the highway: The fly isn't flying 70 MPH, he's just buzzing through the air INSIDE the car, which is moving relative to the outside air. Many here have pushed cast bullets to the point they don’t even print on target anymore because the group is so large. I've seen it, and under all sorts of velocities and conditions. I've also witnessed it one time from the spotting scope, and the boolit just flew out the muzzle on a wide line, the shooter was a very experienced J-word benchrester and he had no explanation for the flier, in fact said the sight picture was the same through recoil as the other shots. The definition I posted in quotations and italics in this thread in above posts and the other thread explain it quite well and is a direct quote from a ballistics manual. It is not my theory; again should you bother to research it you would find out. “Donut patterns”; what is the “distance to the target” are you talking about? Just how many helical spirals do you perceive happen between muzzle and 100 yards, or 200 yards? Not sure. Could be thousands if you're talking about the little, first-order wobbles caused by boolit balance problems every rotation of the boolit, or maybe several across the distance if you're talking multi-order vibrations. The helical spiral may take several hundred yards if not more to be noticeable (probably why those who say they haven’t seen it haven’t seen it), it may take far more range than that and as explained it may not make a complete spiral to the maximum range of the bullet. Ok, there you go Larry. Now you understand why I don't believe your RPM theory. You said "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." Well, if the helix is several hundred yards long it will affect the point of impact, but not the group size. Or if it does affect group size, it will print a doughnut, not a random scatter. I don't see doughnuts on my 200 -yard targets at speeds where the groups blow up, I see random scatter, as many in the ten ring as anywhere else. As already explained; it depends. By “donut” patterns” you have perceived something happening that I have not said does and, in fact, explained doesn’t. No, there will not be “donut patterns” on the target at any reasonable range we shoot cast bullets at and I’ve never indicated there would be “one or two spirals of the helix between muzzle and target”. You perceive that apparently and your perception is not correct. Perhaps my perception is incorrect. But based upon what you said that I quoted above, I don't see how there could be any other conclusion. If the boolit begins to follow a growing spiral path due to RPM exceeding the balance limit of the boolit, then the boolits would print in a ring, unless there were some that were perfect and hit in the center of the group like they're supposed to, and some that are only slightly imperfect, so print in between the fliers and the perfect ones, making an even dispersion. What I HAVE observed is boolits flying either fairly straight into a good group, or flying fairly straight into a large group, and flying wildly from the start, so you're right, and I am one who just hasn't observed a predictable spiral being the result of blown groups by boolits exceeding the RPM threshold.
I’m sure Pwdawg or others on the PP forum can satisfy you needs for pictures. However, why don’t you test them and find out for yourself rather than hypothesis how it can't be? I have already explained on this thread and others the difference between a PP’d cast bullet and a naked one. Pwdawg is probably the most experienced here at shooting PP’d cast bullets at HV and he has also told you the difference on this thread. You are free to believe what you will but the facts supporting the differences are readily apparent. No, there aren't any facts about reasons WHY the PP blows your RPM theory out the window. Suppositions, yes. Lots of results that prove PP works wonders, yes. No facts that explain exactly why a PP makes a regular boolit immune to the effects of centrifugal unbalancing forces or whatever you think makes the groups blow at the same speeds with unpatched boolits.
“Here's a good test of the theory. Take jacketed bullets and deform them, either by bending the noses slightly, or by drilling small holes in the side of the jacket to create an unbalanced condition. Then observe the RPM limits and patterns on the targets of THOSE. If the theory is true, similar limits should be observed as we commonly note with cast boolits. They should shoot accurately to 120k-140K rpm, then go to hell suddenly, just like cast. If the group dispersion is more linear with regard to rpm increase, then we can assume that there is something else going on with cast than just the effects of imperfections showing up catastrophically at a certain RPM.?"
I already ran that test for Bass Ackwards several years back. I used M118SB ammunition in a 10” twist .308W rifle. I shot 10 shot groups at 50 yards, 100 yards and 200 yards with unmodified bullets. I then drilled on .3 gr of the bullet out on the side of 30 more rounds. I then fired the same 10 shot groups at 50, 100 and 200 yards. The expansion of the regular unaltered M118SB was predictable and linear between all ranges. The non linear group expansion of the altered M118SB was also predictable; a small difference between 50 and 100 yards and a large non linear expansion between 100 and 200 yards. Excellent. Now that makes sense. I missed that test. The same things that happen to cast boolits exceeding the RPM threshold happen to Jwords, can I can assume you shot goth sets at the same RPM and that RPM was above the threshold for equivalent cast boolits in that gun? If so that seems pretty conclusive to me, only it's still a stretch to assume that it's launch damage causing the threshold, for powder burn rate can be tweaked all around, boolit fit can be changed drastically, and still the same RPM/accuracy threshold exists for that gun. The question now is HOW are the boolits becoming unbalanced and what can we do to prevent it, or is it muzzle exit problems or something else entirely? I see your observations that the threshold exists and that imbalance can explain it, but it seems simple enough that removing or extending the balance limit would increase the RPM limit, and for some reason it's very difficult to do so at a certain point. Is this point the resonant imbalance of your tire analogy? If the RPM threshold is merely a balance limit, it would stand to reason that one merely has to load beyond that limit a bit to smooth things out a bit, just like "driving through" a vibration point with a bad tire, the vibration point, or point where the imbalance makes a noticeable difference, is very narrow, and calms down on the other side of it. Seems that if the RPM threshold was due strictly to a balance issue, it would just be a bump in the RPM spectrum, but that's where my knowledge of external ballistics reaches it's threshold, and the forces acting on the boolit are more complex than on a tire.
I also, as reported to DrB on his thread, conducted basically the same experiment using the 311291 in an ’06 one load was below the RPM threshold at 1912 fps (136,400 RPM). The 10 shot groups with that load at 50, 100 and 200 yards were .7”, 1.3” and 2.5” restively. That is quite normal linear expansion. The next load was the same bullet loaded over the RPM threshold at 2500+ fps (179,400 RPM). The groups at 50, 100 and 200 yards were 2.55”, 4.7” and 14.5” respectively. What we see there is little non linear expansion between 50 and 100 yards but a very large non linear group expansion of 14.5” at 200 yards. I have run similar test so many times with a variety of cartridges and twists over the years. The results are the same; exceed the RPM threshold and groups open in a non linear fashion as range increases. I know that years of observation of facts is why you developed your theory, but you know that if you added a patch to that boolit you could shoot the same groups you got at 1900 fps at least 2600 fps, or whereever your safe pressure limit was for the gun and load combo, and I can't figure out how the paper makes the boolit that much more perfect in flight, as it would have to be for those results according to your theory.
If you would care to conduct your own tests in an attempt to prove your own thesis/theory correct then feel free to do so and report back your findings. Better than using a jacketed bullet why don’t you do the same test with a cast bullet? That would be more relevant here as we are discussing the RPM threshold as it relates to cast bullets. As to jacketed bullets I suggest you read any one of the later Hornady loading manuals (try page 29 of the current, 8th Edition Horandy manual). Again, not my “theory” here but quoting directly from the ballisticians at Hornady;
“ As long as the bullet is in the barrel it rotates around its center of form but when it leaves the barrel it spins around its center of gravity and this causes it to veer slightly off its intended course at a tangent to the spiral described by its center of gravity as it went up the bore.” No kidding. The question is how much.
Sound familiar, like what I stated/quoted and have been saying all along? Gear, you may be smarter than me, DrB may be smarter than me and some others here, but do you think you or any of them are smarter than the ballisticians at Hornady who have put that out in millions of reloading manuals? Again, it’s not my “theory”, it is ballistics. So, by the laws of balllistics, and by your own theory, all a person has to do is cast a perfect boolit and launch it without damaging it and it will shoot as well at the same velocities as jacketed bullets? How come, by your own admission, it's virtually impossible to do so above the soul-killing and all-encompassing RPM threshold? If imbalance was the only factor, or even the most important factor to this limit, it seems that it would easily be overcome by careful casting, culling, and loading techniques, and would show up gradually, not like a brick-wall barrier at a certain point in the RPM range. But it doesn't seem to be that way, that's why I keep throwing things out there trying to get to the bottom of this, hoping you can educate me a little bit on why this limit occurs, and what can be done about it.
I have conducted enough tests to prove the RPM threshold does exist, particularly with cast bullets, and what happens when it is exceeded. No, I think you've proven that there is a limit to practical accuracy of plain cast boolits at a reasonably common point, but I don't think you have shown any proof that the failure is because it's just spinning too fast. You've shown a fairly close relationship between RPM and group dispersion, but never really proven what happens to cause it. Yes, you've proven that an unbalanced boolit/bullet CAN cause it, but not why the inverse isn't really true, that a near-perfect boolit would be immune to this threshold, same as a good jacketed bullet. I suggest you conduct some of your own test then study up on ballistics before hypothesizing erroneously.
Larry Gibson
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