If the barrel has rifling of one turn in nine inches, the bullet will make one turn in nine inches at any speed. Further, rpm decreases only slightly over the time the bullet is in flight. There is very little air resistance to rotation of a bullet. But you're right, the "buzz-saw" cartoons we saw during the misinformation campaign against Black Talon ammunition were completely false.Originally Posted by geargnasher
Last edited by Tatume; 08-06-2011 at 01:34 PM.
Gear, I used a 60 gr Hornady in my 220 Swift long ago for long range. It was a thing to see when it hit a crow at 400 yards, wings came out of the tree!
I was shooting at paper one day and did not see a hole. I walked down to find the paper full of little shrapnel holes. Back stepping a foot, I found one blade of grass cut off.
Now the helix thing. I watched thousands of bullets go down when I shot IHMSA.
Using the 240 gr bullet in the S&W 29 always showed the helix and it was pronounced. The bullet was super accurate at all ranges and would consistently shoot 1/2" at 50 meters. The only thing we ever seen was if distance was changed a little the POI would shift about as much as the helix.
Just going to a 250 gr bullet eliminated the helix.
The strangest boolit was from the 30-30 TC. Watching it being shot at 200 meters, the boolit would curve way to the right halfway down and it looked like it was going to hit the next ram in line, then it would swing back and hit the ram aimed at. Wind or no wind or from any direction did not change that. This boolit needed different windage settings for different ranges.
Yet with a scope on the gun from bags I could hit pennies at 100 meters. It was strange to shoot pennies at 100 meters from a pistol with a cast boolit but I could do it over and over.
Last edited by 44man; 11-20-2011 at 12:41 PM.
Larry, your persistence and gluttony for punishment is admirable. Nothing wrong with that friend!
You mention obturation in the explanation and speak of imbalances. I can agree with at least part of that whole heartedly. Controlling obturation by using a properly sized boolit, IMO, will always give better, more predictable results that depending on obturation mashing the boolit into putty and then hopefully fitting the grooves. Same thing for alignment. An off centered boolit isn't likely to be more accurate than a well centered one.
As for the RPM theory itself, it makes sense to me that there is a point where a spinning object exceeds it's capability of remaining stable. Where and how that occurs...got me.
Watching bullets and boolits do the strangest things is hard to believe. Yet none of the goofy stuff ever affected accuracy and most shot better then bullets going in a straight path.
The same thing about a WFN not being able to shoot distance, it is just not true. They were just shot wrong.
To shoot 1/2" down to 1/4" groups at 100 meters with a 10" barrel Contender 30-30 and cast is not possible either---or is it?
In my considerable cast bullet experience (CCBE??) with MY ACWW alloy, as a bullet exceeds 120,000 RPMs and heads toward 150,000+ RPMs, two things happen simultaneously: (1) groups start becoming patterns; and (2) bullet holes in the paper target start containing commas.
The commas are caused by the ridges between lube grooves separating from the bullet. Think of a Q shaped bullet hole. Larry is 100% correct.
Last edited by Junior1942; 08-06-2011 at 08:51 AM.
I have always believed under spun bullets are the worst. They give up accuracy with range too fast.
I have recovered a number of 311284 bullets from my berm and none had missing ridges. These were fired at just over 3000fps with a 1 in 10 twist. They were supported in the barrel with a paper patch that prevented distortion due to acceleration. The bullet noses were mangled but the shank was near perfect. That is 216,000rpm if my math is correct. I use a slow powder that gives me near 100% load density and the most gentle acceleration possible, maybe that is why one powder shoots better than another. Accuracy was just under MOA.
45 AUTO! Because having to shoot someone twice is just silly!
Incorrect. The number of turns per distance traveled is variable because it is a function of TIME. If a boolit makes one turn in nine inches after leaving the muzzle, and it hits a target downrange going half the muzzle speed, the boolit will still be rotating almost the same number of turns per minute as when it left the muzzle, but going half the distance in the same amount of time, so it will turn almost one revolution in 4.5 inches as it enters the target.
Take a cordless drill with a bit in it. Hold the trigger wide open and jam the bit into a board as fast as you can, then do it normally. Note the effect.
Gear
Gear
The paper patch on the cast bullet is like a jacket; it does support the bullet. Pdawg's post in this thread is absolutely correct. I have shot enough PP'd cast bullets to know that also.
The PP does support the bullet and prevent collapse of the lube grooves. A collapsing lube groove or grooves on one side can also cause the nose to slump to one side (a well observed/documented fact with unsupported nose bore riders). I’m sure that Pdawg will tell you here as he has reiterated in other threads that the faster you push (increased acceleration) the harder the alloy must be to prevent unwanted and uneven obturation, setback, sloughing, etc. Remember also that the obturation, setback, sloughing, etc most often do not happen evenly around the bullet. However I believe much of the success with softer AC’d alloys some are using at HV is because they do tend to obturate, setback, slough, etc. more evenly around and throughout the bullet during acceleration. They maintain a better balance that way.
The point being here is the PP does support the cast bullet and prevents unwanted unbalancing of the bullet during acceleration which is why it can be driven faster, at higher RPM than the regular unsupported cast bullet.
How “significant” do you think the helical spiral is? DRB seemed a bit confused (he never answered my query one way or the other) as to bullet rotation (the spin to stabilize the bullet) and the helical spiral. The spiral may take several hundred yards to complete. It may be less, it may be more depending on the degree of imbalance of the bullet and the amount of centrifugal force to act upon that imbalance. If the imbalance is small enough and the centrifugal force isn’t high it may not complete a spiral at all during the bullets flight. I also have watched enough cast bullets in flight at long range and have seen the helical spiral, not a complete spiral but a partial one. On the other hand I have seen complete helical spirals on very long ranges where .50 cals were in use. One can observe tracers at long range begin to spiral around the flight path as the tracer element burns out and the bullet becomes unbalanced.
Go back to my above description and in the quoted portion you will find;
“ 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.”
That is not my theory, thesis or what ever you or anyone else wants to call it. That is straight from most every ballistics book or manual. It is well known, factual and understood by ballisticians. Unfortunately many here who want to discredit the science of ballistics have not really studied the subject.
Of course there are “other things going on”. All of the “other” things you mention are what causes the bullet to be unbalanced. But then realize that once in flight it is the increased RPM that increases the centrifugal force to act upon those imbalances in the bullet that causes inaccuracy or those pesky little flyers. We can shoot those bullets up to a certain velocity/RPM quite accurately but at a point in the higher rate of acceleration we create enough imbalances for the higher RPM to adversely affect accuracy in flight. That is where the RPM threshold is.
Let’s go back to the balanced tire vs the unbalanced tire. As we look at the rotating tire imagine that we are looking at a cross section of the bullet. The center of the hub is the center of form. With the balanced tire we can increase the speed which increases the RPM. Since the tire is balanced the center of spin and the center of gravity of the tire coincide with the center of form. Correlating that by increasing the speed of the tire is the same as increasing fps of a bullet which increases the RPM of both. Thus with a balanced tire we can increase the speed/RPM to a very high rate and the tire will spin smoothly. If it were a balanced bullet it would shoot accurately.
Now let us consider the unbalanced tire. If you not ever driven a car with an unbalanced tired then go pop off the WWs off on of the front tires and drive it up to 70 mph or so and you will experience the phenomenon. No, don’t really do that but those who have driven with unbalanced tires are well aware of what I am describing. As we observe the unbalanced tire spinning we can see that up to a certain speed/RPM it spins smoothly. However, as we increase the speed/RPM at a certain point it begins to wobble. With my old ’56 Ford that was around 30 mph. This is because with the unbalanced tire the center of form remains the same at the center of the hub. The center of gravity is moved out because the tire is not balanced and the center of gravity is rotating around the center of form. This also moves the center of spin away from the center of form which is what was causing the initial wobble at a certain speed/RPM. The centers of spin, form and gravity most often are not coincidental in the unbalance tire or an unbalance bullet either.
As speed/RPM increases the tire begins to hop, with the old ’56 Ford this was around 50 – 55 mph. This is the point where the centrifugal force over comes the rotational stability and the helical spiral begins, i.e. the RPM threshold. Note that the helical spin is at a much, much slower rate that the rate of spin of the tire. As we increase the speed/RPM of the unbalanced tire even more we see the “hop” (helical spiral) of the tire gets larger and much more pronounced. The rate of rotation of the helical spiral is much less that the rotational spin of the tire. The very same phenomenon happens with cast bullets and even jacketed bullets.
Where does, or at what speed/RPM does the “wobble” and the “hop” begin? That obviously depends on the size of the tire, the degree of imbalance, how for the centers of spin and gravity are from the center of form and how far they are from each other. There is no exact speed or RPM. The do know that around 25 – 30 MPH the tire will begin to wobble and at some speed/RPM it will begin to hop. It’s the same with a cast bullet; around 120,000 – 140,000 RPM they will begin to “wobble” and at a higher RPM they will begin to “hop”. The “wobble” is where group accuracy begins to deteriorate and the “hop” is where it goes completely bad as in patterns instead of groups.
A completely unbalanced tire will wobble and hop at a low speed/RPM. If we partially balance the tire it won’t wobble and hop until a higher sped/RPM is reached. The perfectly balanced tire (centers of form, spin and gravity coincide) won’t wobble or hop at all regardless of the speed/RPM because the centrifugal force is radiating equally in all directions. It is the same with bullets; we mitigate the wobble and hop by keeping them as balance during casting, loading and acceleration as possible. The better balanced the bullet the higher the speed (fps) and RPM we can shoot them accurately at.
If we had 10 perfectly balanced cast bullets on exit from the muzzle, given all other things being correct, then we could shoot them very accurately regardless of the RPM up to the structural strength of the alloy. Some here have and do claim to cast and shoot perfectly balanced bullets. They do up to a point. All will admit that at some point accuracy goes. So guess what? That’s the point at which their perfectly balanced bullets are not perfectly balanced anymore and they have found the RPM threshold with their bullets and components.
Larry Gibson
You're right, sorry.
Gear
You hold a common belief that is incorrect. The RPM of a bullet is determing at exit from the muzzle. It is a function of twist and velocity and is pretty much "fixed" on muzzle exit. The Bullets RPM slows down very, very little over a bullets flight, even at maximum range. It is not a function of nor affected by velocity decay down range.
Originally Posted by Tatume
If the barrel has rifling of one turn in nine inches, the bullet will make one turn in nine inches at any speed. Further, rpm decreases only slightly over the time the bullet is in flight. There is very little air resistance to rotation of a bullet.
Tatume, you were correct.
Larry Gibson
Time is important. Revolutions per MINUTE.
However the helix a boolit exhibits might not be due to imbalance at all. I have seen it with the best bullets made.
Bullets will go to sleep after velocity drops and to just say it comes into balance might not be correct.
Bullets not spun up enough will be good at close range but will stray farther and farther at longer ranges.
I am afraid to blame it on out of balance bullets.
Take the most perfect bullet made and it will exhibit these traits.
What's amazing, is how much energy it takes to get a boolit up to the terminal RPM. Up to a third of the amount given to the VELOCITY. ... felix
felix
44man
Bullets with yaw and nutaions are the ones that can "go to sleep". That is not the same affect as the helical spiral caused by centrifugal force on an imbalance. It may appear to be so at short range but the two are different. The yaw and nutation causes settle out and the bullet "goes to sleep" with accuracy being better within 100 - 300 yards. With the helical spiral it gets larger as range increases (non linear group expansion as range increases) and the bullet does not "go to sleep". Two different affects. The most "perfect bullets" that exhibit that are from over stabilization (causes yaw) or a muzzle crown defect (creates the nutation).
Larry Gibson
Larry, your grasp of basic physics is much weaker than I ever imagined. Velocity and RPM are both a function of TIME (rounds-per-minute, feet-per-second), where time and rotational velocity are considered constants (in non-relativistic physics). Actually, velocity itself is a functon of time, whereas acceleration/deceleration would be the second derivative.
Now, you say that the boolit's rotation per distance is fixed, right? Like the barrel itself is three hundred yards long, and the boolit tracks the same rotational path from muzzle to target, regardless of velocity. You also say that, for practical purposes, the boolit's rotational velocity doesn't decay, right? Those two things can't exist simultaneously unless the boolit is traveling the same speed at the target that it was in the muzzle. Savvy? There are four factors: Rotational velocity, forward velocity, (actually a formula of increasing deceleration), distance, and time. The laws of freshman algebra dictate that if you change one thing (in this case, forward velocity) you must balance the equation, in this case by increasing the rotational velocity with regard to time/distance.
Let me put it another way. You're drilling cull burros with a 220 Swift. Your muzzle velocity is 3000 fps, the twist is one in nine. The burro is two feet across. If you shot the burro point-blank at 3000 fps, the bullet would make 2-2/3 revolutions through the burro. Your impact velocity is 2000 fps at, say 300 yards, but the rotational velocity is still a function of time, and is still turning practically the same amount of RPM, only velocity (distance traveled in a given time period) is LESS. Therefore, at 2000 fps, the boolit that was traveling at 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, and will be making one rotation in SIX INCHES instead of one rotation in NINE INCHES that it was at the muzzle. When the bullet passes through the burro, it will make four revolutions going through instead of the 2-2/3 it would have made at point-blank.
Your error is that you forget that both rotational velocity and RPM are variables,or functions (fX), directly related to time and distance NOT fixed values, and even excluding the derivatives involved with negative acceleration and taking only two-foot snapshots of distance at muzzle and impact points you can see that the function of time is the factor you and Tatume forgot about.
Gear
Larry, I watched thousands of bullets and never seen "yaw", only the helix around the original flight path. Bullets never shown other then the base to me. No wobble of the bullet.
Yaw is real, no doubt but this was not the case. There was no degradation of accuracy.
I showed the 30-30 boolit and how it shot. I ran out of gas checks one time so I loaded them anyway. Every single boolit went through the paper sideways at 50 yards. I would call that yaw. I went outside the limits of boolit length to twist to velocity. There was nothing wrong with the boolit except it did not fit anymore. Yes, the lack of the GC length changed the boolit into a scud.
I've seen yaw. Not in flight but the imprint it makes on paper. The holes have a slight 'feathering' on one side or the other, mostly random.
I'd suggest that the amount of yaw that causes the spiral patch is very small and would not be visible. By 'yaw' I mean the slight boolit misalignment with the flight path that has the same rotational speed as the helix itself and is an effect of gyroscopic forces. (Support a spinning gyro on one side and it will slowly 'spiral').
The yaw Larry is refering to is the yaw caused by an internal imbalance of the boolit due to uneven mass distribution and has the same rate as the spin. That would cause a spiral path that opens up exponencially. That's the yaw that makes the non-round holes on paper.
I would think that the likelyhood of complex spiral paths would exist with an imbalanced boolit.
Rest In Peace My Son (01/06/1986 - 14/01/2014)
''Assume everything that moves is a human before identifying as otherwise''
As far as yaw goes, I've shot some decent groups with oval holes, and often the ovals were indicating yaw in the same direction, in other words true yaw, not wobble unless the wobble was miraculously synchronized at impact.
On to the Helix. I thought of a perfect analogy that just about every american male between the ages of 6 and 90 should have witnessed at some point in their lives: The spiral of a well-thrown football. Footballs have a built-in embalance at the laces. When thrown with a good, balancing spiral rotation the football will trace the path of a helical spiral as it wobbles about it's center of mass, which is off the center of axis by about an inch. This may be compounded by the forces perpendicular to the line of departure imparted by the quarterback's fingers as he rolls his fingers downward to wind the ball into the spin as he releases it, but the ball has a natural tendency to "go to sleep" after a few yards and settle into a consistent helical path. At some point, say if the football were fired from a pumkin cannon at speed great enough to exceed the RPM threshold of the football, it might go wildly out of control. This analogy, together with your "tire" analogy, show two things: That unbalanced rotating objects have limits to stablity, and that the helix they follow is directly related to the rate of revolution or a order therof. Also, the diameter of the helical path is less than half the cross-section of the object up to a point, then the path can go wild. Does that pretty much sum up your theory?
I think you could make a lot more sense with your theory if you considered a factor called "resonant frequency". You talked all around it with your tire analogy but didn't ever mention it. I've forgotten more about road force variation, match-mounting, dynamic and static balance combined with damping rate and unsprung mass vs. sprung mass of vehicle than most folks probably ever even thought of, so let me explain what I mean about resonance. A tire/wheel/suspension assembly moves as a unit. It's interesting to note that most dynamic road force variances (think soft spot or knot on a tire) usually occur at 30 mph, regardless of tire size, wheel size, suspension type, or type of vehicle. A 10K lb 4x4 diesel pickup with 37" MT Boggers will start to shake at the same SPEED (albeit a different FREQUENCY) that a Honda will if there is a knot on the tread. This factor will usually occur again in the next order of the harmonic, that being TWICE the rotational velocity, and in the case of a tire in contact with the road, twice the vehicle speed. At the next order, 120 MPH, the tire explodes. It's RPM threshold being exceeded. Compare that to a static embalance, which is really what we have with boolits. A tire-wheel assembly that is out of static balance will usually start to vibrate at 35 mph, smooth out, and do it again at 70. This seems more related to tire size than road force variances, I think because it's more direct and constant of an embalance than tire tread variances. On a small tire a balance problem might show up at 25, 50, and 100, while larger tires might tramp at 45 and nobody has reason to find out if they do so again at 90. What I'm driving at here is that an unbalanced boolit will behave in a very predictable manner up to a certain point based upon it's resonant properties, and if you want to gain some credibility to your theory and make it useful, it would be a simple matter to develop an equation where a person could determine first the resonant frequency of a given boolit, then enter values for twist rate, muzzle velocity, etc. into an equation and get a viable "threshold number" that they can expect to achieve with accuracy.
You also don't explain why the helix, if it is reponsible for blown groups according to your theory, doesn't make circular doughnut patterns on the targets. If a boolit only makes one or two spirals of the helix between muzzle and target, then that isn't responsible for blown groups at all. It might explain why groups get printed in different places on the targets at different distances, but not blown groups. I can understand that a wild loss of stability at a certain point in the RPM spectrum would blow groups, but if it's a predictable, ever-growing helix it would only move the point of impact, not print a group the size of the helical sprial at one distance, unless the helix was making a revolution about every ten feet and the ES of the velocity was equal to that 10 feet or close. In that event the group patters would be doughnuts, anyway, as I already mentioned. Bascially I don't buy your helix theory of embalance causing non-linear group dispersion above the RPM threshold because, based upon what we do know to be true and documented, the patterns on paper don't reflect it. Also, I don't buy it because an identical boolit that's PP'd can be fired at higher velocity and don't pretend to tell me that it isn't subject to the same external forces that an unpatched one is. Paper won't save a boolit nose from slump, especially if the nose isn't patched, and I challenge anyone to show me recovered samples of PP'd GG boolits vs. unpatched GG boolits fired from the same gun at the same velocity where the patch prevented GG collapse or any other damage being done to the boolit during it's trip down the bore except possibly more secure engraving. This overwhelmingly convinces me that RPM has very little to do with accuracy deterioration from 100k to 200k rpm, that there is something else like barrel whip, muzzle exit tip, or deformation at muzzle exit. The only other thing I can think a PP does to increase usable velocity is gas seal and helping the boolit fit the throat so it doesn't deform on launch by creating a cushion around it.
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.
Gear
....Which offers an explanation of why any kind of jacket, be it copper or paper, offers better performance by increasing the bearing strength of the engraved portion so the unit can resist the torque forces imparted by the lands without deforming too badly.
Gear
44man, how far was the paper target? Did you do any tests to determine at what distance the boolit came apart? And how far it was effective in crow after it had come apart?I was shooting at paper one day and did not see a hole. I walked down to find the paper full of little shrapnel holes.
That supports 44man's account of the S&W 29 and the 30-30 TC.I've shot some decent groups with oval holes, and often the ovals were indicating yaw in the same direction,
Rest In Peace My Son (01/06/1986 - 14/01/2014)
''Assume everything that moves is a human before identifying as otherwise''
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