Not to sound retarded ( my head is hurting from all this. It's been a while since school) but can you share the formulas you're using? I've got one differed out but would lime to make sure im understanding correctly.
Wes
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Not to sound retarded ( my head is hurting from all this. It's been a while since school) but can you share the formulas you're using? I've got one differed out but would lime to make sure im understanding correctly.
Wes
Okay, I read the response further down the page and this is just confusing. Twist is not RPM, time has nothing to do with twist. But you aren't saying that a 1/9" twist boolit, no matter what the speed, is somehow going to be going appreciably slower than 1 turn in 9" are you? I can see some rotational slowing but it's not much in, say, 100 yds. So please clear this up for me because what you said makes zero sense to me.
to the time to introduce the "expert"at Sierra Ballistics cited by the great "starmetal" in support of his idea that the twist rate wasn't fixed by the the rifling twist but got greater and greater as it whistled down range in that E.Tennessee air.. Maybeso some should call the physics teacher at the local High School.. Onceabull
Aren't the two time measures completely disconnected, with no relationship to each other. One being RPM, which is a result of twist and bullet velocity when the bullet exits the barrel. The RPM doesn't change much, and the velocity no longer has an impact, out of the barrel.
Two is bullet velocity in FPS. That can decrease significantly over a long range.
Since there is no direct connection, out of the barrel, the RPM decreases little, the velocity decreases much more.
Tying them together confuses the issue.
I have heard such also.
While I haven't tried it in a deliberate sense,,, I loaded up some 35 grainers designed for the Hornet once in my Swift,,,,, guessing 4000 fps.
At initial sight in, at the range of first line of sight crossing of about 38 yds iirc, after the first shot I could see, what I would call a vapor trail of lead surrounding the bullet hole. It did have a slight spiral pattern, that means anything.
I can't say more velocity added, would have resulted in total disintegration. But suspect it is possible, as they were not designed for that velocity, with their light jackets and a fair amount of exposed lead.
I didn't persue it further to find out, as they did fine, for near vaporization of the 13 Stripped diggers, at 3500 out of the .223. Which is still close to a 1000 fps faster than intended.
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. A thrown football, regardless of the quarterback, comes no where near the RPM we are talking about. Not a good analogy at all. 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.
BTW; the bullet will make more turns in the donkey but the RPM of the bullet remains the same. 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.
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. 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. I have already explained that the reason is the variables.
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.
Again; the helix is not my theory. The helix is a ballistic fact. If you would research it in a ballistics book or manual you will find that out. 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.
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. Many here have pushed cast bullets to the point they don’t even print on target anymore because the group is so large. 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? 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. 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.
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.
“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.
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.
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.”
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.
I have conducted enough tests to prove the RPM threshold does exist, particularly with cast bullets, and what happens when it is exceeded. I suggest you conduct some of your own test then study up on ballistics before hypothesizing erroneously.
Larry Gibson
Put a Hornady 55 gr SX or the Sierra 50 gr Blitz out of a 9" twist or faster above 3200 fps or so and you will many times see a grey "smoke" trail behind the bullet or a puff of grey at 30 -50 yards where the bullet comes apart. The Speer 52 gr HP out of a 7" twist AR at 3200 fps will many times make a "banana peel" hole through a 25 yard target and won't make it to a 100 yard target.
This has nothing to do with the RPM threshold as such. It has to do with the centrifugal force over coming the structural integrety of the very thin jackets and soft lead cores.
Larry Gibson
No, Bret, it's going faster. Now before you get freaked out by that, understand that it's all relative to time.
Take a trip with me on the back of a boolit, using some made-up but realistic numbers: 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. Farther down range, 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.
I don't know why this is such a difficult thing to get. If the barrel went all the way to a 300-yard target with the same twist the whole way, the boolit would have to be going the same forward velocity at the target as it was at the "muzzle" to make the same number of rotations in a given time period. In other words, for the boolit to be maintaining it's same rate of spin at the target that it does at the muzzle (which it essentially does), yet lose velocity (which it most certainly does, and a significant amount), the number of turns traced in a given amount of travel must increase.
Here's another analogy: A propeller on a small aircraft. Sitting on the runway with the brakes set and ready for launch, the prop is wound up tight at takeoff RPM, then the brakes are released and it begins to accelerate forward, maintaining the same RPM as the plane continues to accelerate down the runway. From the plane's perspective, the prop is turning the same speed the whole time, but if there were oil dripping off the prop blades as it went from a standstill to takeoff, the dripmarks on the runway would go from a puddle at standstill to ever-increasing sling marks, and at 150 mph the marks would be much more widely spaced than at 10 feet after launch.
If that doesn't click, how about this: oil is dripping at a constant rate from a leaky pan plug on you car 'cause the lube monkey you took it to for an oil change didn't replace the cracked Nylon gasket. The drip marks on the ground will be closer to each other at 5 mph than they will be at 55 mph. Agree? Of course they will be. The drip was a constant, the forward velocity was not. Same with boolits spinning in the air. If each rotation of a boolit was an oil drip, the drips will be closer together the slower the boolit goes, and as the boolit decelerates downrange, the boolit will be making more revolutions in a given distance, even though it's revolving at virtually the same rate (like the constant oil drip) as it was at launch.
The problem seems to be that some people have the concept that because twist rate and boolit rotation is fixed, it will be so along the entire boolit's path, just like tires on a car in contact with the road. Not the same thing. Here's what you're probably thinking: A tire will rotate at different speeds as the road speed changes, they are fixed to each other. As the car slows, so does the tire, and it doesn't matter if you go a mile at 100 MPH or a mile at one MPH, the tire will make the same number of revolutions. This is not the same thing as the oil drip or boolit in flight. Notice that if the tire makes a mile at one mile per hour, it is turning SLOWWWWW. If it makes the same mile in the same number of revolutions, but at 100 miles an hour, it is turning FAST. A boolit is rotating at the same rate whether going forward at 3000 fps or one fps, but the number of rotations it makes in a foot is much less in 3000 feet than of travel than it is in one foot of travel.
Gear
"This has nothing to do with the RPM threshold as such. It has to do with the centrifugal force over coming the structural integrety of the very thin jackets and soft lead cores."
I know it's off the topic in relation to cast Larry,,,,,, but.
It is the centrifugal force, that in essence ruins the bullets integrity, caused by running past the rpm threshold of the bullet design So in that sense, they are directly related? Looking at what you noted with 2 different faster twist rates at lower velocity, compared to what I seen with slower twist at higher velocity,,, it seems logical.
As for the cast rpm threshold issue at hand, I haven't the experience to comment, so I'm just reading at this point.
Gear used the term spin rate. That is not the same as RPM. Well, it could be because RPM is also spin rate. But it is the spin relative to a time frame. Spin rate relative to distance is still spin rate and that will increase down range as the boolit slows its velocity. It's RPM will not increase and Gear did not say it would. There's no reason not to speak of spin rate in terms of distance, after all we speak of rifling twist rate in terms of distance and in his example it is relevant as the buzz saw effect would become more apparent as the boolit slows before striking its target because the spin rate would have increased. Confusing isn't it!:roll:
Well, that not quite what I meant Larry, but I guess I can see what your saying with your context.
Now as for the rpm threshold with cast, I'm still having a hard time wrapping my head around that totally.
Up to a point, your thoughts make sense to me, if a boolit with some minor balance issues from the mold, and is running at high rpm's, I could see some accuracy issues as a result. But I don't understand how the high rpms are creating a balance problem in flight, unless it's so fast, or rather the cenrtrifugal force is so great, to start having affect on the boolit. Which seems to me, is what your thoughts on threshold, is more about? Yet then when the PP boolit came into the discussion, and shows good accuracy at higher rpm's, ,,,,you lost me at how the paper "jacket" eliminates the balance or centrifugal issues. As when it leave the muzzle, the paper comes off, having no more association with the bullet, at it's higher rpm's well beyond the threshold you speak of. And yet still have very good accuracy, with a softer boolit/alloy to begin with, compared to what is in your notations. Cast is cast, is it not? Not taking into account specific alloy needs for the pressure at hand with a naked boolit.
I guess what I'm saying is, I'm lost. As at first, your arguements for the threshold has some reasonable rationale behind them. But the PP boolit, logically removed the rationale for me, as both boolits are naked in flight.
How many turns does a bullet fired at 3000 fps make in 100 yards (I'll make it easy 1 turn in 10 inches) -360? (actually it isn't about rpm - yet) And now how many turns does that same boolit spinning at that same rpm. rate (doesn't slow down remember) but traveling at 2000 fps make in 100 yards? The answer is that it turns one turn in 6 inches, roughly. I believe this is what gear is talking about. Rpm remains constant, distance traveled per unit time becomes less and less as the boolit slows down. therefore distance taken to make one revolution is less. If I've got it right this is what geargnasher meant.
In all of this remember that the greatest force acting on a boolit after leaving the barrel (barring colliding with something) is air resistance. The pressure on the frontal part of the boolit is squared more than five times (each time the speed doubles) when it is accelerated from approx 100 fps to 3000 fps. I don't possess the fluid engineering nor the math to get this exact, but I believe it is close. What needs to be remembered about this squaring is that the square derived from the first doubling is then squared again and then that is squared again and the magnitude of the resulting pressure is truly alarming. Thus terming gaseous engineering "fluid dynamics'.
How close my math comes is immaterial though. You all probly have shot in a 30 knot crosswind and watched the boolit strike a foot or more downwind from where you pointed it at 100 yards. Remember that this is a paltry puff compared to the wind in the face of the boolit (which is about 2160 mph approx.). Short of an unstabilized boolit (ie. "gone") this wind resistance will cause more deflection on a non uniform frontal area than any wobbling effect could cause by itself regardless of cause (poor launch, unbalance. rpm, or indigestion). None of these would have any, or virtually no effect at all in a vacuum.
This by itself amounts to a comment made by DrB saying that to if rpm theory boiled down to saying that a boolit can be spun too fast, then there would be no question (or words to that effect), loosely paraphrased. But larry has in the past claimed that air deflection was not an issue in terms of what he was advocating. Air deflection is what everything relates to. Ironically, like the rpm theory there is also nothing to do about it (except aerodynamic design). Where they are dissimilar is that we are apparently transiting beyond the RPM barrier.
At least I'm not the only one. Nobody can convince me that a piece of notebook paper can be strong enough to prevent lube groove collapse, nose slump, or other launch damage. It can't even prevent such in a base-first sizer under a few PSI, how can it do it in a barrel under tens of thousands of PSI? My conclusion is that the PP isn't raising the limit of "RPM" by supporting the boolit better, or making the boolit more defect-free in flight than any other boolit, it functions by some other means to raise the "threshold", probably merely making a better gas seal and adding some strength to the driving bands. If that's the case, then defects of the boolit aren't responsible for creating the RPM threshold of plain boolits to begin with, there is some other factor at work here creating those limits, probably trailing ege failure, uneven jetting on muzzle exit, or some sort of boolit damage during firing from which the PP protects the boolit. Who really knows for sure? I sure don't.
Gear
leftiye
Excellent synopsis that is correct. The confusing part is Gear seems to think the rotaional velocity (RPM) does change with regarde to the bullets velocity slowing down at longer range. This is perhaps confusing because he stated.
"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."
Aparently he did not learn about "constants" in freshman algebra. The rotaion velocity (RPM) remains constant as velocity of the bullet slows down. What you say may have been what gear meant but if so he was confusing a lot of us. You are quite correct, thanks for the input.
Larry Gibson
Onesonek
"Larry thinks the paper protects the boolit well enough during launch to account for the great increase in RPM potential, and it might, but what about outside the gun?"
Consider that the PP does protect the cast bullet, thus there is less unwanted imbalance done to the cast bullet during acceleration (launch). The less unbalanced the bullet is (or the more balanced the cast bullet is) The less the centrifugal force of higher RPM has to act upon and the bullet is more accurate at higher velocity. Basicly simple in that the PP protects and supports the cast bullet preventing obturation, setback, sloughing, etc. to a much higher velcoity. May not be able to convince gear of that but him not being convinced does not make it a proven fact. Perhaps he has some scientific reason why pwdawg can shoot the 311284 PP'd at 3000 fps with excellent accuracy but he nor any one else can/is shooting the same 311284 naked at 3000 fps with equal accuracy?
Larry Gibson
Back in 1988 I brought a new Colt H-bar. Had 7:1 twist. Shooting 55grain UMc brand it could not hit anything. I assumed at the time the twist rate made the bullet come apart. As most AR's of the time used 1:12 twist.
I wonder why they came out with the fast twist AR's to use with heavier bullets?