RPM theshold discussion

[Bret4207]

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. Because math and me don't see eye to eye! 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

I think I understand what you're saying, that the starting twist remains relatively constant and that the speed changes making the twist relatively faster. It still seems like a lot of other mathematically provable points that are real hard to prove in actual practice. Too bad we don't have a way to show this in simple pictures for non-math believers like me.

Thanks for the explanation. Skeptically yours, Bret

[44man]

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?

That supports 44man's account of the S&W 29 and the 30-30 TC.

I was just shooting 100 yards with the .220 Swift. That drove me nuts all the time because the old Winchester pre 64 had a 1 in 10 twist and loved the 60 gr Hornady. I could never shoot less then 1" at 100. It was over spun at the velocity I shot. It had to be shooting a helix. I sighted this gun at 350 yards and never shot at chucks under 200, I would actually walk away from them another 200 yards to get at least 400.
One day while sighting in I put 5 shots in 1/4" at 350 yards but it would NOT shoot at 100. There could only be one explanation! The RPM's did not change much but velocity dropped, the helix went away, the bullet went to sleep.
This was the reason I loved the .220 over the 22-250, a faster twist for the heavy bullet for better long range results. The .222, .223 and 22-250 shot better at close range with the slower twists.
Now the 29, The helix was VERY FAST around the flight path like touching a gyroscope. As it went downrange, it got less fast until I would lose sight of the bullet so I suspect at some range the bullet would go to sleep. It is strange that 10 gr more bullet weight eliminated the helix. It was not seen from the Ruger at all even with a 240 gr.
You can see what close range BR shooters do, they use a slower twist to eliminate the helix but want a faster twist for long range.
I do not know what causes the corkscrew around the flight path but there is some relationship to velocity, not out of balance bullets.
The question is, what stops the helix? Why do bullets go to sleep? Do the RPM's actually slow just enough? What is the relationship to velocity?
I don't like math or theory, I go by experience in all that I have seen. I would never try to explain the what and why of it.
Regardless, the stopping of the helix is the point of the bullet going to sleep. Whether out of the muzzle or at some point down range.
This stuff turns into the 1000# of canaries in a plane and does the plane lose weight if they all fly.
My way works. Increase velocity until groups close. Keep increasing velocity until groups start to open then back down to the best spot. To keep increasing velocity after passing the correct spot will only tell you that bullet will not shoot there. Calling it an RPM threshold or any other name is a waste of time. Nothing you ever do will make the bullet shoot out of it's accuracy point.
To have a boolit shoot super at 1500 fps and take it to 3200 fps expecting it to shoot is just folly. To try and explain it is worse. The gun you have and the rate of twist will dictate what you can do.
The best illustration is the guy that buys a revolver and looks for the highest velocity possible, then takes the same boolit and wants to shoot it dead slow. The gun has a twist rate built in, it works best where it works but handgun shooters forget twist.
Take your .454 and shoot it at 1600 fps and then want the same boolit to shoot at 800 fps is very stupid. Get a different boolit.

[Junior1942]

Point: the paper patch received most of the rifling indentations, not the bullet surface. The bullet left the muzzle much less deformed than a normal bullet.

Point: dud artillery shells have been observed still spinning in the cavity they punched in the ground. That's zero velocity with RPMs.

Point: my Q effect. Only one thing could cause it: the bullet throwing off part of the area between lube grooves due to an RPM value higher than the bullet can take. Picture a #. In the # you can see both semi-vertical land marks/indentations and horizontal lube groove indentations. The comma on a bullet hole was caused by the little leaning box in the # detaching itself from the bullet on one side. That comma is the first sign of bullet damage due to too-high RPMs for the bullet's alloy and construction.

All jacketed and cast bullets have an RPM threshold past which they will self-destruct.

[357shooter]

Not sure why the RPM vs FPS connection is such a big topic. There is no connection, but I guess that's just me.

It seems to me the air pressure on the bullet nose, which is magnified by longer bullets, plays a much bigger role in destabilizing a bullet. If i understand it correctly the air pressure by increased velocity is offset by the higher RPM (when it left the barrel, and a direct result of the twist rate) of the higher velocity.

Bullet length and RPM need to be balanced so it will "sleep" at some point. Velocity is a third element, but down the list and allows fine tuning for accuracy. It gets discussed most often because it's the easiest to tune.

[geargnasher]

Not sure why the RPM vs FPS connection is such a big topic. There is no connection, but I guess that's just me. The relationship is direct: RPM of a boolit is directly related to twist rate and forward velocity. Not sure what you meant.

It seems to me the air pressure on the bullet nose, which is magnified by longer bullets, plays a much bigger role in destabilizing a bullet. If i understand it correctly the air pressure by increased velocity is offset by the higher RPM (when it left the barrel, and a direct result of the twist rate) of the higher velocity. Lots of things going on with that. Air pressure on the nose, particularly uneven pressure due to a nose deformation, might cause a boolit to yaw, but the air pressure will apply to the side of the boolit's base with a large amount of pressure, too, since the boolit will yaw about it's fore-and-aft center of gravity. these pressures sometimes balance themselves out in a yaw condition, explaining why one or more keyholes can appear within the dispersion of a normal group, and also why keyholes can be wild fliers depending on (I think) boolit shape and the drag profile it presents when yawing through the air. I suppose that a boolit rotating faster due to higher velocity will be "more" stable and resist deflection more, but you have to remember that the boolit is still spinning, and any defect on the nose is imparting equal outward forces on the air pressure as the boolit rotates.

Bullet length and RPM need to be balanced so it will "sleep" at some point. Velocity is a third element, but down the list and allows fine tuning for accuracy. It gets discussed most often because it's the easiest to tune.

This begs another question: Somebody schooled in long-range techical ballistics please explain what "going to sleep" really is. Is it a point where, as forward velocity decreases, but rotational velocity remains virtually the same, that the boolit becomes relatively MORE stablized by spinning more turns in a given distance, and so eliminates the helical path it had been following? If the Helix, like Larry suggests, gets larger at a non-linear rate above the RPM threshold, isn't it reasonable to assume that it will get smaller, or taper DOWN at a certain distance WITHIN the rpm threshold?

Gear

[Larry Gibson]

onesonek

"But I don't understand how the high rpms are creating a balance problem in flight......"

The higher RPM do not create a balance problem in flight. The centrifugal force of the higher RPMs have a greater affect on the imbalances of the bullet. The faster we accellerate a cast bullet in the barrel the higher the probability of unwanted obturation, setback, sloughing, etc.. If the bullet leaves the barrel balanced then the centrifugal force radiates equally in all directions. If the bullet is unbalanced with more weight on one side of the center of form then the centrifugal force will act more in that direction. At a certain point of increased RPM the centrifugal force becomes great enough to cause the bullet to go off on a tangent and begin the helical spiral. That is the the RPM threshold.

The PP is a jacket and does protect the cast bullet from that unwanted obturation, setback, sloughing, etc. as the bullet accellerates in the bore to a much higher velocity. Thus the PP'd cast bullet is leaving the muzzle in a much better ballance condition than the naked cast bullet at any given velocity. The PP coming off the bullet on muzzle exit has nothing to do with whether imbalances were done to the bullet in the bore during acceleration. Point is; the PP protects the cast bullet during accleration from imbalaces being done to it while in the bore. The PP's cast bullet leaves the bore in a more balanced condition than a naked cast bullet given the same velocity. Ergo, since the PP'd bullet is more balanced it will be more accurate above the RPM threshold of the naked cast bullet.

"Cast is not cast?"

No, not all "cast" bullets are the same. Wrap a PP around a cast bullet or a metal jacket (Speer "casts" the cores in the jackets in the Hot Core line) and the "cast' bullet becomes better protected during accleration and will maintain balance to a much higher velocity level than will a naked cast bullet.

"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."

Simply understand the damage (imbalancing) is done during acceleration in the bore not during flight. During flight is where the adverse affect on the bullet takes place that causes inaccuracy.

Larry Gibson

[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?

The SAW was also a 5.56 and one of the requirements was to have tracer burn to 600 meters. The 5.56 tracer is a very long bullet and required the faster twist to stabilize it. That's why the 7" twist. The standard M855 (62 gr FMJBT) is adequately stabitlized in a 9" - 10" though. Most M193 (55 gr FMJBT) do quite well in 9 - 10" twists also. There have been problems with some M193 in 7" twist barrels though. That was with the longer 'A2s 20" barrels. with the current shorter barrels of the M4 series the velocity/RPM is lowered so much it's not a problem. Lethality has become the problem.

Larry Gibson

[Larry Gibson]

"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.

Two different issues; bullets coming apart in flight has to do with the structural strength of the bullet. The RPM threshold has to do with how incresing centrifugal force acts upon imbalances causing inaccuracy. While the centrafugal force is a common denomonater with both the two are not the same. Same as a wind will blow a bullet off and the same wind will cool you down. Same wind, two different things.

Larry Gibson

[geargnasher]

onesonek

"But I don't understand how the high rpms are creating a balance problem in flight......"

The higher RPM do not create a balance problem in flight. The centrifugal force of the higher RPMs have a greater affect on the imbalances of the bullet. The faster we accellerate a cast bullet in the barrel the higher the probability of unwanted obturation, setback, sloughing, etc.. If the bullet leaves the barrel balanced then the centrifugal force radiates equally in all directions. If the bullet is unbalanced with more weight on one side of the center of form then the centrifugal force will act more in that direction. At a certain point of increased RPM the centrifugal force becomes great enough to cause the bullet to go off on a tangent and begin the helical spiral. That is the the RPM threshold.

The PP is a jacket and does protect the cast bullet from that unwanted obturation, setback, sloughing, etc. as the bullet accellerates in the bore to a much higher velocity. If this were a Wikipedia entry there would be red flags all over the place saying "DOCUMENTATION NEEDED". Thus the PP'd cast bullet is leaving the muzzle in a much better ballance condition than the naked cast bullet at any given velocity. The PP coming off the bullet on muzzle exit has nothing to do with whether imbalances were done to the bullet in the bore during acceleration. Point is; the PP protects the cast bullet during accleration from imbalaces being done to it while in the bore. The PP's cast bullet leaves the bore in a more balanced condition than a naked cast bullet given the same velocity. Ergo, since the PP'd bullet is more balanced it will be more accurate above the RPM threshold of the naked cast bullet. Well explained as usual, but still too many unsupported statements of "fact" that don't add up for me. You still haven't touched on how a half-patched boolit can shoot as well a a jacketed one when the same boolit, UN-patched, won't shoot nearly the same velocity. I don't buy the nose deformation/resulting imbalance thing causing the rpm threshold at all because of this simple fact.

"Cast is not cast?"

No, not all "cast" bullets are the same. Wrap a PP around a cast bullet or a metal jacket (Speer "casts" the cores in the jackets in the Hot Core line) and the "cast' bullet becomes better protected during accleration and will maintain balance to a much higher velocity level than will a naked cast bullet. Sounds good, but a copper jacket supports the unsupported nose core (to a point) better than paper can, even if you wrapped paper clear over the nose and twisted it to a point, yet patching the bands seems to extend the useful velocity range of a boolit to nearly the same as a copper jacketed one. Why?

"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."

Simply understand the damage (imbalancing) is done during acceleration in the bore not during flight. During flight is where the adverse affect on the bullet takes place that causes inaccuracy. Crystal-clear explanation of the idea. I don't think Onesonoak or I have any difficulty following your explanations, but I for one am skeptical that the paper patch is that good at preventing the sort of deformation you say is responsible for the increase in threshold. Now do I have an alternate explanation? I wish I did. Maybe someone does, but I don't think preventing off-center nose slumping is a principle cause of the threshold increase offered by the paper patch.

Larry Gibson

Onesonoak, I know that was addressed to you, but I'm responding in general to Larry's statements, not meaing to step on anyone's toes here. Forgive me if I have.

Gear

[Larry Gibson]

Gear

obviously you've made up your mind without any testing of your own. Instead of Hypothosizing why don't you conduct some tests of your own?

In all your hypothosizing you have asked one good question regarding PP'd bullets. This is not a thread about shooting PP'd bullets BTW. It is about the RPM threshold on naked cast bullets. Keep that in mind will you? I will answer your question though;

"You talk about setback, slumping, etc. being prevented by PP and thus raising the RPM threshold over 30% by enabling the launch of a more perfect boolit, but how does the patch prevent the nose from slumping unevenly in the bore any more than an unpatched boolit, when in some cases the nose of the patched boolit isn't even patched?"

Have you ever seen pictures (there is one in the NRA Cast Bullet Handbook) or looked closely at recover cast rifle bullets, especially those with long bore riding noses, and seen rifling marks on one side of the nose? If so you will also note that the nose os not bent but has basicly tilted to the side. Also if you look closely you will see the nose tilted because the lube grooves collapsed under that prtion of the tilt. When lube grooves collapse it can be partially on only one portion of the circumference. It is not an even collapse allthe way around. This can be especially the case in WQ'd bullets where there are spots in the bullet that are harder than other spots.

If the lube grooves collapse on on side of the bullet the can and does tilt in that direction. The PP supports the driving bands and lube grooves and keeps them from collapsing to much greater degree thatn the unsupported lube grooves of the naked cast bullet. That's why.

Larry Gibson

[Larry Gibson]

This begs another question: Somebody schooled in long-range techical ballistics please explain what "going to sleep" really is. Is it a point where, as forward velocity decreases, but rotational velocity remains virtually the same, that the boolit becomes relatively MORE stablized by spinning more turns in a given distance, and so eliminates the helical path it had been following? If the Helix, like Larry suggests, gets larger at a non-linear rate above the RPM threshold, isn't it reasonable to assume that it will get smaller, or taper DOWN at a certain distance WITHIN the rpm threshold?

Gear

Perhaps if you would read a ballistics book (I've already recommended a very good beginning one to you) you'd understand. With rifles that exhibit the "going to sleep" phenomanon, such as 44mans, the cause is yaw and precessions. To quote from the book I suggest you read;

"The initial yaw as the bullet leaves the muzzle is governed mostly by stability (44man's was a fast 10" twist). The direction of yaw is haphazard or random and can be anywhere aroind the 360 degrees of the muzzle. This makes target dispersion larger in cases of considerable yaw with low stability (in 44man's case the bullet was over stabilized).......The yaw may be extremely high with the use of poor and mis-matched equipment (a 10" twist for the Swift which normally has a 14" twist). The force on the bullet is not on the center of gravity but on the front of it. the bullet is trying to tumble in flight. The initial wobble that is yaw and precesion can last up to 200+ yards before settling down."

This yaw and precession and the bullet going to sleep has nothing to do with the RPM threshold, it is a completely different action. Basically with 44man's 220 Swift with the fast 10" twist the over stabilized bullets had yaw and were affected by precession on exit from the muzzle and it took a couple hundred yards for the rotaional stability to over come the yaw and precession and the bullets settled down or as is commonly refered to; "went to sleep".

Larry Gibson

[geargnasher]

Gear

obviously you've made up your mind without any testing of your own. I haven't made up my mind, but until I can get satisfactory and proven answers to some inconsistencies with your theory, I can't accept it as truth.Instead of Hypothosizing why don't you conduct some tests of your own? Much of my skepticism evolves from my own testing and observations, particularly recent experiments with HV PP.

In all your hypothosizing you have asked one good question regarding PP'd bullets. This is not a thread about shooting PP'd bullets BTW. It is about the RPM threshold on naked cast bullets. Keep that in mind will you? Don't try to squirrel out of this one, Larry, it won't work. Paper patching is a loading technique for shooting cast that alters the properties of the boolit, same as using filler, thicker case necks, slower powder, whatever. It is possible to shoot THE SAME boolit either patched or not, so it's interesting to discover the cause of why merely adding the patch has such a drastic effect upon your theory. I will answer your question though;

"You talk about setback, slumping, etc. being prevented by PP and thus raising the RPM threshold over 30% by enabling the launch of a more perfect boolit, but how does the patch prevent the nose from slumping unevenly in the bore any more than an unpatched boolit, when in some cases the nose of the patched boolit isn't even patched?"

Have you ever seen pictures (there is one in the NRA Cast Bullet Handbook) or looked closely at recover cast rifle bullets, especially those with long bore riding noses, and seen rifling marks on one side of the nose? If so you will also note that the nose os not bent but has basicly tilted to the side. Also if you look closely you will see the nose tilted because the lube grooves collapsed under that prtion of the tilt. When lube grooves collapse it can be partially on only one portion of the circumference. It is not an even collapse allthe way around. This can be especially the case in WQ'd bullets where there are spots in the bullet that are harder than other spots. I fully understand what nose slump is, no need for the elementary reiteration of it. I've seen pics, and I've noted it many times in my recovered boolits, particularly Keith-style revolver boolits that were launched at a rate where the static inertia of the nose exceeded the yield strength of the alloy. You still haven't even touched on how the paper patch, WHICH DOESN'T EVEN TOUCH THE OGIVE OF THE NOSE, can prevent this slump and increase the RPM threshold.

If the lube grooves collapse on on side of the bullet the can and does tilt in that direction. The PP supports the driving bands and lube grooves and keeps them from collapsing to much greater degree thatn the unsupported lube grooves of the naked cast bullet. You'll never convince me that paper can prevent the collapse of lube grooves. I can make a patched boolit accordion-up in my base-first sizer just as easily as an unpatched one. It's just paper, like wadding up the paper wrapper when sliding it off of a soda straw, it has very little strength in that dimension. That's why. No. Something else is going on. Some other factor that the PP affects which is not present in unpatched boolits, thus making the difference in accurate RPM/velocity potential. If it can be discovered, it might be applicable to plain cast boolits, and the RPM threshold eliminated or greatly extended.

Larry Gibson

Here are some of MY observations, food for thought in that there are a LOT of things acting to limit cast boolit accuracy/velocity performance, not just one thing called RPM. Plain-based, cast boolits have an accuracy/velocity threshold for some reason. It seems that .30 caliber PB boolits start to shoot bigger groups somewhere between 1400-1600 fps in ten-twist rifles, regardless of cartridge. They also start to lead the bore around 1500-1600 fps. Now add compacting filler, the leading can be eliminated to upwards of 2K fps pretty easily. Add a paper gas check without filler, the leading is also eliminated to at least 1800 fps in the one gun in which I tried it. Accuracy, however, had the same limits of around 1400-1600 fps. Add a copper gas check, accuracy is restored up to at least 2k fps in the same guns with the same alloy, add filler and it goes up a bit more, harden the alloy and accuracy/velocity potential goes up even more, and still more with slower powder. Scrap the gas check and use a paper jacket, the accuracy/velocity potential goes up at least by the same order as adding a copper check to a PB boolit. Difficult to say that it's merely boolit deformation inside the barrel during launch that's causing the external ballistic changes, I think it has to do more with barrel harmonics and muzzle exit, because if it was RPM/balance alone, that would be easier to eliminate with more perfect boolits, and HERE'S ANOTHER ONE, breech seating. Most of the guys that breech seat boolits use slow twist rates and comparatively low velocity. I tried breech-seating in .30-06 once, a few years back when I was hell-bent to get over 2400 fps with a regular cast boolit while maintaining 1.5 MOA accuracy. The guy who was doing my case prep for me (because I didn't have the tools) suggested it and even turned a seater on his lathe. I found it made no real difference, even after many tweaks to the load to compensate for the different dynamics of the case volume. while in no way conclusive, my thoughts on this experience were that boolit deformation on launch wasn't what I was battling at that point, my brass and boolit had been tweaked to fit my gun so well by that point that there was something else going on. Ultimately I never achieved the 1.5 MOA goal with any consistency much above 2400 fps, but I did get close, and could maintain 2 moa out to 2700, but change the temperature, time between shot strings (barrel heat), or the phase of the moon and it went south real quick. I wish I still had that tool, and wish the man that made it and helped me was still here, might do some more tests.

Gear

[Larry Gibson]

Gear

Let me suggest a simple test to you. Take your '06 (I assume it has the standard 10" twist?), a naked GC'd cast bullet of 170 - 200 gr cast of and alloy of 15 - 20 BHN, size and lube the bullet appropriately for your rifle and load in 1 gr increments using 4895 and a 1/2 - 3/4 gr dacron filler (you can load sans the filler but consistency will be better with the filler) from 25 gr to 35 gr. I suggest a minimum of 7 shot test strings with each powder charge but 5 shot strings will suffice for this test. Shoot them for group at 100 yards and chronograph them. Report back showing us the group sizes (scanned or pictures would be nice) and the average velocity for each charge. That will demonstrate whether there is an RPM threshold or not and where you find it with those componants in your rifle.

Try to test from a solid bench rest in relatively windless conditions to eliminate as much of shooter error and wind drft as possible. BTW; there are lots of .223s with twists from 7" upwards of 14' out there and also numerous .308Ws with twists from 10" to 14". You could borrow a couple of those with various twists and test them side by side and see if accuracy doesn't go south at a lower velocity in the faster twists.

Shouldn't be too hard for you to do an actual test of this to prove it one way or the other instead of just hypothesizing why the RPM threshold can't be. I have conducted test after test using just such different twist rifles proving the RPM threshold is real and valid with cast bullets. Why don't you conduct tests to prove to us your hypothesis that the RPM threshold can't be.

Larry Gibson

[357shooter]

The relationship is direct: RPM of a boolit is directly related to twist rate and forward velocity. Not sure what you meant.

Gear

In the barrel that's true, as soon as the bullet exits the muzzle the RPM and velocity no longer have a cause and effect relationship. Without the rifling making the direct connection, there isn't any direct connection.

[Larry Gibson]

In the barrel that's true, as soon as the bullet exits the muzzle the RPM and velocity no longer have a cause and effect relationship. Without the rifling making the direct connection, there isn't any direct connection.

Very well put 357shooter.

Larry Gibson

[grouch]

Larry, I know you've been woking on this for a long time, but it seems to me more logical to consider "rim speed " than RPM because it should better relate to centrrfucal force than RPM does unless you limit your discussion to a single caliber.
Grouch

[geargnasher]

Larry, don't you read ANYTHING I say? I said from the beginning that I have observed the same thing happening as you have, but that I don't find the evidence that RPM is the absolute cause of it. Just like I don't find Richard Lee's pressure theory to be absolute either at HV.

If you want to see what I mean about observing the same things you have, check out this recent example, there have been many others over the years similar to it: I have been working with a brand-new, left-handed .30-06 I bought in 2006, on of the last ones made in New Haven. It's a *** really, rough everything and poor fit-finish. It is a one-in-ten, I checked with a tight patch. The bore is .301" and the groove is .309", the chamber neck is cut to almost .345" which makes fitting brass impossible, even with .311" boolits and military cases. I bought the Lee 312-185 mould to try this gun out, results I must say are typical. All these boolits were water-quenched 50/50 WW/roofing lead, having a month-old hardness of 19.3 bhn on the Lee scale. I started with Reloder 22 because it was the slowest, bulkiest powder I had on hand that would still burn decently at lower pressures. 4831 never worked well for me at lower pressures and 4350 was a bit fast for the velocity I wanted.





This pic shows in the upper left target Reloader 22 at 45, 46, and 47 grains, the lower left shows it at 48, 49, and 50, the lower right shows it at 51 and 52. My order is top, LL, LR in the three dot targets, and top first/bottom last in the two dot targets. Notice how the groups here were best right around 48 grains, like the target on upper right where I tried 48 grains with and without BPI Original shot buffer. Velocities for the 48 grain load without filler were 2136, 2094, 2082, 2076, and 2139. Lots of ES, but most accurate by far. At 49 grains, where the load started to blow the groups, velocities were 2190, 2183, 2170, 2172, and 2186. Very small ES, very large group, fired after the 48-grain group under the same conditions, only a 10 minute barrel cooling break (standard for me).


Here I tried 50 grains of RX22 with and without Dacron, don't know what happened there.

Here's a powder switch to H4350, one of my favorites. Top was 40 grains and 1/2 grain Dacron, forgot to write down two numbers but three of them were 2066, 2038, and 2071. Middle group was 39 grains and .7 Dacron, getting 1958, 2020, 2004, 2032, and 2083, the first three were overlapping in a beautiful triangle, the fourth was high, the fifth came back into the group somewhat. Same thing happened with the top group if you look carefully. I think barrel heat or too much lube (purge fliers) may be causing this. Then I took it to 41 grains, and as you would expect, blew the group. 2127, 2118, 2090, 2103, 2085. Anything over 2K fps starts to open up the groups with these two powders. That's about 144,000 RPM, what a shocker.

Now let me show you another real shocker: Richard Lee says that the reasonable accuracy limit for the strength of a 19.3 BHN alloy is 24, 703 PSI. Not having a PBL, I used my chronograph numbers to approximate pressures from his cast boolit load data, since the charge weights/velocity were off by a grain or two in my gun. Here's what I came up with: right about 47 grains of RX 22 gives somewhere around 23,000 PSI, and 48 close to 24,000, 49 exceeds his recommended allowance for loading just under the boolit strength, and guess what? Blown groups. Just like he predicted in his charts. Now look at the H4350, my chono data was within just a few FPS of the published data, really neat: Just like Lee's data, 39 grains was just over 2,000 fps average in my gun and pressure was 24,200, 41 grains that blew the group was 26,700 PSI and averaging around 2100 fps or 151,200 RPM, while the 40-grain load somewhere around 2050 fps was till holding together with some slight fliers at 25,400 PSI and 147,600 RPM.

So what does this mean? Just at the point that I exceeded the RPM theshold, accuracy deteriorated. Actually I pushed it a bit beyond where you say it usually is, but the boolit's a bit tougher than you said to use, too, so it adds up. The other thing, and this is my whole point, the Lee forumula was exceeded in exactly the same place you say the "RPM threshold" is reached, with the same effect on groups. Could it be that you're theory is the same as Mr. Lee's, but you call it something else? This is not the only time I've seen certain RPM and boolit strength coincide with accuracy deterioration, in fact every time I've measured velocity and strength and compared to Lee's charts, I get the same results with standard loading techniques, not doing anything fancy for case prep, boolit fit, or muzzle exit protection.

Go figure.

Gear

[geargnasher]

Larry, I know you've been woking on this for a long time, but it seems to me more logical to consider "rim speed " than RPM because it should better relate to centrrfucal force than RPM does unless you limit your discussion to a single caliber.
Grouch

I wasn't going to bring it up just yet, but since you did I might as well mention that there is, in fact, no such thing as "centrifugal" force. It's called a "ficticious force" because it's action on an object is totally different depending on which particular inertial reference frame the observer is placed.

Gear

[Larry Gibson]

I wasn't going to bring it up just yet, but since you did I might as well mention that there is, in fact, no such thing as "centrifugal" force. It's called a "ficticious force" because it's action on an object is totally different depending on which particular inertial reference frame the observer is placed.

Gear

Is that so? If it has an "action" then how can it be "fictiicious? Besides, you the shooter, are always in the same "inertial reference" as the "observer. If it is "ficticious" then how is it your body wants to slide across a bench seat in a car if it takes a corner fast? I think you've researched too much on Wikpedia, try a real book or manual on ballistics.

Larry Gibson

[Iron Mike Golf]

...Somebody schooled in long-range techical ballistics please explain what "going to sleep" really is....Gear

Disclaimer: I am not a ballistician.

From what I have read and understand, "going to sleep" is a dampening of the helixes (I think "spirograph pattern" due to dynamic stability.

http://www.nennstiel-ruprecht.de/bullfly/fig12.htm

As I understand it, this is caused by bullet yaw at the moment it leaves the muzzle. Dynamic stability causes the diameter of the pattern to decrease over time (and range). Gyroscopic stability resists changes to the direction of flight due to off-axis aerodynamic pressures.

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Larry,

In the same article is a diagram relating gyroscopic stability to dynamic stability:

http://www.nennstiel-ruprecht.de/bul...ria.htm#header

Seems to me that you can affect the overall stability of a bullet by changing the rotational velocity and and get into unstable conditions by either spinning the bullet too slow or too fast.

Also, in that article, it is stated "rotational velocity is much less damped than the transversal velocity (which is damped due to the action of the drag)" on this page:

http://www.nennstiel-ruprecht.de/bullfly/fig14.htm

The author author states that dynamic stability is based on 5 aerodynamic coefficients and "because these coefficients are hard to determine, it can become very complicated to calculate the dynamic stability factor, which varies as a function of the momentary bullet velocity. " (emphasis mine)

That makes me think defining (and calculating) a threshhold is a difficult proposition. It would be a threshhold for a particular bullet, a particular load, at a particular range.

Jeff