RPM Test; a tale of three twists, Chapter 2

[Larry Gibson]

Larry;
Do you have the numbers where you could send them to me? Excel maybe? I do better understanding the numbers.
Thanks;
joe b.

Joe

Sorry, I do not have the numbers collated on a spread sheet. Look at the Oehler test print out in the response to 44man and you will get an idea of the volume of numbers and the job that would be to spread sheet them. I have 22 such record print outs for the first test alone. I could scan them all or copy them and mail them (easiest).

Larry Gibson

[Larry Gibson]

it would seem to me that if boolit deformation is causing the better b.c
that by switching to lino will bear that out.
or point in its direction.
hate to say it but the "better" boolit may not be

Except with perhaps a rare occasion that leftiye and I discussed the BC of a deformed bullet will be worse. We will see how the linotype 311291s fare in Chapter 3. Knowing the BCs of RN & FN bullets decrease with the increase of velocity it will be interesting to note the difference in that increase between the two alloys. If perchance the BC change is close to the same then it is telling the #2 alloy held up very well with little deformation. Of course it could be completely different and right now I don't know. We shall see.

Apparently we lost part of your message. What do you mean with; "hate to say it but the "better" boolit may not be"?

Larry Gibson

[Larry Gibson]

I just love it when you guys get into these deep technical discussions. I bring my wife in here and show her proof positive that other people are afflicted as bad or worse than me.

Awesome show, keep it up.

Keith

Keith

If you've got the "affliction" leap on into the discussion

Larry Gibson

[felix]

Larry, use a side test with 4227/2400 at about 23-25 grains with either boolit and to see if the little wiggles go away, or strongly diminish, during the ignition stage. That will tell you the story without calling Oehler. ... felix

[Larry Gibson]

Larry, use a side test with 4227/2400 at about 23-25 grains with either boolit and to see if the little wiggles go away, or strongly diminish, during the ignition stage. That will tell you the story without calling Oehler. ... felix

felix

Good idea, I had planned on using those powders also at the end of this test. Someone else had requested a test with "faster powders".

Larry Gibson

[runfiverun]

larry
what i meant was the harder boolit may not show this anomaly
to "set back" and show the better or as good as bc.
i have run into the [ problem ] of going to a harder boolit did not really gain
me more. accuracy or what have you at least not enough to justify the extra cost.

[Larry Gibson]

larry
what i meant was the harder boolit may not show this anomaly
to "set back" and show the better or as good as bc.
i have run into the [ problem ] of going to a harder boolit did not really gain
me more. accuracy or what have you at least not enough to justify the extra cost.

That has been my general experience with bore riders in the quicker twists of 10" and faster. This is why I mostly just shoot bore riders cast of ACWWs at 1800-1900 fps in the 9 1/2-10" twist barrels. However with slower powders I have pushed the RPM threshold in a couple of those but not very much. I've gotten away with some pretty decent accuracy in the 2000+ fps range years back with the two Lyman Lovern bullets cast in linotype in 8x57 which is pushing the RPM threshold. In 12-14" twist barrels of several calibers I have run some pretty decent accuracy at higher velocity with the harder bullets. However all have really been within the RPM threshold. Also I've not pushed a good bore rider like 311291 in a 14" twist before when cast hard. Will be interesting to see if it fares any better than the #2 type alloy.

Larry Gibson

[runfiverun]

i just read my own post
i need to use ing and commas and stuff.
my elementary school teacher would be proud?

[Bass Ackward]

Larry,

I think you draw some interesting conclusions about pressure curve with the twist rates that are not correct. Listen to my grandson.

Watched my grandson with his pedal car in the driveway. He was sitting still and I cut the wheels hard to the right and asked him to start out. He said grampa, you need to straighten the wheels out some and then turn if you need to otherwise it's too hard to pedal.

Load from 11 to 15 grains of Red Dot. That should max out between 1700 and 1800 fps which is the ideal RPM range. See if you get different accuracy from the three twists even in the ideal RPM zone. If the accuracy is different in the ideal zone, then the only variable has to be pressure. Please post those group sizes.

[runfiverun]

bass
straightening the wheels would be like straightening the
twist in a bbl wouldn't it?

[leftiye]

Dern smart kid!

[Larry Gibson]

Larry,

I think you draw some interesting conclusions about pressure curve with the twist rates that are not correct. Listen to my grandson.

Watched my grandson with his pedal car in the driveway. He was sitting still and I cut the wheels hard to the right and asked him to start out. He said grampa, you need to straighten the wheels out some and then turn if you need to otherwise it's too hard to pedal.

Load from 11 to 15 grains of Red Dot. That should max out between 1700 and 1800 fps which is the ideal RPM range. See if you get different accuracy from the three twists even in the ideal RPM zone. If the accuracy is different in the ideal zone, then the only variable has to be pressure. Please post those group sizes.

Bass

With all due respect to your grandson; you just aren't getting it. I don't "draw interesting conclusions". The facts on the pressure curves are there. Perhaps you failed to look at the graph on group size vs pressure and the graph on group size vs velocity. You're the one who disagrees with RPM threshold with regards to high velocity loads. With the facts clearly beginning to demonstrate there is a RPM threshold with regards to high velocity, you now want to revert back to medium velocity? Give us a break, at least try to keep your story straight.

As to your test with Red Dot; define "different accuracy". The test has already demonstrated that pressure was not what caused the proportionally greater group sizes. It was the increased RPM of the faster twist that caused the proportionally larger groups.

Also when have we ever said 1700-1800 fps was the "ideal RPM range"? Since RPM is a function of twist and velocity (or have you forgotten?) at 1700-1800 fps; the 10" twist will be doing 122,500 - 129,700 RPM, the 12" twist will be doing 102,000 - 108,00 and the 14" twist will be doing 87,500 - 92,600 RPM. Which one of those 3 different RPM levels do you consider "ideal"? Since the RPM is in fact varied, how is pressure then the “only variable? It isn’t, is it.

Further more, since the RPM threshold (the topic of this test and this discussion, remember?) is 125,000 - 140,000 RPM and involves inaccuracy induced when RPM overcomes the rotational stability of the bullet in or above that threshold I just don't see where your test is relevant.

Larry Gibson

[Larry Gibson]

bass
straightening the wheels would be like straightening the
twist in a bbl wouldn't it?

It doesn't work out like Bass seems to think it does. Unfortuneately for many, many years others have believed that pressure was the culprit also. You will read that over and over again in numerous writing and publications. All of them ignore the effects of RPM. Pressure is certainly partially to blame as it is what creates the defects in the bullets (given a quality cast bullet) during accelleration. However, it is RPM that adversely affects the bullet during flight creating the inaccuracy. If it was just pressure then explain how it is that when the pressure is the same in the 10" twist and the 14" twist the 10" twist is still proportionately more inaccurate? The answer is simple, RPM.

Bass just doesn't see that as apparently do numerous other people who've believed for years that it was pressure.

Pressure the same, accuracy is not the same; 10" twist has worse accuracy than 14" twist.......hmmmmm.........RPM higher in 10" twist than 14" twist........accuracy worse in 10" twist.........pressure is the same.....hmmmmmm. Not to hard to understand.

Larry Gibson

[Bass Ackward]

Larry,

I try to consider everything. I suppose RPM forces in the bore explains why lube sizing of bullets lets go worse in colder weather But in the warm it does OK with pressure being constant. Why rifling height makes a difference too. But pressure is still there as well as RPMs.

My tests with defacing bullets showed the worst results when the base was disturbed. And we have history on crown research too. I have shot plenty of good bullets at low RPMs badly and quite a few good bullets at high RPM well. So RPMs can't explain everything.

If RPMs explains HV, it has to apply in some regard at low? RPMs is used as an excuse to explain failure when the security of cast comfort levels are surpassed.

What ties pressure and RPMs together is hardness as it defeats both. And why we hear match hardness to the velocity of interest. Not to the pressure of interest, not the RPMs of interest, but velocity. So maybe both are at work.

But think my granson is wrong huh? Take your car and cut your wheels in the angle of a 10 twist and slowly step on the gas from a dead stop. Feel how much more gas it takes to get your car moving than with the wheels straighter. Everything has an effect, don't blind yourself to it. If this could be defined simply, then we would have answers now.

[felix]

The base of the boolit must be DYNAMICALLY the same at the muzzle during the entire duration of each boolit exit. Otherwise, we are really sucking wind on this RPM endeavor. The duration is defined as the time it takes when the pressure between the boolit base and the barrel becomes zero. "Dynamically the same" means the base remains perfectly round for all pressures/twists without regard to base expansion or contraction at exit. This has to explain why boat tails and flat based projectiles can shoot identically out of the same gun, but in most guns they do not. Obturation style is paramount. ... felix

[sundog]

Boolits don't have steering wheels, in and of themselves, if they are good boolits to begin with. The steering wheels are 'induced' by such things as poor alignment, poor throat to boolit fit, poor land/groove consistency, poor boolit exit or poor crown, etc., basically all those things we've been discussing.

So, the more twist you put on the boolit (one factor that can affect it's 'steering wheels') the more chance you have of inducing failure - inability to get started.

Don't forget, the barrel is a swaging device. If the boolit is not swaged perfectly, then you get what you get, which is less than perfect performance.

I'm not taking any sides on any of this, just thinking out loud. I know the things that work for me, so anything I can learn new from y'all is just like eating chocolate pudding (which to me is 'pretty good stuff, Maynard!')

[felix]

Yeah, it's tough to make RPM a true independent variable when considering this mess of variables which seem to depend on each other so strongly in our discussions. ... felix

[Larry Gibson]

Bass Ackward

Okay bass, lets go through this one more time.

I try to consider everything. I suppose RPM forces in the bore explains why lube sizing of bullets lets go worse in colder weather But in the warm it does OK with pressure being constant. Why rifling height makes a difference too. But pressure is still there as well as RPMs.

I don't think you are trying to consider everything. I think you are searching, sometimes even grasping, for a something to validate something you (your not alone as there are many others) have believed for many years. I believed it for many years also. But the more I looked at it the less pressure answered the question. How many times have we heard or seen it written; “I load 311359 in my .30 carbine to 40,000 psi and it shoots great. But when I load it to the same pressure in my 30-06 it shoots terribly." Or as published in the current Lyman manual; "Load Lyman's 311291 in a 30-30 case at about 40,000 psi. In a good rifle it will shoot alongside jacketed ammo of the best quality, if properly made and loaded. Now load it to 40,000 psi in a .300 Magnum. The only thing it will shoot along side of is a blunderbuss-if you're lucky." Everyone wants to say it is pressure but yet they constantly give examples where it is not pressure. None of those who say it is pressure even consider that it might be something else. That is because everyone else says it is pressure so it must be. But it is not pressure. I did not dream this idea of an RPM threshold up myself. There have been several others over the years who've said the same thing.

Have you ever consider that it is the cold making the lube harder as the reason it "let's go"? Probably not. I and numerous others conducted tests back in the '70s on that very subject. The lube residue in the bore gets harder in cold weather. Ever notice in cold weather with a fouled cold bore the first 2-3 shots are the flyers? When the barrel has warmed up the rest of the string shoots to group.

Lube failing and the height of rifling all have effects in the bore. Once again the RPM effects the bullet outside the bore.

My tests with defacing bullets showed the worst results when the base was disturbed. And we have history on crown research too. I have shot plenty of good bullets at low RPMs badly and quite a few good bullets at high RPM well. So RPMs can't explain everything.

If RPMs explains HV, it has to apply in some regard at low? RPMs is used as an excuse to explain failure when the security of cast comfort levels are surpassed.

[B][I never said, nor has anyone else, that RPM explains "everything". The RPM threshold explains why, with regular cast bullets, we can get very good accuracy only to a certain level. After that level regardless of what is done accuracy is not very good. It is because the pressure has caused defects in the bullet during acceleration and the bullet is then unbalanced during flight. The RPM then over comes the rotational stability of the bullet during flightand inaccuracy results.

The effects of RPM do apply at lower pressures. Cast the bullet of just soft lead and how fast can we shoot it before accuracy fails? How fast can we shoot PB bullets before accuracy fails? The answer is increased pressure has caused defects in the bullets during acceleration at a lower level and RPM during flight causes the then unbalanced bullet to be inaccurate. How many times have I told you that all bullets are imbalanced even jacketed bullets? When the center of spin does not coincide with the center of form or the center of gravity the bullet is unbalanced. RPM will adversely affect the accurate flight of every unbalanced bullet. How much that affect is depends on the amount of imbalance. That is why we shoot bullets into groups instead of one hole.

"RPMs is used as an excuse to explain failure when the security of cast comfort levels are surpassed." Let us substitute "pressure" for "RPM" and we have your excuse. However I am proving the effect of RPMs so it is not an excuse. Also, as you have failed to notice, I also am proving the adverse effects pressure has on the bullet during acceleration. /B]
What ties pressure and RPMs together is hardness as it defeats both. And why we hear match hardness to the velocity of interest. Not to the pressure of interest, not the RPMs of interest, but velocity. So maybe both are at work.

"So maybe both are at work." Perhaps you are getting the idea. Cast a harder bullet of better design that resists deformation during acceleration and you can push the RPM threshold for that bullet up to a much higher velocity. Your work with the LBT bullet is good proof of that. Just increasing the hardness of a regular cast bullet also allows you to push the RPM threshold but not nearly as much.

But think my granson is wrong huh? Take your car and cut your wheels in the angle of a 10 twist and slowly step on the gas from a dead stop. Feel how much more gas it takes to get your car moving than with the wheels straighter. Everything has an effect, don't blind yourself to it. If this could be defined simply, then we would have answers now.


Never said your grandson was wrong, it was he I was giving "due credit" to. If you are inferring that pressure with a given load is higher in quicker twist barrels then you are correct. The M43s measurements clearly show that. However the comparison of groups was with equal pressures in all three barrels. That comparison clearly shows the proportional increase in group size is larger with the faster twist.

I do not blind myself. It is you who have the blinders on. You actually see the relationship that pressure has during acceleration on the integrity of a cast bullet. You do understand how that acceleration from pressure causes defects in the balance of the bullet. What you fail to see is that it is the centrifugal force of RPM that adversely affects the bullets flight and causes the inaccuracy. If you would quit grasping at straws to prove it is pressure when there is ample evidence it is not pressure then you would understand the relationship of the two. You would understand the cause and effect of each and have a better understanding of the cast bullet in flight. Take the blinders off Bass.

Larry Gibson

[Larry Gibson]

sundog

Quite correct; it is all of those things that cuase the defects in the balance of the bullet while inside the barrel. It is the RPM during flight outside the barrel that causes the inaccuracy. The greater the defects, the higher the RPM the greater the inaccuracy.

Larry Gibson

[Larry Gibson]

Bass

One other reminder; you've not answered the question, "As to your test with Red Dot; define "different accuracy"." with regards to your test with Red Dot. Are you now not wanting to proceed in that direction?

Larry Gibson