RPM Threshold

[felix]

BR work (for real) was always balls-to-the-wall, and then back down and even back up to get the harmonics to play fair (depending on the day). Then we bitch and moan about a barrel destroyed after 700 rounds. Load adjustments at the range only to find the load for that day. Looking back on all that makes one go bezerk over the waste (if you don't make the gains under the table by winning). I came close, but no cigars. Always a better weatherman on the job that hour. Moral of the story: "Don't take your guns to town". ... felix

[Larry Gibson]

Bass

That's clear enough, any reason we can't see some of the groups and bullets designs. We know you have a camera and can post pictures? And is that 50 shot test (it's only 10 five shot groups) with your bullets and loads covering 1700 to 2600 fps too much to ask?

but the question I really have to ask; what the hell does......

"I have bullet designs with low velocity loads that shoot 3/8s to 1 1/2" in several calibers and bore diameters at 100 yards. I have bullet designs with HV loads that shoot from .2" to 1 1/2" across several calibers and bore diameters at 100 yards. Sometimes, they are the same design, sometimes not and one design will perform better at one end of the scale or the other. But that is just because I haven't done my part and found either a better HV or low velocity load."

.....have to do with answering why our intrepid cast bullet shooter can't get the same accuracy at 2400 fps that he gets at 1800 fps with his 311291 out of his good shooting '06?

Your answer is apparently to tell him to get new custom moulds, change alloy, rebarrel his rifle to a larger capacity cartridge, use slower burning powder, put pressure on the barrel and tune the "cusp" out of it, etc. ad nauseum.

I simply tell him he won't get there from here with 311291 and his 10" twist '06 because the RPM at the higher velocity is causing the inaccuracy.

That clear enough.

Larry Gibson

[Bass Ackward]

Larry,

Your opinion is valued by me. And because of such, how about doing me a favor? Do one more test and report on it for the board. You are uniquely the candidate here because you are the only one that has attested to have multiple twist rates in the same caliber that I know have been smoothed by shooting. So both should be excellent for this test.

I would like for you to slug your 10 twist and 14 twist for me with a caveat. I would like you to use a good heavy weight oil like say gear weight if you got it to ensure that there is no galling. Use the same heavy oil in both bores so the condition is exactly the same. The thicker the oil, the better the test results will be. If you patch one to remove the excess, then do the other EXACTLY the same, but use a small jag and or patch so fit is very light to leave considerable oil in the bore. Put a tight fitting patch in the chamber to prevent anything going into where you don't want it.

Pound a lead slug into each barrel just to get it to fully engrave and allow the use of a bore guide to protect the muzzle of coarse, but don't push it through yet. Just start one in each barrel.

The test is to start with the 14 twist and push that slug through just as fast as you can. Then before you lose that feeling in your mind, immediately grab the 10 twist and do the same thing. Warning: one is going to fly, and the other will take more time and EFFORT.

Please report on that test and tell us:

1. Which one took the most force to push the slug through?
2. And how significant was the difference?

Then I will ask you a question that you alone will uniquely be able to answer cause you felt what your bullet feels. In fact, this is very educational for anyone that has this capability to test. Feel free to answer these same questions.

[Calamity Jake]

I don't consider myself to be a dummy by any means and I understand what is trying to be
accomplished here, or at least I think I do!!
And I am learning a lot of good stuff.
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But all this RPM BC is making my head spin!!

Have a good day!!

[subsonic]

balanced bullets = BR condoms @ 20 cents (???) per. A deep pockets requirement. ... felix

I mean undrilled, reasonably balanced, not BR stuff, although it would be interesting with the BR too.

[Larry Gibson]

Bass

It is done. I had some 323471s cast of pure lead (old Paco loads) so I ran them through a 314 then 311 die. Lubed both the 10 and 14" twist barrels with some very heavy military gun grease that I always use when slugging bores. Makes the slugs go through slicker than snot. I have two vises mounted on my work bench so both rifles were set up as requested. With both rifles mounted on the bench at the same time I was able to push the slug through one barrel then imediately push the slug through the other barrel without losing that "warm fuzzy".

1. Which one took the most force to push the slug through?

Hard to say as the 14" twist barrel is 5 1/2" longer. The perception was the 14" twist barrel took more force but in reality they were pretty close.

2. And how significant was the difference?

I was pushing longer with the 14" twist which made it appear different. Not any perceptable difference when the slug was moving.

What is your question?

Bear in mind, once again, that in the test we are not comparing the accuracy between the barrels. To do so would only determine the accuracy between the 3 rifles. We are only comparing the accuracy of each barrel to itself. As each barrel produces a different RPM at a given velocity we are looking for the loss of accuracy in that barrel with identical loads as used in the other barrels. If one barrels best accuracy is 2 mos and the other 2 are 1 moa it doesn't matter. What we are looking for is at what velocity each individual twist loses accuracy. Thus if groups begin to open (losing accuracy) at 1913 fps in the 10" twist, 2166 fps in the 12" twist and 2526 with the 14" twist (all with the same loads) then the reason would be RPM is accentuating the defects in the bullets since the RPM for all 3 velocities is 130,000. Now I don't expect the results to be that linear. The Higher acceleration of the faster loads is going to create more defects in the bullets balance and the 14" twist should lose accuracy at the lower end of the RPM threshold. The slower twist, though it does not accentuate the imbalances in flight as does the 10" twist, will have more defects (imbalances) at the higher velocity (higher accelleration or more pressure) to deal with. This is why it is a threshold (125-140,000 RPM) and not a "limit".

Larry Gibson

[leftiye]

I won't put ALL OF MY EGGSES in any basket, no matter how good it is. All of the other factors that affect accuracy still affect accuracy, don't they? Don't they? Don't they? You can't actually say that this RPM limit operates on its own at all anyway.

[Pat I.]

I think Larry's test will be excellent but I can't believe Larry and John are the only two guys that have tried working up high velocity loads. I've tried but found that it took a slower twist to get higher speed with the best accuracy. Doesn't anyone else have experiences to share?

Pat

[Larry Gibson]

I won't put ALL OF MY EGGSES in any basket, no matter how good it is. All of the other factors that affect accuracy still affect accuracy, don't they? Don't they? Don't they? You can't actually say that this RPM limit operates on its own at all anyway.

You are correct with your own answer to your question. There are indeed many things that effect accuracy. However consider that "all of the other factors that effect accuracy" (assuming you are talking alloy, hardness, lube, fit, design, obturation, accelleration, pressure, etc.) take place in the barrel. RPM only effects the bullet in flight outside of the barrel. The "effects" you are thinking about have already happened to the bullet in the barrel before RPM does it's thing which is to accentuate those "effects" that have already happened outside of the barrel. The faster the RPM the more it accententuates those "effects" (actually defects and imbalances). It is the increased RPM which causes more pitch, yaw and wobble in the unbalanced cast bullet thus decreasing accuracy.

Pitch, yaw and wobble happen when the bullet has left the barrel. I think some here have a hard time differentiating between what happens to the cast bullet during the internal ballistic phase vs what happens to it during the external ballistic phase. Hope this explanation helps you and perhaps you can see I've not put my eggs in one basket. I understand there are two baskets. and my eggs are in the second basket. There are three baskets actually, if we want to get into terminal ballistics also. I don't as terminal ballistics is not part of this question.

Larry Gibson

[Larry Gibson]

Pat

Most of them do it's just that they don't realise it. Look back at many, many posts where they state the velocity of their "most accurate" loads and you'll find they are within or below the RPM threshold. Those who have gotten accuracy above the RPM threshold all admit, or choose not to respond, that their bullets are more accurate in or below the RPM threshold. The proof is there in their own shooting, they just don't see it.

I, too, have gotten what I consider to be reasonbly good accuracy with cast bullets (with only a few designs) at 2400 -2600 fps in the .308 or '06 with 10" twists. However those same bullets ALWAYS shoot more accurately down in or under the RPM threshold. The higher the RPM is above the threshold the less accurate the bullet will be. Maybe the accuracy lost won't be much (if you've the correct design, alloy, fit, hardness, size, acceleration, etc) but accuracy will be lost none the less. I'm sure everyone else's experience is the same.

Larry Gibson

[38 Super Auto]

Lot of drop in those 22.7 miles.

You might have to use the tang sight to capture that sight picture.

[Bass Ackward]

Larry,

Your results were way different than mine. My question was going to be RPMs or pressure?

When I did it, if I pushed through easy, then both slugs seemed the same. The oil wasn't supposed to make it slick as snot, but to offer non-gauling resistance so the difference would be obvious. If I pushed hard or fast, then the slower twist was dramatically less effort and faster than the fast twist. I did it with 45 caliber 14 and 20 twists I had on hand.

When I was a kid, Ol Earl (friend) told me to find a smooth wall. Face it perpendicular. That is a zero twist. Turn to a 45 degree angle and that simulated a 10 twist and try to walk forward along the wall. Then he said to cut that angle in half and repeat. It seems stupid now, but it was eye opening as a kid and was appartent that twist rate was an impediment to forward motion and the at some point the wall was merely aguide. Different powders would be required to produce the same pressure curve. Shotguns with no twist rate could launch heavier loads with much faster powders than something WITH a twist rate.

And since lead is only so strong, the slower twist rate would have the higher velocity level. And if you can run a higher velocity level, you don't have to rotate it as much either. Therefore, the fast twist would have much higher harmonics from the pressure and the slower twist would be much easier to control. The wall for lead is short (Why taller rifling helps) and the pressure driving it forward wants to push the core straight ahead while the wall wants it to wait and rotate. You need to fight or prevent that stripping. Not rotational force stripping or damage. So it's not rpms to me, but pressure.

That was what I was hoping for you to see.

[BAGTIC]

The only thing I know about "RPM" is a note in the FAQ section of one of the bullet maker's sites. The question was about why do .22 bullets fragment when fired. The answer was that a commercial bullet (80 grain?) fired in a military barrel rifled for the FMJ Ball (62 grain?) would fly apart. Apparently it's common knowledge among bullet companies.

As for the rest of the "RPM" subject, I am totally ignorant...
and might even prefer to stay that way.
CM

Not likely as the military loads 77 grain Sierra boattails in some of its ammo. Besides the heavier bullet will be propelled slower reducing the spin rate.

There used to be a .22/.257 wildat made by someone in California's central valley was was designed for heavy bullets at high velocity. I can't remember the name. I saw some 80+ grain commercial bullets (Hornady?) made and labeled for for it. It must have worked.

[BAGTIC]

Modern bullets shoot much better than those of 50 years ago. Closer tolerances and tighter quality controls.

The higher the rpm the greater the deleterious effect of any imperfection, void, off center core, cocked bullet, etc., Higher rpm's will shoot accurately they just require higher quality bullets.

[BAGTIC]

The only thing I know about "RPM" is a note in the FAQ section of one of the bullet maker's sites. The question was about why do .22 bullets fragment when fired. The answer was that a commercial bullet (80 grain?) fired in a military barrel rifled for the FMJ Ball (62 grain?) would fly apart. Apparently it's common knowledge among bullet companies.

As for the rest of the "RPM" subject, I am totally ignorant...
and might even prefer to stay that way.
CM

Not likely as the military loads 77 grain Sierra boattails in some of its ammo. Besides the heavier bullet will be propelled slower reducing the spin rate.

There used to be a .22/.257 wildat made by someone in California's central valley was was designed for heavy bullets at high velocity. I can't remember the name. I saw some 80+ grain commercial bullets (Hornady?) made and labeled for for it. It must have worked.

[Bullshop]

BAGTIC
That would be Keneth Clark, and the cartridge was the 224 Clark. It was based on the 257 roberts imp. case. I built one with a 1/9" twist and a 30" barrel that easily did 4100 fps with the then new Siera 69gn bthp match.
BIC/BS

[BD]

224 Clark, (.22/257); 220 Howell, (22/30'06) and the 223 Vais, (22/6.5x55), were all designed around the idea of high SD .22 bullets moving along at a pretty good clip. The ones I've seen all worked pretty well with an 8 or 9 twist. The 77 grain Sierra or Nosler BTHPs shoot very well out of my 8 twist DCM, as do the 80 grainers if I load'em one at a time. I say the idea of "commercial" bullets flying to bits in "military" barrels is bunk.

And if Bullshop's had the 69 grain Sierras over 4,000 in a 9 twist, then I have to assume that the jacket construction is playing a significant role in addition to rpms, as I have watched 40 grainers dissappear in similar circumstances.

BD