RPM ACCURACY THRESHOLD

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CB posted this 25 November 2007

I'm reading references to an “rpm threshold” beyond which accuracy diminishes. Can't find anything on the search places. Can somebody tell me what it's all about or direct me to a source? Has top do with twist and mv etc.

Thanks;

joe b. 

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RicinYakima posted this 25 November 2007

Never heard of such a limit, so when you get any information post it here. Ric

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shooter93 posted this 25 November 2007

I've been looking for the stuff and formula myself....I'm curious because it may very well have some merit...a point where rpm alone is just to great for the bullet to hang together. I've seen it with j-bullets. I built a fast twist gain twist swift Ackely to shoot 75, 80 gr. bullets. On a whim I laoded 50's as fast as I could. I had some cross the screens WELL over 4,000 fps but they never made the 100 yd. target...they just came apart.

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shooter93 posted this 25 November 2007

There also seems to be a correlation between the two...both with cast and J-bullets if you study match results.....we know the velocity they are shooting...bullet weight and the twist rate....the twist rates vary which I'm assuming at this point is a rpm based decsision.

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Idaho Sharpshooter posted this 25 November 2007

And the answer is...variable.

Draw a circle. Put crosshairs on the first one. That junction represents the geometric center of the projectile. Now, figure in production tolerances of .002” variation in jacket thickness. Make the bullet. Now put a dot as close as you can to the crosshair junction without touching it. With an imperfect jacketed bullet, or a cast one with a microscopic void, the dot represents the center of weight. See where this is heading...? Inside the barrel that bullet is spinning perfectly round. Once it leaves, those two different centers force the bullet to rotate slightly out of exact crosshair center trying to do a balancing act between those two bullet centers, which, if driven fast enough; will rupture the jacketed bullet, and cause the cast bullet to veer off course at some point. The faster you spin the bullet with velocity or twist, the tougher that bullet jacket or casting has to be. Finding the RPM is fairly straightforward. Take the twist rate and divide by twelve. Twelve inches in a foot translates into 1 revolution. Take a 1:12” twist in a 24” barrelled 308W with a velocity of 3000fps. Per revolution inside the barrel is 3,000RPS. Times 60 seconds per minute (60x3000) the RPM is a heady 180,000RPM. Shoot that bullet at 1600fps and the RPM is 96,000. Guess which is easier on the bullet? Guess which is (generally) more accurate a load? Matching a bullet length (actually bearing surface is a more accurate method, but we have trouble getting an accurate reading) and using the Greenhill Formula is still the best place to start. I have a friend at Picatinny Arsenal who does projectile calculations for the DoD. He wrote most of the stuff Sierra uses these days. He just says that minimum RPM is best, as long as it stabilizes the projectile by about 35 feet from muzzle. There's more to it than that, but it's too late on a Sunday night to get into Transonic Instability and Laminar Flow Boundaries.

Rich

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shooter93 posted this 26 November 2007

I think that's correct Idaho shooter. There has to be a point where rpm's would alter the shape and flight of any projectile...the more variables involved...and you'd have the most with cast bullets...the greater or sooner the effect. I think this is what many people refer to as.."over Stablized"...a term which makes no sense. It is either stable or not, it's then a matter of the drgree of not being stabilized. Back many years ago Ed Shilen was working with the 222...222 mag and then the 222-1/2. He was looking nit just for load densities but also 100% load density at a specific velocity range. Then he started on different twist rates...often non typical ones for caliber which you see more of now adays. Now this was j-bullets but everything that applies to them also applies to cast bullets but in greater terms. If suoer hard lube are lost unevenly once a bullet exits the barrel...and they can and do....and this effects stability and accuracy then ANY inaccuracy in the bullet will do the same to one extent or another. Using your example of a small air pocket at some rpm the bullet would become wildly unstable. And it does seem the 100,000 rpm range give or take seems to be the threshold for maximun accuracy with cast bullets most are used to using.

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CB posted this 27 November 2007

There are a lot more posts on this topic on CAST BOOLITS, where the CBA shooters, twists and RPMs are mentioned.

I went through the CBA 2006 National Match results, made a spreadsheet showing the TWIST, MV and RPM of the entrant's guns.

            Taking out those with gain twist, then the SS Breech Seaters, the RPM varied from 158,400 down to 96075, with an average of 125578.

            The workbook is attached.

 

There's a lot on the 30BR and short bullets and slow twists at http://www.6mmbr.com/30BR.html

 

I suspect that short light bullets are easier to shoot accurately, hence the references to 100-125 grain .30 caliber bullets.

 

Greenhill says that in 30 caliber, a 10” twist will stabilize a bullet 1.42” long, which would weigh ~250 grains. It says that a 15” twist, for examploe, will stabilize a bullet .95” long weighing ~166 grains. All Cast bullets.

I think it's clear that 30 caliber twists are generally “faster than needed", that slower twists with shorter lighter bullets are easier to shoot and torque on the bags less.

My impression is that gain twist and slow twist barrels are “easier on the bullet” as the bullet accelerates in rotation.

joe b. 

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CB posted this 29 November 2007

Here's a summary of the 2006, 2007 CBA NM RPMs, and the workbook.

2006 CBA NM

RPM

Average            125,578

>150,000                     1

140,000-150,000            6

130,000-140,000            3

120,000-130,000            10

110,000-120,000            9

100,000-110,000            4

<100,000                     1         

 

2007 CBA NM

RPM

Average            132,246

>150,000                     2

140,000-150,000            6

130,000-140,000            12

120,000-130,000            6

110,000-120,000            7

100,000-110,000            9

<100,000                     2                     

           

Eliminated = unknown and PBB.

joe b.

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billwnr posted this 30 November 2007

I'd venture a guess and say it's not RPMs that causes a breakdown in accuracy but gas cutting on the sides of the bullets. As RPMs increase, so should chamber pressure. At a point the pressure is to high for the alloy.

I'm making my judgement based off the table in Lee's latest reloading book that relates how hard of an alloy is needed for levels of chamber pressure.

Just a thought and I think this is why Joe thought he had to eliminate PBB from the table as they are susceptible to gas cutting at a very eary pressure level.

Joe, I'm not criticizing here.  Just trying to offer my opinion that this may be being looked at from the wrong angle.

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CB posted this 30 November 2007

billwnr wrote: I'm making my judgement based off the table in Lee's latest reloading book that relates how hard of an alloy is needed for levels of chamber pressure.

The notion that pressure and bullet hardness and accuracy are related through some formula is nonsense and has been shown to be so.

Nonsense.

joe b. 

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billwnr posted this 30 November 2007

Thank you for your humble opinion.

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billwnr posted this 30 November 2007

Mr Humble One, one more thing I would propose as a candidate for considering is some of the rifles have shallow angles on the throats and some of the Production class rifles have the factory throat angles. This may affect the “max RPM” thought you are fixated upon.

The “skid start” of the factory throats cause problems.

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CB posted this 30 November 2007

Joe Brennan wrote: The notion that pressure and bullet hardness and accuracy are related through some formula is nonsense and has been shown to be so.

Nonsense.

joe b. 

The notion that bullet rpm (a calculation almost 600 times actual flight-time) as relevance to accuracy is nonsense and established twist rate formulas show so.

Nonsense.

Dan W.

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Idaho Sharpshooter posted this 30 November 2007

The beauty of this forum is that you can say anything you want to, and never be called to task for it. You can, however, show yourself to be a fool rather easily. Joe, I think you are on the threshold....

Rich

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CB posted this 30 November 2007

Idaho Sharpshooter wrote: The beauty of this forum is that you can say anything you want to, and never be called to task for it. Rich Fortunately (or unfortunately depending on your view point) this isn't the only forum that suffers this disease. I think it's a pandemic.

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CB posted this 01 December 2007

Idaho Sharpshooter wrote: The beauty of this forum is that you can say anything you want to, and never be called to task for it. You can, however, show yourself to be a fool rather easily. Joe, I think you are on the threshold....

Rich

Rich;

Probably you are right, I am a fool.  

The matter of applying formulas to bullet hardness, arriving at pressure numbers and somehow equating that with accuracy can be understood best by looking at these:

1984 “Jacketed Performance With Cast Bullets"  by Veral Smith

TFS 81 Sep-Oct 1989  “Match Wheelgun And Load Preparation" 

TFS 86-3, July-August 1990

1991 “Bullet Making Annual” article, Pg 17

TFS 96 Mar-Apr 1992 “Technical Dialogue"

TFS 102- 4 Mar-Apr 1993 Interpolating Pressure for Correct BHN

TCB 116 Jul-Aug 1995  “More on Chamber Pressure and BHN"

TFS 131-10 Jan-Feb 1998 “Still More On Chamber Pressure And BHN"

Handloader 226 December 2003 , starting on pg.6, 

2003 Modern Reloading, Second Edition, Richard Lee

For a comprehensive treatment of the topic, see “Cast Bullets For Beginner And Expert", Second Edition, 3.3 Cast Bullet Hardness Requirements.

As far as I can determine this notion was first presented by C. E. Harris, who has written about it on this forum in the recent past. Everything written on the subject since seems to be based on the original article. Ed has also written about “The Load", in the past and here on this forum.

I have pointed out, here, on this forum, in great detail, that the pressure/hardness/accuracy rule propounded is wildly inconsistent with pressures, accuracy and bullet hardness of “The Load".

Now a fool such as myself would think that this inconsistency calls into question the validity of both propositions, that the hardness business and the “The Load” business should be at least examined in detail.

This particularly because  "Cast Bullets For Beginner And Expert", Second Edition, 3.3 Cast Bullet Hardness Requirements provides substantial evidence that the  hardness notion is nonsense.

But I am a fool, and you are a genius. You're not alone here.

Best of luck;

joe b.

 

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billwnr posted this 01 December 2007

Joe, I won't pussyfoot around this morning. I really don't care what you know or think you know...but the way you express yourself you are a #$^*( pain in the butt. I'm surprised people continue to associate with you. One thing that gets me is how you come up with some wingding idea, propose it..someone else improves it..and then you claim it as your own.

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Idaho Sharpshooter posted this 02 December 2007

Whatever...

Rich

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CB posted this 02 December 2007

billwnr wrote: Joe, I won't pussyfoot around this morning. I really don't care what you know or think you know...but the way you express yourself you are a #$^( pain in the butt. I'm surprised people continue to associate with you. One thing that gets me is how you come up with some wingding idea, propose it..someone else improves it..and then you claim it as your own. "Joe, I won't pussyfoot around this morning. I really don't care what you know or think you know...but the way you express yourself you are a #$^( pain in the butt."

Thanks for writing in, I was beginning to think it wasn't working!

"I'm surprised people continue to associate with you."

I think some can't help themselves, they're so mad!!

"One thing that gets me is how you come up with some wingding idea, propose it..someone else improves it..and then you claim it as your own."

I like to claim all the credit for all ideas and innovations, regardless of who actually came up with the idea. Why share the credit with others?

joe b.

Inventor of the inclined plane, wheels on luggage, the internal combustion engine, trigonometry, short pants and the Baldwin apple.

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Idaho Sharpshooter posted this 02 December 2007

Joe makes the distinction between symbiotic and parasitic relationships...

I can understand why some knowledgeable/experienced shooters choose not to share information here now.

Perhaps the answer if for the CBA to copyright the forum, threads and responses, leaving commercial usage to only the poster or the CBA.

Rich

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CB posted this 02 December 2007

Okay gents, this seems to be going to a place I don't want it to go..

Everyone is entitled to their opinion and views. I think that perhaps this has gone far enough.

I acknowledge everyone's viewpoint, but this has turned into a spittin match, and I don't allow no spittin here.

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