Twist rates for dummies

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beagle6 posted this 3 weeks ago

After reading some of joeb's answers and explanation of Greenhills Formula I am totally baffled. It seems to me(at least) that the stability of a bullet has to due with how fast it is rotating, not the twist that got it rotating. If a 30 caliber bullet leaves a 12 inch twist barrel at 3000 f/s, it should be rotating 3000 times a second. Let's say the bullet is stable at that rotational speed. Now lets send the same bullet through the same bullet, or a lead bullet of similar shape and weight, through the same barrel at 1500 f/s like we do with our cast bullets. It is now only turning 1/2 as fast but we know from experience that it will shoot well at that velocity. Do bullets have that much tolerance in rotational velocity or am I missing something? I make no claim to mathematics or engineering backround but would like someone to explain errors in my thing.

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beagle6 posted this 3 weeks ago

I need to edit my original post. Ignore " through the same bullet" in the 5th line down. Also, the last word should be "thinking".

Just too many interruptions when I was typing it out. I apologize.

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Redleged posted this 3 weeks ago

Hi. I think the answer lies somewhere in between where you would to take into account GH's stabilization formula (and it's derivative offspring,) as well as the rotational threshold of cast bullets. Larry Gibson wrote about this on other forums (here's an example.) In my own observations developing cast loads for my newly rebored 260 REM, 1:8" twist, shooting a 140 gr GC bullet of Lyman #2. I started getting unexplained fliers and sporadic/inconsistent groups as I pushed the velocity over ~1600fps (about 142,000 RPM.)  I think your comment about 1500 fps being much more accurate bears this out. I know there are certain bullet types and methods where you can exceed this (e.g. NOE 30XCB 165gr FNGC cast with precision.) In addition to stabilization based on bullet length, weight, and barrel twist, as well as the RPM limit, I also think that pushing a cast bullet too fast through a barrel with a quicker twist than optimal creates a shear stress on the cast lead bearing surface that it might not have on a jacketed bullet. Ed

Growing old is mandatory, growing up, however, is totally optional!

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joeb33050 posted this 3 weeks ago

After reading some of joeb's answers and explanation of Greenhills Formula I am totally baffled. It seems to me(at least) that the stability of a bullet has to due with how fast it is rotating, not the twist that got it rotating. If a 30 caliber bullet leaves a 12 inch twist barrel at 3000 f/s, it should be rotating 3000 times a second. Let's say the bullet is stable at that rotational speed. Now lets send the same bullet through the same bullet, or a lead bullet of similar shape and weight, through the same barrel at 1500 f/s like we do with our cast bullets. It is now only turning 1/2 as fast but we know from experience that it will shoot well at that velocity. Do bullets have that much tolerance in rotational velocity or am I missing something?

Yeah, you're missing something, you're ass end to. Greenhill wrote in the black powder, 1500 fps world. In that world, a .3 dia bullet of a certain maximum length in a 12" twist barrel  is stable. It's also stable at 3000 fps, and 8000 fps and a zillion fps.

But, stability is a teeny bit affected by velocity, not affected much at all. EXCEPT in the i'm-almosty-unstable region, where the bullet goes from stable to unstable in a small v delta. Because, stability isn't affected my v very much.

As for rpms, good luck. I've been trying to understand the LG rpm story for 15 years, with no luck.

 

I make no claim to mathematics or engineering backround but would like someone to explain errors in my thing.

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Ken Campbell Iowa posted this 3 weeks ago

the stabilization of spinning objects is pretty spooky  ... although children play with spinning tops that refuse to fall over ...  if you spin them faster and faster they get harder to move .. they like to stay where they are ... bullets are like tops ... 

dr. mann was fascinated by tops ..... and got interested in why bullets don't all fly into the same hole ... even when they are highly stabilized.   ( he finally decided that bullets really fly reliably to where they are directed as they leave the muzzle ... the problem is they are aimed at different places on the target just as they leave the muzzle ) ....

we used to mount our toy car wheels on a ball bearing axle and whizz them up to about 40,000 rpm with an air gun .... it was very hard to change the axial direction of such a spinning mass.  spooky ...

****************

anyway, greenhill gives the least amount of spin to stabilize a bullet .... you can spin it more, it just becomes more stable ....  if a bullet is stable at 1000 rpm, you can drop it from 5000 rpm to 3000 rpm and it is still stable .

top a the mornin to y'all

ken

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Larry Gibson posted this 3 weeks ago

I'm going to stab a sacred cow here;  if Mann was correct then how is it that given the exact same load a 150 gr 30 caliber Speer, Sierra, Hornady or Nosler SP is much more accurate (shoots smaller groups) out of the same 30 caliber rifle as any M2 150 gr bullet?  The reason they shoot smaller groups and are more accurate is because they are more balanced and are less affected in flight by the centrifugal force created by the RPM on the imbalances. Given the same load the barrel "nodes" would be relatively the same for all of the various bullets.  Mann's assumption, as stated by Ken, is only partially correct. His assumption fails to consider what other forces act upon the bullet while it is in flight.  Considering the title of his book I always thought that assumption of his was odd.

Also once a bullet is stable it is stable.  There isn't any "more stable".  Over spinning a bullet beyond stabilization only increases the adverse affect the centrifugal force has on any imbalances in the bullet during flight.  There is an abundance of evidence demonstrating the best accuracy is achieved by a bullet having just enough spin (RPM) for stabilization.  Read any modern work on ballistics, study what bench rest shooters use (both cast and jacketed bullets) and look at what the winners are using.  A simple explanation, with easy to understand pictures, can be found in the last few Hornady reloading manuals. 

Yes, some do have a problem understanding because it can be "spooky" as Ken mentions.  However, it is still science, not a "story".

LMG 

Concealment is not cover.........

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joeb33050 posted this 3 weeks ago

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joeb33050 posted this 3 weeks ago

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joeb33050 posted this 3 weeks ago

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beagle6 posted this 3 weeks ago

WOW! That is a lot of good information. Thanks to all for taking the time to answer my question.

beagle6

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Larry Gibson posted this 3 weeks ago

Every bullet has a minimal rotational velocity required for stabilization.  It is the twist rate of the rifling, the velocity and the length of the bullet that determines the minimal rotational velocity required.  Yes, there is a "constant" in the formula also that is based on the density of the material of which the bullet is made.

Simply put for a given bullet; the faster the rifling twist the less velocity is required for stability.......or the slower the twist the higher the velocity.  A 311284 will become stable at a lower velocity in a 10" twist than in a 12" twist.

LMG

Concealment is not cover.........

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Scearcy posted this 3 weeks ago

The thing that has always perplexed me, Larry, is that the twist rate and the velocity are not proportional. If I double the velocity, I can not cut the twist rate in half and still get the same result. Intuitively it seems as though for any bullet, there should simply be a magic rpm above which the bullet is stable. However it is clear from my own experience that as the angular velocity decreases (although at a much slower rate than the linear velocity) after firing, a bullet that was stable at the muzzle can become unstable in flight. Also while it is true that a bullet can wobble its axis a fair amount while still maintaining accuracy at the target, I believe the bullets wobble has to increase it drag, accelerate velocity loss and ultimately lead to instability. Underlying all of these confusing facts, it is also seemingly evident that two bullets of exactly the same length and same alloy don't necessarily require the same twist rate. The RCBS 95 gr .245 RN and the NOE 105 gr .245 FN are a case in point. They are the same length but the RCBS bullet is measurably easier to stabilize.

None of this is meant to be argumentative. I am simply taking this opportunity to air some of my own rambling thoughts about twist rates and such.

Jim

 

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beagle6 posted this 3 weeks ago

I was a bit apprehensive about starting this discussion but now I'm glad I did . There was a lot of good information submitted and and any time there is an exchange of knowledge by such experienced people, it is beneficial to all.

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Ken Campbell Iowa posted this 3 weeks ago

... a man has got to know his limitations ....

i can see why i can shoot 2 moa with a good cast bullet rig ... heck, hitting a baseball at 100 steps is pretty good stuff ...  smacking a target i can barely see without a 10 power scope is very satisfying ...  not into the same hole, but into the same baseball ...

but i never could see why my 788 remmy in 44 mag shot 16 moa with 10 different loads and molds and visibly perfect castings  ....  it made me sad to have to sell this otherwise really fun gun to a collector ... kinda like putting a beloved dog down for a terminal disease ... ( fwiw it shot 3 moa with sierra mj ) ...

how could it be that bad ... then we read about joeb's mysterious occasional large cast groups for unfathomable reasons from respectable rifles ...

maybe we should be looking at horrible groups to uncover secrets of cast ( un ) predictability ... why are they 12 moa .... not 120 moa ? ...

we could form an elite research team of very large group shooters ... i remember the guy that bot my remmy 44 ...

ken

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M3 Mitch posted this 3 weeks ago

Well when a gun will shoot jacketed accurately but not cast, my first thought is that the cast bullets in question are too small in diameter.  It could be that the 788 in 44 Mag had a throat angle that was not at all conducive to cast bullet use, it's *possible* that adjusting the leade would have helped - but, Ken, you are a damn good gunsmith and I would think you would have tried this sort of stuff.  

My own experience, and what I have read on here and in FS, lead me to believe that really bad accuracy is almost always a bullet fit problem, and that in these cases going to a different powder is a waste of time.  And bullet fit problems are almost always that the bullet is too small in diameter - in my limited experience anyway.

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joeb33050 posted this 3 weeks ago

Here's a table showing how stability varies as V and Twist.

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joeb33050 posted this 3 weeks ago

SF < 1.0  Bullet is not stabilized

SF > 1.0  Bullet is marginally stabilized

SF > 1.3 = Bullet is fully stabilized

SF > 1.5 = Bullet is maximally stabilized

 

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joeb33050 posted this 3 weeks ago

Charlie Dell's formula:

(3.5 x MV1/2 x Caliber2) / Bullet Length = Required Twist Rate

Note the MV squared, twist rqd and stability are affected little by V.100 fps ^1/2 = 10, 900 fps ^1/2 = 30; 1600 fps^1/2 = 40, etc.

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joeb33050 posted this 3 weeks ago

Homer Powley:

(20.62/((BL/DIA)^2.25)SQRT(WT/((1-(V/5705))SF)))

 

= twist; 

SF= ((WT(20.62(DIA^2.25))^2))/((TW*(BL^2.25))^2))/1-(V/5705))

= stability factor.

 

Now let the stability factor wizards begin.

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beagle6 posted this 3 weeks ago

joeb

In Charlie Dell's Formula, is bullet length in inches or calibers?

beagle6

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joeb33050 posted this 3 weeks ago

Inches. I'd be happy to send the EXCEL workbooks for all of these.

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