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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joeb33050 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.

Stability varies and as the square of twist.. Double the twist and Sg quadruples. 

Stability varies (almost) linear. Increasing velocity increases Sg by a small, (almost) constant amount.

(Sg vs. V is a sorta log function, but quite flat >1500 fps.)

 

 

 

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Scearcy posted this 3 weeks ago

Joe

I would like to have EXCEL workbooks for these. I'll send a PM with my email address.

If memory serves the SF target for optimal accuracy is thought to be well below 1.5. There must be opinions regarding the best range of SF.

Thank you Joe. You make me dust off mathematics I haven't used in many years. Being a numbers geek, I find it to be fun.

Jim

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Larry Gibson posted this 3 weeks ago

Searcy

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.

No, they are not proportional.  Sometimes what we think there "should be" just isn't.  Even in the relative simple formula for determining stability of bullets there are variables.  Those variables are accounted for in the constant.  If we accept the reality of what really is then we begin to see many erroneous assumptions, myths and old wife's tales for what they are or what they aren't.  Unfortunately many prefer to believe those erroneous assumptions, myths and old wife's tales instead of learning as we advance our knowledge of what is really occurring.

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.

Loss of accuracy at longer ranges is most often assumed to be from a loss of rotational velocity (angular velocity?).  However most bullets loss accuracy at longer ranges is not because of a loss of stability due to degradation of rotational velocity but because other forces act upon the bullet causing it to lose stability.  What happens to 38 SPL WCs out of 18 3/8" twist barrels near or beyond 50 yards is a good example.  What causes the loss of stability is the uneven air resistance on the flat face of the bullet overcomes the dynamic stability of the bullet.  Increasing the twist (increases the RPMs and thus the dynamic stability of the bullet) with a 16" twist barrel will keep the bullets flying "stable for another 25 + yards but the air resistance will overcome the stability of the bullet and cause the nose to yaw and then the bullet to tumble.

We also must understand that stability, even though necessary for accuracy, is not the only thing that underlies the causes of inaccuracy.  There are many other influences on accuracy that have nothing to do with stability of the bullet.  Let's take my previous example of the 150 gr 30 caliber jacketed bullets for instance.  The typical 150 gr M2 bullet from the military 30-06 gives typically  2 - 4 moa accuracy (10 shot groups) at 300 yards.  The bullet holes are all nice and round thus the bullets are stable.  Yet a 150 gr Speer, Hornady or Sierra SP over the exact same load at the same velocity out of the same rifle will give 1 moa +/- groups at the same 300 yards.  The bullet holes are nice and round, the same as the M2 bullet holes, thus they are also stable.  There must be other things/forces at play that make the M2 bullets less accurate than the commercial bullets which tells us that stability is not the only criteria for accuracy.  Factually we can shoot some very poor groups with bullets that are stable.

This is certainly an interesting conversation.

LMG

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

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Ken Campbell Iowa posted this 3 weeks ago

...  just to keep up the sales of aspirin .... maybe the slower twists also are more gentle to our delicate squishy bullets as they " take the rifling " ....  gentle leade angles, gentle twist ..

even the mj hunter benchrest shooters use the slowest twist they can get by with ...  17 twist and 115 gr. 30 cal ..  22lr match shooters use a lot of 17 twist barrels ...

which brings up gain twist barrels ....  some shoot well, but doesn't the bullet undergo constant re-squishing as the twist rate increases ??     a friend tried a $$$ custom pacnor gain twist on his very good 22lr match rifle ... didn't shoot better ...  but  just one data dot ...

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

my mental model of a bullet entering a short-leade rifling is similar to one slowly sticking one's finger into a fan ....  the tip in both cases is trying to twist everything behind it in an argumentative fashion ...

that is why long tapered leads ( ~0.5 degree included )  and matching tapered bullets tend to work well with mushy bullets ...  the bullet only has to move forward 0.00002 inches before it is engraved about 80 per cent as much as it is going to engrave.   

fwiw, mental models of how things work can be skewed by wishful thinking ... ed harris has it right ... again  ...

ken

 

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Larry Gibson posted this 3 weeks ago

Ken

In reality the reason the slower twists have proven to give better accuracy is simply because they do not spin the bullet as fast given an equal velocity.  That means there is less centrifugal force created with the lessor RPMs to act adversely on any imbalances in the bullet during flight.

Having recovered 30 XCB and 311466 cast bullets from 10, 12, 14 and 16" twist there was no additional discernible additional damage/deformation from the bullets shot out of the faster twists given the same load/velocity.  However, there was a definite discernible difference in accuracy (10 shot groups at 100, 200 and 300 yards), especially at high RPM/velocities.  The slower twists always produced the better accuracy.

LMG

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Larry Gibson posted this 3 weeks ago

 For those interested who have access to Robert A. Rinker's Understanding Firearm Ballistics on pages 141 - 143 there is an excellent and simplified explanation of the Greenhill Formula.

LMG

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

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joeb33050 posted this 2 weeks ago

JUST DID THIS, INTERESTING-AT LEAST TO ME

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Ken Campbell Iowa posted this 2 weeks ago

thanks joe ... now all that remains is for " someone " with a 68 gr. 224 bullet to add a data line from real life range results ....  i suppose it would have to be based on " wildly unstable " , not just tipping bullet holes .

we need a research grant ... i am thinking a tunnel of ?600? yards of very heavy water mist would stop a bullet undamaged ...

ken

 

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frnkeore posted this 2 weeks ago

Joe,

What .98 long, 68 gr bullet are you using for that graph.

Frank

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frnkeore posted this 2 weeks ago

The main reason that there is such a large accuracy difference, between the M2 and commercial bullets (especially match grade), is the quality control (especially jacket QC) and construction differences, between the two. There will also be a difference in CG, that may make a difference in accuracy.

Frank

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joeb33050 posted this 2 weeks ago

Joe,

What .98 long, 68 gr bullet are you using for that graph.

Frank

Hornady 68 gr hpbtm; this is the heaviest/longest bullet I've used in my Jacketed Bullet Test. In a 9" twist 22-250 bbl, 16.5" long, it goes sideways/very tipped just below 1250 fps. My plan is to slowly reduce v and see how tipping progresses.

The graph is the same SHAPE for any bullet length/weight, not specific to any bullet.

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Larry Gibson posted this 2 weeks ago

The main reason that there is such a large accuracy difference, between the M2 and commercial bullets (especially match grade), is the quality control (especially jacket QC) and construction differences, between the two. There will also be a difference in CG, that may make a difference in accuracy.

Frank

You are essentially correct.  The fact is the end result of that lessor quality control is the M2 bullets means they have more imbalance in them than do the much better made commercial bullets. Thus the greater imbalances in the M2 bullets give the centrifugal force more to act on.  It's as simple as excellent quality (more consistent in jacket concentric thickness in particular) bullets are more accurate all other things being equal.  It applies to cast bullets also. 

The CG (basically the center of mass) is well behind the CP (center of pressure) on all bullets I mentioned.  The CG in an M2 bullet is a tudge bit farther behind its CP than in the commercial bullets.  That does mean the yaws and nutation's caused by the centrifugal force acting on the imbalances would have a slightly greater effect.   But then we have seen some lots of M2 bullets that shoot very accurately so how is it that the CG is the cause of inaccuracy?  Why is it Hornady M80 bullets (150 gr FMJBTs) shoot more accurately than most milsurp M80 bullets?  The reason is simple, as noted, the bullets, particularly the jackets, are much better made with less imbalances. A purview of the section on bullet accuracy in Hornady manuals explains it.

The goal is to get the bullet out of the muzzle as balanced as we can.  Otherwise the more unbalanced, the less accurate.

LMG

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

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frnkeore posted this 2 weeks ago

As a hole, the shape of a bullet, dictates the CG. Tapered  jackets and cannalures as well as lead density have a minor effect on jacketed. Lube grooves and their position on cast. Imperfections on both.

As Joe points out, most stabilization programs don't allow for CG in their programs.

While I can't say how much the position of the CG can effect accuracy, I can say the the position of the CG can and will effect the stability of any given bullet, in any given twist rate.

The stability program that I use calculates the appoximent GC, by bullet shape and also calculates the amount of increased twist that it takes to keep a bullet point on, in the transonic range, where the building and uneven sonic waves play hell with the bullet.

I've posted the Geoffery Kolbe program in the past, more than once, on this forum and got no questions or feed back. I can only assume that most think it's BS or don't know how to input the variables but, it's the most accurate program that I've found and will answer questions that many scratch their heads about.

Before I could afford a computer I knew the GH formula and used it but, there cam a day that I found that the famous 150 constant didn't dictate stability. A good friend that I got into Schuetzen shooting built a rifle with my info. I was new to the sport (1985) and I thought all 32/40 where 16 twist so, I told him to get a Douglas 16 twist barrel and the same mold that I used a 1.165 long Ron Long.

Well when we start shooting together, his bullet tipped noticeably, mine didn't and never had. So, I measured my twist, it was 15! What to do, I had cost him a lot of money in my ignorance! I then took his mold and cut .050 off of the base, his bullet straighten up and was competitive with my rifle.........SAVED.

I then used that length bullet in the GH formula changing the constant to 175 and used that number until I got one on a computer.

My second learning experience about 1990:

In my 15 twist rifle, I was able to stabilize the 323471, well enough to shoot a .43 group at 2100 fps. The 323471 is 1.215 long. A friend that I shot with, Stan D. (can't spell his long name) but, he was well known, here in the PNW, that was making bullet molds, he offered to make a custom mold for me. It is 1.15 long and a ogive of .38, weighing 209 gr. The 323471 is 1,22 long with a ogive of .57, weighing 220 gr.

You would think that the 1.15 long bullet, would easily stabilize in the 15 twist, if the 1.22 long Lyman does. It was NOT to be! The 1.15 long bullet had noticeable tipping and I couldn't use it. The CG is the difference.

Regarding yaw, I don't think it effects the BC very much at super sonic speeds, so long as it stays fairly point on. In SS speeds the shock wave is more important that the air waves. It's the point of the bullet that pushes the shock wave, the flatter the point, the harder it is to push the shock wave and the more velocity you loose. That is why spitzers excel at SS speeds, at least regarding BC.

If I could get a drawing of Joe's 68 gr bullet, I can run some figures for him to compare with his velocity experiments.

Frank

 

 

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frnkeore posted this 2 weeks ago

These are the graphs for my shortened and original length Bullets. The original length is 1.16, the shortened (by cutting off .050 from the base band only) 1.11. The spitzer nose is .53 long, with a .075 meplat.

First is the 1.16 in 16 twist, Second, 1.11 in 16 twist.

The last graph is for the 1.16, in a 15 twist.

Frank

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Scearcy posted this 2 weeks ago

I just want to say this thread has gone well beyond twist rates for dummies - good stuff!

Jim

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frnkeore posted this 2 weeks ago

This is how Joe's, Miller/GH program deals with the same bullet/twist combo. Note that the Sg figures, for both, say the the bullet won't be stable, especially for the 1.11 long/16 twist.

And the last picture is the match results that I shot with the 1.11 long/16 twist combo.

Frank

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RicinYakima posted this 2 weeks ago

Empirical results outweigh theoretical musings.

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joeb33050 posted this 2 weeks ago

Here are the solutions:

.321"Dia, 190GR, 1.11" long, 1450FPS, 16"twist

GREENHILL 13.9"

DELL 12.4"

POWLEY WITH SF = 1.3, 17.7"

MILLER WITH Sg = 1.243, 16"

Greenhill and Dell suggest that a faster twist is required, Powley and Miller suggest that a 16" twist is at least marginal.

Same story for the 190 grain bullet.

However, I'm missing Frank's point. Please explain.

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frnkeore posted this 2 weeks ago

My point is a comparison of the different stability programs and their accuracy, when it comes to bullet shape and there for CG.

The one I use, indicates the the 1.11 bullet is fully stable and the accuracy, shows that it's fully stable.

A Sg of 1.243, is not fully stabilized and the bullet should not be able to create the accuracy that it demonstrates, in the match results.

I don't believe anyone would choose a twist/bullet combo in the 1.24 range, either. Miller doesn't suggest anything under 1.4 Sg.

If you change the GH constant to 173, it gives a twist of 16. I found when I shorten that mold, that I could use 175 as a safe constant for my twist calculations, for Spitzer bullets. Spitzer bullets have their CG closer to the base that other common designs.

The program that I use calculates the CG of a bullet, when producing a Sg factor.

Frank

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