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Thread: RPM Threshold barrel twist/velocity chart

  1. #1
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    RPM Threshold barrel twist/velocity chart

    RPM threshold twist/velocity chart

    I’m posting this at request for an easy reference to see the velocity range where the RPM threshold will most likely be found based on the twist of the barrel.

    The RPM threshold is that point where accuracy begins to deteriorate when the RPM is sufficient to act on imbalances in the bullet in flight to the extent the bullet begins a helical arc in flight or it’s flight path goes off on a tangent. It is best noted when working up a load as velocity increases flyers begin to happen. Then as velocity is further increased the total group size increases sometimes to the point some bullets fly so far off they miss the target. A further indication the cast bullets at or over the RPM threshold is (or some of them in a load that is on the edge of the RPM threshold) the non linear dispersion of the group size as range increases.

    Let us keep in mind the RPM threshold most often falls in the 120,000 to 140,000 RPM range with regular lube groove cast bullets. Exactly where the RPM threshold will be in fps depends on numerous factors; alloy, bullet design, fit, sizing, lube, GC’d and seated square, powder burning rate and the length of the barrel, etc. The RPM threshold may be lower than 120,000 RPM by careless casting and loading techniques or when using very soft alloys with very fast burning powders. Conversely, the RPM threshold can be above 140,000 by careful casting and bullet selection and preparation along with careful accuracy enhancing loading techniques, especially those for cast bullets at high velocity such as using slow burning powders that ignite easily and burn efficiently at lower pressures. The trick is to get the cast bullet to exit the muzzle as balanced as possible with as little deformation to it during accelleration. The more balanced the bullet is and the closer the axis of rotation coincides with the center of mass on exit from the muzzle and during flight the more accurate the bullet will be and thus, the higher the RPM threshold will be.

    The RPM threshold is not a set “limit” of RPM or velocity. Best accuracy will be just under the RPM threshold or lower. Useable accuracy can be had above the RPM threshold if the ranges are not long and the accuracy requirement is not small. Keeping .223 cast bullets on a silhouette target out to 200 yards for example or keeping hunting cast bullet accuracy at say 4 moa if the max range to be used is 50 – 100 yards.
    Again; the RPM threshold will generally be found between 120,000 to 140,000 RPM with regular commercial cast bullet designs and loading techniques most cast bullet shooters use.

    In the chart below I’ve computed the fps for various common barrel twists for 120,000 and 140,000 RPM. For other twists in between anyone shouldn’t find it too difficult to interpolate. These fps figures should give you an idea in what fps range your loads, as you work them up, will probably bump into the RPM threshold and when accuracy will probably begin to deteriorate. Some pundates will crticise this chart saying they, or someone else, gets accuracy above the figures in the chart. For those who understand how to push the RPM threshold up with higher velocity cast bullet loads that can indeed be the case. However, as mentioned, the chart is for the majority of cast bullet shooters who do not care to push the RPM threshold up but simply want to understand where and why accuracy will probably deteriorate with their regular cast bullet loads. This chart was done for them.

    RPM……….120,000……….140,000

    Twist……….FPS…………..FPS

    7”…………1166…………..1361

    8”………….1333…………..1555

    9”………….1500…………..1750

    10”………...1666…………..1944

    11”………...1833…………..2139

    12”…………2000………….2333

    14”…………2333………….2722

    16”…………2666………….3111

    18”………….3000…………3500

    Larry Gibson
    Last edited by Larry Gibson; 07-29-2013 at 01:43 PM.

  2. #2
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    RPM Threshold; A Tale of Three Twists, Chapter II

    Here the 2nd chapter which was posted on 5 April 2008.I just changed a few remarks at the endhaving completed other tests in the last 5+ years.The proof of the RPM Thresholds existence isthere.If you have questions please readthe original thread ; http://castboolits.gunloads.com/showthread.php?28807-RPM-Test-a-tale-of-three-twists-Chapter-2/page2

    Many good questions were asked and answered.The usual arguments are there also.There is no need to rehash those on thisthread because the proof is here.Icompleted a couple more tests and did not post the results because they confirmed what is here. My testing and load development then took the turn to see just how fast I could push a regular cast bullet of a ternary alloy with the 14” twist Palma rifle and maintainaccuracy of 2 moa or less and maintaining linear group dispersion out to aminimum of 300 yards.

    I have succeededwith that and you have all seen the results of the 311466 cast of #2 alloypushed to 2600+ fps.I can harden thebullet with CU and push to close to 2680 fps.I can also go to a slightly lighter weight 311465 and push 2700+fps.That is about the limit with thecase capacity of the .308W.Yes I can usea faster burning powder and increase velocity but accuracy goes as the psi(measured with the Oehler M43 in that Palma rifle) climbs above 42,000psi.It appears that psi may be a “limiting”factor as that may be causing “plasticization of the bullet. I don’t know that for sure yet but the quickertime/pressure curve definitely damages the bullet more and lowers the RPMThreshold so accuracy does not hold to 2600 fps with the 311466.

    My next step in this high velocity quest with accuracy isthree fold; 1st is to use a slower twist of 16”, 2nd isto use a longer barrel of 30” and 3rd is to use a case with largercapacity.The case for that needs tohold RL19, AA4350, H4831SC or RL22 right at 100% load density while keeping the311466 at or under 40,000 psi to achieve a velocity of 2700 upwards of 2900+fps.That would still keep the bulletbelow the RPM Threshold and the plasticization psi level. Additionally the case should have the longerneck of the 30-30 or ’06 in lieu of the shorter .308W case neck.This will keep a properly designed castbullet with the GC at the bottom of the case neck and the ogive just on the leadewith a short nose. The 2 current .30cal cast bullet designs available that fit this criteria are the Lyman/Loverin 311466and the LBT 311-160 cast bullet designs.Both also have 65%+ bearing surface.

    The cartridge for the next step is the 30x57/30 XCB whichis basically a short chambered 30-06 with a tight neck.This cartridge was designed with these goalsin mind.If the case capacity is notenough it can be increased by simply rechambering with the same reamer a bitdeeper in increments until case capacity matches the desired goal of velocityand psi at 100% load density.Cases areeasily formed and shortened standard ’06 dies are used for forming cases andfor loading.I am in the process oflocating a quality barrel of correct length and dimensions to continue thequest.

    As you all know in the past any time the RPM Threshold ismentioned the pundits come out in force to discredit me, let me say that again….todiscredit me.The existence of the RPMThreshold is proven.For those who can’tget their heads around it simple study it and perform a few testsyourself.You will find the RPMthreshold.For those who still don’tunderstand it is not a “limit” then I suggest you read the sticky on the RPMThreshold as I define it there.It isnot hard to understand.For those whowish to argue using the same old non proven arguments you’ve used for years withme in a further attempt to discredit me then please don’t waste your time orours.I will not respond.What would be more beneficial and appreciatedwould be for you to conduct your own thorough test and post the results.However, should anyone have an honestquestion pertaining to these test results I will entertain that.

    Larry Gibson


    RPMTest; atale with three twists

    Chapter 2;
    Test 1 [311291 of 2/1alloy]

    Yesterday broke clear with the promise of some warmth and little wind so Ipacked up the three rifles, the M43 PBL, the
    test ammoand the usual other necessary accoutrements for the range and set off theTacoma Rifle and Revolver Club to conduct the first test. Theprimary goal of this test was to see if we coulddetermine what causes the 311291 cast bullet to loose accuracy at a certainlevel. On arrival at TRRC I proceeded to set up. The benches there are verysolid benchrest designed and made. It was about 46-48 degrees in the shade ofthe firing line but was into the 50s in the sunshine. Wind was coming out of 11o’clock at 1-3 mph. The target distance was 103 yards. The testing was begunusing the 10” twist rifle and then the 12” twist rifle and finally the 14”twist rifle. The barrels were cleaned between every two 5 shot groups with 2foulers fired before testing was resumed. All data was collected via the M43using pressure recording, muzzle screens and down range screens. Besidesinformation on the rifle, load and testconditions the M43 provided data on the following information;

    Data recorded for each shot;
    • Velocity at the muzzle screens
    • Proof variance of muzzle screens
    • Time Of Flight between muzzle screens and down range screens (in front of 100yard target)
    • The down range velocity
    • Proof variance of down range screens
    • Ballistic Coefficient
    • Peak average pressure (psi.m43)
    • Area under the pressure curve
    • Rise of pressure curve
    • Actual pressure curve

    Summary of shot data for recorded shots in the group;
    • Average velocity at muzzle screens
    • Average Proof variance of muzzle screens
    • Average TOF
    • Average down range velocity at down range screens
    • Average proof variance of down range screens
    • Average Ballistic Coefficient
    • Average peak pressure
    • Average area under the pressure curve
    • Average rise of pressure curve
    • Standard Deviation of each of the above data averages
    • The high reading of each of the above data fields
    • The low reading of each of the above data fields
    • The Extreme Spread of each of the above data fields.

    The M43 also provided the additional data on Standard Atmospheric Ballistics;
    • Bullet path from muzzle to 250 yards based on data entered and the actual BC
    • 10 mph wind deflection
    • Computed muzzle velocity (fps)
    • Energy (ft-lbs)
    • Power factor
    • Recoil of the rifle
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    The testing was uneventful except for one low shot that hit one of the downrange screens….ooops! It knocked a chunk of the plastic off but didn’t actuallyhurt anything. As the groups enlarged I did have a few rounds that hit on theedge of the window and didn’t read. This cut some of the group data to 4 shotsinstead of 5 and one group to 3 shots of recorded data. The first
    test waswith the 311291 cast of 2 parts WW to 1 part linotype. This gives an alloy thatwith the bullets air cooled the hardness of the bullets is similar to Lyman’s#2 alloy. That has long been a standard for cast bullets. As mentioned inChapter 1, the cases for all three rifles were fire formed to the specificrifles and “match prepped” as such. The primers used are WLRs. Two powders wereused. H4895, a medium burning powder, was used with a Dacron filler in 2 grincrements from 26 gr to 38 gr. This was expected, and did, to give velocitiesfrom 1700 fps or so up through 2500 fps. The second powder tested was H4831SC,a slow burning powder, loaded in 2 gr increments from 40 to 46 gr to give from90 to 100% loading density. The only sorting done with the 311291 bullets wereto inspect them for wrinkles, voids of non fillout. None were weighed forsegregation by weight. The gas checks used were Hornady’s. They were pre-seatedwith the Lyman GC seater on a Lyman 450 with the .311 H die and then lubed inthe .310 H die. The lube used was Javelina. At no time during the test wasthere any indication of leading or “lube failure”.

    All told in
    Test 1 I fired 75 shots forrecord plus 10 foulers through each rifle for a total of 250 shots . Afterreturning home it seemed a daunting task to sort through the data, measuregroups and put it into some format that is easily presented on this forum. Icould list all sorts of numbers in various manners but that would just getconfusing. From the listed data the M43 provides on each shot plus the averageslet me tell you I’ve got lots of numbers! I decided instead to put thepertinent data onto graph form. That is a “visual” way to present informationand it gives valid comparisons which are easy to see and make comparisons from.It is easy enough to pull additional information of the graphs if you want it.However the little squares of the graph did not scan well so if you want somespecific information don’t hesitate to ask. I couldn’t get the graph on thiscomputer to work right so I resorted to graph paper and hand plotted them.

    Without further ado we might as well get to the meat and potatoes of the
    test.Graph #1 is a comparison of velocity and pressure. There was considerableconsternation from some forum members that pressures would not be “exact”between the rifles. I stated that, disregarding the fact that there is alwaysvariation of pressures, even with the same load in the same rifle; thepressures need not be the same in each rifle. In fact they were not. When wegraph out the velocity/pressure of the same increasing loads out of differentrifles what we expect to see is a linear relationship between them. The linearlines for each (red = 10” twist, blue = 12” twist, green = 14” twist) shouldrun fairly parallel. This gives us a valid comparison of the time pressurecurves of each rifle with the other rifles time pressure curves. That’s exactlywhat we see in graph #1. As the pressure increases the velocity increasespretty close for the 10 and 12” twist rifles but the 14” had some problems. Wealso see a slight divergence as velocity increases. This is expected as the 12and 14” twist barrels were longer than the 10” twist barrel so velocityincreased more as pressure was increased. Thus the comparison between therifles is valid as the linear progressions are close to the same. Were one ofthem radically different then it would be obvious a comparison wasn’t valid.However there is a slight anomaly with the 14” twist. We could pontificate asto why and probably come up with numerous reasons, most of which would probablybe wrong. So let’s what the data can tell us regarding that anomaly.

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    Last edited by Larry Gibson; 06-26-2014 at 09:08 PM.

  3. #3
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    The answer to the velocity/pressure anomaly with the 14” twistis rather simple and is demonstrated in graph #2. The relationship betweenpressure and velocity is encompassed in internal ballistics so we merely needto look at that data showing the consistency of the loads, i.e. how consistentthe powder burns. Consistency of a load (given a teststring of several shots) is most often expressed in Extreme Spread of velocityand Standard Deviation of the combined averages of velocity. SD tells us what aload may do but ES tells us what that load did do. Since I am interested inwhat the load did do I compared the ES consistency of the loads with thepressure. In graph #2 the loads of the 10 and 12" twists all had ESs of 50fps or less. That is pretty good consistency given the spread of the loadsvelocities of 1700 fps through 2500 fps. The 14” twist had some early problemswith the powder burning efficiently. We see the ES for the 2nd and 3rd test loadswas considerably higher than the same loads in the 10 and 12” twists. Thataccounts for the small anomaly in the pressure curve of the 14” twist on graph#1. The other, and perhaps more important, piece of information graph #1 givesus is the time pressure curve of the same loads in the different twists. Obviouslythe curves are pretty close together and linear. Thus the time pressure curveor acceleration is very close to the same for each rifle.
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    Next let us consider the question; if the time pressure curvesare the same then any deformation to the bullet due to acceleration will beclose to the same. Thus if the deformation to each bullet is the same at thesame rate of acceleration then any change to the form of the bullet will resultin a change to the Ballistic Coefficient. Following that then won’t any changesto the BC be the same for each twist since any deformation of the bullet shouldbe the same? To find the answer to that question we merely compare the BCs ofthe 3 different twists as the velocity increases (hence the accelerationincreases and deformation of the bullet increases). Graph #3 provides thecomparison of the BCs vs the velocities of each load in each twist. Let usremember that the BC in this case is a measured BC from the actual flight ofthe bullets not a guestimated one from some chart. These actual BCs measuredthe bullets ability to fly through the air efficiently. The higher the BC theless deformed and more stabilized the bullet was. It is readily apparent thatthe BCs stayed pretty much the same for all three twists during acceleration atall velocities and pressures. It is interesting to note that the BCs of thebullets from the 10” twist retained the highest BC at the highest velocity(acceleration). This is just the opposite what it would be as believed by someon this forum. The BCs from the bullets from all three twists stayed very closetogether and linear across the wide spectrum of velocity (acceleration) from1700 to 2500 fps which obviously shows the acceleration remained constantregardless of the twist of the barrel.

    Attachment 108102







    So this is what we now know now about the same loads in the 3different twists; the time pressure curve is very close to the same, theacceleration is very close to the same and the BCs remain very close to thesame.



    Let’s now take a look at the results on target. After all whatwe are looking at in conducting this test isthe accuracy at higher velocity and why that accuracy goes bad. Graph #4 showsus the group sizes vs pressure. Whoa there! Something is amiss….if the timepressure curves are the same, the acceleration the same and the BCs are thesame; then if the groups get larger as we increase velocity shouldn’t thegroups get larger by proportionally the same amount? [Note; by “proportionalamount” is an amount to compare the accuracy of each twist to each other. Theproportional amount factor of increase is found by dividing the increased groupsize by the smallest group with each rifle.] However, what we see is that thegroups do not get proportionally larger as velocity increases. The inaccuracyof the 10” twist increases by a factor of 5.38 while the inaccuracy of the 12”twist increases by a factor of 3.14 and 14” twist increases by a factor of 2.08.

    Attachment 108103





    Hmmmmmm……pressure curve is the same, deformation of the bulletfrom acceleration is the same so then why doesn’t inaccuracy increase the same?Especially since graph #4 shows the group size vs pressure. But wait…there’smore (sorry, just couldn’t resist!). Doesn’t every one say that it is pressurethat destroys accuracy? We do see that accuracy with all three twists isdecreasing with the increase of pressure. If pressure was the only reason forthe decrease in inaccuracy then the inaccuracy should be proportional and wefind it isn’t. We also see a much greater increase of inaccuracy with the 10”twist than either the 12 or 14” twists. We also see the 12” twist’s inaccuracyto increase more rapidly than the 14”s inaccuracy. Again, if it was pressurethat increased the inaccuracy then why doesn’t the inaccuracy of all threetwists increase equally as the pressure increases? It seems there is somethingother than pressure adversely affecting accuracy and to a much greater extent.



    Okay, let’s look at it one more way just to be fair. Graph #5 compares accuracyto velocity. Something wrong here again….that dreadful 10” twist is once againbeing more inaccurate by a greater proportional amount than either the 12 or14” twists. How can this be? We know the acceleration is the same; the BCs arethe same so the deformation of the bullet is the same yet the 10” twistsinaccuracy is disproportional to the 12 and 14” twists. It should be the sameamount of inaccuracy for each twist if pressure was the problem, right? Thelines for each twist should be linear right? Yet the proportion of inaccuracy isnot the same between the twists nor are the lines linear. Have we missedsomething? Is there another game afoot? We’ve a good handle on the internalballistics. We know about the terminal ballistics as the group sizes are selfrevealing. But have we really looked hard at the external ballistics (thebullets flight)? We know the bullets are stable, we know the BCs are gettingsmaller as the velocity increases telling us there is some deformation from theacceleration. We know the 10” twist had the highest BC at the highest pressureand velocity so why isn’t it as accurate as the 12 and 14” twists?


    Attachment 108104



    Let us look at graph #6. It is a comparison of group sizes vs
    RPM. Notethe very, very obvious adverse affect that the increasing RPM hason the accuracy of the 10” twist. That red line really climbs up there! Alsonote that area of RPM where the majority ofaccurate groups fall; it is in or below the RPMthreshold. Also note that in or at the top end of the RPMthreshold is where accuracy begins to deteriorate.

    Attachment 108105



    The tests with H4831SC seemed to be headed the same way but were inconclusiveas top velocity was only 2287 fps with 100% loading density. The 10” twistvelocity was 1928 fps through 2287 fps with groups running from 2.4” to 3.3”.
    RPM was138,900 to 164,700. Conversely the 14” twist went from 1906 fps to 2265 fps.Groups ran .95” to 2.2”. RPM was98,000 to 116,600. The highest peak pressure was 39,600 psi.M43. Thus Icouldn’t get into a high enough pressure/RPM range with all three twists tomake any comparison.

    I am not going to conclude that there is an
    RPMthreshold as the test is not complete. Ishall wait until I conclude the testbefore giving a firm conclusion. However, we see from the test so far that veryfirm evidence is being found to make a definite case that the RPM threshold isalive, well and readily producible.

    Note; a change of the testing direction was done after the original thread wasposted. I switched from the 311291 (177gr) to the 311466 (155 – 160 gr). Thereason for the change was to increase velocity with a change to slower burningpowders and to use a bullet with a design more conducive to HV accuracy.

    Larry Gibson

    Last edited by Larry Gibson; 06-17-2014 at 11:48 PM.

  4. #4
    Boolit Master
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    Thanks for your time, effort, and generosity in sharing your current results, Larry. I'll be interested in the rest of your test results when they're available.

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    Waiting to see what you can do with 1:16 twist 30x57, thanks!
    Last edited by swheeler; 06-18-2014 at 10:00 AM. Reason: inflamitory
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    Thanks Larry!
    The solid soft lead bullet is undoubtably the best and most satisfactory expanding bullet that has ever been designed. It invariably mushrooms perfectly, and never breaks up. With the metal base that is essential for velocities of 2000 f.s. and upwards to protect the naked base, these metal-based soft lead bullets are splendid.
    John Taylor - "African Rifles and Cartridges"

    Forget everything you know about loading jacketed bullets. This is a whole new ball game!


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    Thanks Larry
    thanks very much for all the information --lots to think about
    Book marked for future ref

    I can see from your graphs where the Nodes are velocity ver group size
    where the therotical velocity range will give you the therotical smallest group size
    if you leave the nodes (or velocity range) group size increases for a small chance in velocity you get a big change in group size

    Keep up the trigger time and keep sharing the information you find doing it

    thanks Again

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    Boolit Master 35 shooter's Avatar
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    Thanks so much for the effort your obviously putting into these tests and for sharing the info.I think that's the first time i've ever seen a ballistics lab set up at the range before.
    The graphs are very telling. I'll be watching this with great interest all the way to the end results. The big surprise to me so far was that the 10 twist didn't hold a bit closer to the 12 twist. I mean i expected those results somewhat, but when it went south it went in a hurry. I just expected it to be a "bit" more proportional...wow!

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    There are a lot of reasons why I love this site. the hours of reading and the vast experience of the members who share with us their trials and tribulations but most of all its articles like Larry has written here and who despite all the mud being flung by others still manages to produce these absolute gems for our edification and pleasure. Thank you Larry , I for one appreciate your informative and well reasoned articles. Now to try and improve my groups .
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    Good job Larry, glad to see results. You have brought up another point to think about. Something that would be of great interest. We see what over spin is doing, what if the tests were repeated at long range, 300 or farther, to see if accuracy with each twist changes and at what point a boolit might go to sleep.
    You would not need the instrumentation since you have the info, just shoot groups with each load.
    I would like to know if the 1 in 10 twist will start to shoot a better group somewhere and if the slow twists will increase. Make another nice graph.

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    44man - As I've noted in earlier posts, the Hunter Benchrest folks have gone to progressively slower twists. While I was shooting the winners went from 15" to 16" to 17" twists in .30 cal. rifles, 130 to 135 gr. bullets. Those folks shoot out to 300 yards in competition, with group sizes (100 yards) of .1" or less (5 shot) for the competitive shooters. The accuracy holds to at least 300 yards. Based on the Greenhill formula, bullets of 180 gr. would not stabilize in the super slow twists, but those lighter bullets sure enuf do. It has been noted also that linear velocity decreases rapidly while spin velocity decreases very slowly. Larry's tests don't change the fact that a stable bullet pretty much stays stable until it is stopped by something. On the other hand, his data clearly shows that you can "overspin" a bullet. In the case of lead boolits, overspin apparently can and does result in boolit deformation. The metal simply isn't strong enuf to withstand the centrifugal forces beyond a given rpm threshold. It would be curious to see what quenching does to the rpm threshold. Quenching allows higher pressures without boolit deformation/skidding/"whatever" occuring, but does it significantly alter the rpm threshold ? I don't recall any discussion of this, but suspect it would alter the rpm threshold. Whatcha think, Larry ? Pilgrim
    Last edited by Pilgrim; 06-18-2014 at 11:41 AM. Reason: added clarification re: yardage

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    looks like your 12" twist rifle is a very accurate one....

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    Thanks Larry! I think............what did you say?

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    Outstanding, Larry. I'm copying for reference...
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    Good show Larry. Thanks for the effort and time this took.

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    Thank you Larry for all the time you put into this, very informative and keep it up.

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    I add my thanks also Larry. Always look forward to your post's.

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    Quote Originally Posted by 44man View Post
    Goodjob Larry, glad to see results. You have brought up another point to thinkabout. Something that would be of great interest. We see what over spin isdoing, what if the tests were repeated at long range, 300 or farther, to see ifaccuracy with each twist changes and at what point a boolit might go to sleep.
    You would not need the instrumentation since you have the info, just shootgroups with each load.
    I would like to know if the 1 in 10 twist will start to shoot a better group somewhereand if the slow twists will increase. Make another nice graph.
    No need to "make another graph".....I'll show you the targets. In the past we have discussed the difference between the bullet "going tosleep" and "crossing the RPM threshold". The going to sleep has to do with bullet stability in flight on exit from the muzzle; basically when yaw and pitch settle down. Most often it is conceded that bullets go to sleep at a relatively short range; usually prior to 100 yards. However, some do believe bullets "go to sleep" at longer ranges. While I have never seen this either by shooting smaller groups of a sufficient sample at longer range or by watching bullet traces over manyyears of High Power shooting and long range target interdiction at 200 - 1000+ yards I do ask those who believe to show me. None have been able to produce an example on demand. None the less let us discuss it.


    Additionally in our previous discussions we have noted that a bullet crossing the RPM Threshold has nothing to do with stability of the bullet either initially on muzzle exit or after it has crossed the RPM Threshold. The non linear expansion of group size as the range increases is another measure the bullet/load has crossed the RPM threshold. The non linear expansion may not be much or it may be a lot as it depends on the imbalance and the amount of centrifugal force (amount of RPM) there is to act upon it. Obviously non linear expansion of group size as range increases is counter to the "bullet going to sleep" theory.

    Let me show you an example. We have the target results of 2 separate loads shown below using the 311291pushed by 4895 out of the .308W test rifle with the 10" twistbarrel. Ten shot test groups were fired at 50 yards, 100 yards and 200yards with each test load.


    The 1st groups are with 28 gr of 4895 at a velocity of 1912 fps (muzzle velocity)at 137,664 RPM which is just under the upper end of the RPM Threshold. Note the 50 yard group minus the 4 foulers has 9 shots in .7” (I called the flyer down and away). The 100 yard group of 1.55” is of linear expansion to the 50 yard group (should be close to twice as big). The 200 yard group also is very close to linear expansion when compared to the 50 and 100 yard groups. Thus we see that even though this load is close to the upper end of the RPM Threshold the group expansion is linear. Also note all the bullet holes are round indicating excellent stability, even at 50 yards.

    Attachment 108122


    The next groups are with a load of 38 gr of 4895 under the same 311291. The velocity is 2515 fps with 181,080 RPM which is obviously above the RPM threshold. If the cast bullets were going to “strip/skid” on the lands or go unstable it probably would have shown with this load. Obviously from the deteriorated accuracy we can conclude there was some damage to the bullet during acceleration. The measured psi was 51,700 btw. As we see there appears to be linear dispersion of the groups between 50 and 100yards. This is why 50 yard testing can be deceiving and perhaps we could erroneously conclude the bullets were “going to sleep” between 50 and 100 yards(?). However, when we shoot a test string at 200 yards we definitely see the non linear expansion between the 100 yard 4.7” group and 200 yard 14.5” group. We may also have erroneously concluded that the 4.7” 100 yard group was “good enough” and perhaps then expect to hit the heart lung area of a deer out to 200yards(?). Actual testing shows the group size was 14.5” (much larger than the heart/lung area) and was really pretty poor accuracy. Also note that all10 holes in the 200 yard target are round indicating no hint of instability ofthe bullets even though the are obviously no longer following the line offlight but are going off on a tangent orinto a slow increasing in diameter helical arc around the line of flight. Looking at that 200 yard target do we really think the bullets will "go to sleep" at a farther range and be more accurate?

    Attachment 108123
    Attachment 108124


    Larry Gibson
    Last edited by Larry Gibson; 06-18-2014 at 02:32 PM.

  19. #19
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    Quote Originally Posted by Pilgrim View Post
    ................ It would be curious to see what quenching does to the rpm threshold. Quenching allows higher pressures without boolit deformation/skidding/"whatever" occuring, but does it significantly alter the rpm threshold ? I don't recall any discussion of this, but suspect it would alter the rpm threshold. Whatcha think, Larry ? Pilgrim
    Ah, Pilgrim......hardening the bullets is one way we can push the RPM threshold upwards and maintain accuracy. The hardening (WQing in this case) while perhaps preventing or attenuating the skid/stripping it also allows the bullet to withstand a higher rate of acceleration before unwanted obturation, sloughing or collapsing of the bullet occurs. Using longer barrels and slower powders also helps push the RPM Threshold up as does using a properly designed bullet.

    Of course if building/rebarreling a rifle for cast bullet use is an option then using the slower twist makes it all so much easier to shoot our ternary alloyed cast bullets at high velocity and maintain accuracy.

    Larry Gibson

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    Quote Originally Posted by runfiverun View Post
    looks like your 12" twist rifle is a very accurate one....
    That it is and a prized possession of mine. I have cleaned the 1000 yard line with it a couple times and won a state championship with it using Redfield Palma sights on it. It is a M70 with a 26" Schneider barrel. Picture shows it with a Redfield 4x12 Ultra on it. I'm going to blubber like a baby when I shoot that barrel out...........

    Larry Gibson

    Attachment 108125

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check