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
2 Attachment(s)
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
Attachment 108092
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.
Attachment 108090