RPM Test; a tale of three twists, Chapter 2
RPM Test; a tale with three twists
Chapter 2; Test 1 [311291 of 2/1 alloy]
Yesterday broke clear with the promise of some warmth and little wind so I packed up the three rifles, the M43 PBL, the test ammo and the usual other necessary accoutrements for the range and set off the Tacoma Rifle and Revolver Club to conduct the first test. The primary goal of this test was to see if we could determine what causes the 311291 cast bullet to loose accuracy at a certain level. On arrival at TRRC I proceeded to set up. The benches there are very solid benchrest designed and made. It was about 46-48 degrees in the shade of the firing line but was into the 50s in the sunshine. Wind was coming out of 11 o’clock at 1-3 mph. The target distance was 103 yards. The testing was begun using 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 2 foulers fired before testing was resumed. All data was collected via the M43 using pressure recording, muzzle screens and down range screens. Besides information on the rifle, load and test conditions 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 100 yard 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
The testing was uneventful except for one low shot that hit one of the down range screens….ooops! It knocked a chunk of the plastic off but didn’t actually hurt anything. As the groups enlarged I did have a few rounds that hit on the edge of the window and didn’t read. This cut some of the group data to 4 shots instead of 5 and one group to 3 shots of recorded data. The first test was with the 311291 cast of 2 parts WW to 1 part linotype. This gives an alloy that with 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 in Chapter 1, the cases for all three rifles were fire formed to the specific rifles and “match prepped” as such. The primers used are WLRs. Two powders were used. H4895, a medium burning powder, was used with a Dacron filler in 2 gr increments from 26 gr to 38 gr. This was expected, and did, to give velocities from 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 from 90 to 100% loading density. The only sorting done with the 311291 bullets were to inspect them for wrinkles, voids of non fillout. None were weighed for segregation by weight. The gas checks used were Hornady’s. They were pre-seated with the Lyman GC seater on a Lyman 450 with the .311 H die and then lubed in the .310 H die. The lube used was Javelina. At no time during the test was there any indication of leading or “lube failure”.
All told in Test 1 I fired 75 shots for record plus 10 foulers through each rifle for a total of 250 shots . After returning home it seemed a daunting task to sort through the data, measure groups and put it into some format that is easily presented on this forum. I could list all sorts of numbers in various manners but that would just get confusing. From the listed data the M43 provides on each shot plus the averages let me tell you I’ve got lots of numbers! I decided instead to put the pertinent data onto graph form. That is a “visual” way to present information and 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 some specific information don’t hesitate to ask. I couldn’t get the graph on this computer 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 considerable consternation from some forum members that pressures would not be “exact” between the rifles. I stated that, disregarding the fact that there is always variation of pressures, even with the same load in the same rifle; the pressures need not be the same in each rifle. In fact they were not. When we graph out the velocity/pressure of the same increasing loads out of different rifles what we expect to see is a linear relationship between them. The linear lines for each (red = 10” twist, blue = 12” twist, green = 14” twist) should run fairly parallel. This gives us a valid comparison of the time pressure curves of each rifle with the other rifles time pressure curves. That’s exactly what we see in graph #1. As the pressure increases the velocity increases pretty close for the 10 and 12” twist rifles but the 14” had some problems. We also see a slight divergence as velocity increases. This is expected as the 12 and 14” twist barrels were longer than the 10” twist barrel so velocity increased more as pressure was increased. Thus the comparison between the rifles is valid as the linear progressions are close to the same. Were one of them 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 as to why and probably come up with numerous reasons, most of which would probably be wrong. So let’s what the data can tell us regarding that anomaly.
The answer to the velocity/pressure anomaly with the 14” twist is rather simple and is demonstrated in graph #2. The relationship between pressure and velocity is encompassed in internal ballistics so we merely need to look at that data showing the consistency of the loads, i.e. how consistent the powder burns. Consistency of a load (given a test string of several shots) is most often expressed in Extreme Spread of velocity and Standard Deviation of the combined averages of velocity. SD tells us what a load may do but ES tells us what that load did do. Since I am interested in what the load did do I compared the ES consistency of the loads with the pressure. In graph #2 the loads of the 10 and 12" twists all had ESs of 50 fps or less. That is pretty good consistency given the spread of the loads velocities of 1700 fps through 2500 fps. The 14” twist had some early problems with the powder burning efficiently. We see the ES for the 2nd and 3rd test loads was considerably higher than the same loads in the 10 and 12” twists. That accounts 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 gives us is the time pressure curve of the same loads in the different twists. Obviously the curves are pretty close together and linear. Thus the time pressure curve or acceleration is very close to the same for each rifle.
Next let us consider the question; if the time pressure curves are the same then any deformation to the bullet due to acceleration will be close to the same. Thus if the deformation to each bullet is the same at the same rate of acceleration then any change to the form of the bullet will result in a change to the Ballistic Coefficient. Following that then won’t any changes to the BC be the same for each twist since any deformation of the bullet should be the same? To find the answer to that question we merely compare the BCs of the 3 different twists as the velocity increases (hence the acceleration increases and deformation of the bullet increases). Graph #3 provides the comparison of the BCs vs the velocities of each load in each twist. Let us remember that the BC in this case is a measured BC from the actual flight of the bullets not a guestimated one from some chart. These actual BCs measured the bullets ability to fly through the air efficiently. The higher the BC the less deformed and more stabilized the bullet was. It is readily apparent that the BCs stayed pretty much the same for all three twists during acceleration at all velocities and pressures. It is interesting to note that the BCs of the bullets 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 some on this forum. The BCs from the bullets from all three twists stayed very close together and linear across the wide spectrum of velocity (acceleration) from 1700 to 2500 fps which obviously shows the acceleration remained constant regardless of the twist of the barrel.
So this is what we now know now about the same loads in the 3 different twists; the time pressure curve is the same, the acceleration is the same and the BCs remain the same. Let’s now take a look at the results on target. After all what we are looking at in conducting this test is the accuracy at higher velocity and why that accuracy goes bad. Graph #4 shows us the group sizes vs pressure. Whoa there! Something is amiss….if the time pressure curves are the same, the acceleration the same and the BCs are the same; then if the groups get larger as we increase velocity shouldn’t the groups get larger by proportionally the same amount? [Note; by “proportional amount” is an amount to compare the accuracy of each twist to each other. The proportional amount of increase is found by dividing the increased group size by the smallest group with each rifle.] However, what we see is that the groups do not get proportionally larger as velocity increases. The inaccuracy of the 10” twist increases 5.38 while the inaccuracy of the 12” twist increases 3.14 and 14” twist increases 2.08. Hmmmmmm……pressure curve is the same, deformation of the bullet from acceleration is the same then why doesn’t inaccuracy increase the same? Especially since graph #4 shows the group size vs pressure. But wait…there’s more (sorry, just couldn’t resist!). Doesn’t every one say that it is pressure that destroys accuracy? We do see that accuracy with all three twists is decreasing with the increase of pressure. If pressure was the only reason for the decrease in inaccuracy then the inaccuracy should be proportional and it isn’t. We also see inaccuracy increases much more with the 10” twist than either the 12 or 14” twists. We also see the 12” twist’s inaccuracy to increase more rapidly than the 14”s inaccuracy. Again, if it was pressure that increased the inaccuracy then why doesn’t the inaccuracy of all three twists increase equally as the pressure increases? It seems there is something other 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 accuracy to velocity. Something wrong here again….that dreadful 10” twist is once again being more inaccurate by a greater proportional amount than either the 12 or 14” twists. How can this be? We know the acceleration is the same; the BCs are the same so the deformation of the bullet is the same yet the 10” twists inaccuracy is disproportional to the 12 and 14” twists. It should be the same amount of inaccuracy, right? The lines for each twist should be linear right? Yet the proportional inaccuracy are not the same between the twists nor are the lines linear. Have we missed something? Is there another game afoot? We’ve a good handle on the internal ballistics. We know about the terminal ballistics as the groups are self revealing. But have we really looked hard at the external ballistics (the bullets flight)? We know the bullets are stable, we know the BCs are getting smaller telling us there is some deformation from the acceleration. We know the 10” twist had the highest BC at the highest pressure and velocity so why isn’t it as accurate as the 12 and 14” twists?
Let us look at graph #6. It is a comparison of group sizes vs RPM. Note the very, very obvious adverse affect that the increasing RPM has on the accuracy of the 10” twist. That red line really climbs up there! Also note that area of RPM where the majority of accurate groups fall; it is in or below the RPM threshold. Also note that in or at the top end of the RPM threshold is where accuracy begins to deteriorate.
The tests with H4831SC seemed to be headed the same way but were inconclusive as top velocity was only 2287 fps with 100% loading density. The 10” twist velocity was 1928 fps through 2287 fps with groups running from 2.4” to 3.3”. RPM was 138,900 to 164,700. Conversely the 14” twist went from 1906 fps to 2265 fps. Groups ran .95” to 2.2”. RPM was 98,000 to 116,600. The highest peak pressure was 39,600 psi.M43. Thus I couldn’t get into a high enough pressure/RPM range with all three twists to make any comparison.
I am not going to conclude that there is an RPM threshold as the test is not complete. I shall wait until I conclude the test before giving a firm conclusion.
Chapter 3 will be to see how I can improve on and perhaps push the threshold with 311291 in all 3 twists. I have some cast of linotype (that’s pretty hard stuff). I have them weighed to a +/- ½ gr. I weighed them “dressed” for summer (that’s with just the GC on, not fully “dressed” with lube too). I plan on using Varget and RL15. Probably won’t get around to testing those until May.
Larry Gibson
To be continued:
