RPM Threshold

[felix]

It might be best to set a delimiter, such as an "x" inch group. So, what is the top speed where that group size is still guaranteed intact? Must choose a group size that all contested guns can do at some minimum velocity, range. Shorter the range the better, to eliminate the environment as much as is possible. Prolly no more than 80-100 yards, eh? I feel there is no real need to show targets in this instance because only honest folks will be participating. ... felix

[FISH4BUGS]

As a machine gun shooter, RPM means rounds per minute. The ONLY factor to consider!

[felix]

Agree whole heartedly when fishing for bugs. I went fishing once for sunfish, and brought up a damn submarine. ... felix

[joeb33050]

Bass

The question here is rather straight forward;

Why is it the average reloader when he gets a regular mould from Lyman, RCBS, Lee or Saeco for his rifle can get accuracy at a certain velocity yet loses accuracy above that velocity?

With the 30-06 for expample he gets good accuracy in the 1700-1900 fps range but above that he gets poor accuracy. He casts the bullets of WWs or an alloy like #2 as mentioned in Lyman's or RCBS cast bullet manuals and uses normal loading techniques as per the manuals. I believe the reason is that above a certain threshold of RPM (the threshold is a range of velocity of 125-140,000 RPM not a specific "limit") accuracy deteriorates with such cast bullet loads as the increased RPM accentuates the inbalances in the bullet (caused during casting, by poor fit, excessive obturation during accelleration, etc.). The increased RPM causes the yaw, pitch and wobble to be increased and accuracy suffers as the higher the velocity the higher the RPM.

I'm attempting to answer that question; is that too hard to understand? I think it is not as you've already conceded that RPM does have an adverse effect on accuracy of cast bullets; " So your test to find best "average accuracy" with what's "reasonably" available off the shelf or steps that are taken is fairly useless. If you want to prove "easiest accuracy" that a slower twist is more .... "idiot proof" and minimizes errors as a reloader and shooter of cast, save your components, I'll give you that." BTW; the use of the slower twist is to keep the RPM under the threshold while attaining a higher velocity. Our example shooter is not going to get another barrel for his favorite '06, he wants it to shoot as is.

Now as to the rest of your last post; yes you can get around the RPM threshold by getting "anal". There are a number of things one CAN do, but the average cast bullet shooter isn't going to do them. He neither has the equipment, the knowledge nor the desire. He just wants to cast bullets, load them in the rifle that he has and go shoot them. He is not going to get a different rifle, new molds, a lot more casting equipment or make some exotic alloy. He just wants to know why his bullets won't shoot accurately above 1900 fps or so out of his '06. And by just casting and shooting regular bullets he is not going to get accuracy above the threshold, now is he. The reason he won't get accuracy above a certain velocity with his 10" twist '06 (or any other rifle of 12" twist or faster) is EXACTLY what we are talking about, isn't it?

The question isn't about getting anal is it, it is about why normal cast bullet loads almost all cast bullet shooters use don't shoot as accuately above a certain velocity as they do below at or below that velocity. THAT is the question I am preparing to answer by shooting over 500 rounds, using 2 different cast bullets, 3 different powders (4895, RL15, H4831SC) loaded in 1 gr increments to give velocties from 1800 through 2500+ fps with each powder and bullet. Identical loads will be fired through 3 accurate rifles of .308 Winchester and each 5 shot string of a particular load will be chronographed and the group measured. Each through a rifle with a with a different twist; 10, 12 and 14".

The test IS NOT to show which rifle is more accurate but at what RPM accuracy deteriorates. Thus the test I am doing is to show at what RPM accuracy deteriorates in each of the 3 rifles with the 2 different bullets loaded over 3 different powders. Given the different twists of the rifles the RPM will be different for each rifle at any given velocity. If accuracy deteriorates in the 10" twist at 2000 fps but doesn't in the 12 and 14" twists then that is an indicator that RPM is the culprit. If at 2200 fps accuracy is still worse in the 10" twist and deteriorates in the 12' twist also but doesn't in the 14" twist then that is even more of an indicator that RPM is the culprit. Then if at 2400 fps accuracy deteriorates in the 14" barrel and we compare the RPM when accuracy deteriorated in all three twist barrels and find that it is close to the same then that would be a good bet that RPM was the real culprit.

So Bass, if you insist my test is "fairly useless" then I challenge you to come up with a comprehensive comparable test to show us how "the HV guys are changing the stereotype." Please SHOW me/us how your '06 holds accuracy from 1800 fps through 2600+ fps and that best accuracy DOES NOT come in the RPM threshold with that 154 gr LBT bullet but at the higher velocity/RPM of 2500+ fps. The choice of alloy, lube, size, barrel bedding or any of the other "anal" things you want to do are ok with me. Pictures of targets and chronograph results please. Shouldn't take more than 10 five shot strings to cover 1800 to 2600+ fps. The guantlet is at your feet. All you have to do is pick it up.

Nothing like a friendly little shoot out, eh?

Larry Gibson

I believe that "guantlet" is more properly spelled "gonetlet". Or perhaps "gontlet". Could it be "gonetlette"? Never mind.
joe b.

[Larry Gibson]

felix

I disagree. If we were testing to see which load was the "most accurate" that might be a good idea. However, what we are looking for here is the accuracy capability of a certain twist over a broad range of velocity. Lets' take the '06 with 10" twist as an example. Let's say we work loads up in 1 gr increments each with 5 shots. The velocity and group sizes look like this;

fps group (inches)

1797 1.2
1869 1.2
1948 2
1981 2.8
2051 4.1
2102 2.4
2183 3.1
2283 3.4
2392 3.7
2438 4.3
2496 5.7
2592 8

What we see is very obvious (these are actual velocities and group sizes fired in a 30-06 with Bass's LBT bullet over increasing charges of 4895 BTW). As velocity increased so did accelleration. As the velocity increased so did the RPM. You can see from the increasing group size that somethig is causing the inaccuracy. I believe this is evidence that RPM is accentuating the defects/imbalances in the bullet. The defects in the bullets may have also increased because of acceleration and increased presure. What ever the reason for the defects it is the RPM that makes accentuates them decreasing accuracy.

The following is using Bass's LBT bullet over RL19;

2431 3.3
2493 3.4
2526 3.5
2588 5.3
2614 4.1
2660 5.25
2724 14

We see here that the slower burning RL19 gave slightly better accuracy through 2526 fps. But the point is; the best accuracy with that bullet in a 10" twist barrel was down at 1790-1950 fps. That is right smack dab in the RPM threshold.

That is why I'm only looking for the RPM when accuracy goes south.

Larry Gibson

[Bullshop]

You guys have got me wondering how all this applies to a boolit in a sabot. My tuned in load with the 06 has a 55 gn boolit doing 4100 fps from a 1/10" twist. How many rpm's is that? In developing the load it went to 4300 fps max velocity according to pressure indications but best accuracy was at 4100 fps comming up from the mid 30's. Thats about what you see from most any bullet load where the best is at just a bit under max, but this is still with a boolit.
So is this rpm over the threshold, and if so why does it work?
BIC/BS

[felix]

Either way looks OK to me, Larry. On the first showing, 2350 would be the cutoff. On the second, I'd say 2500, but I would reshoot the 2588 group just to see if 2600 could be obtained. ... felix

[felix]

Dan, the sabbot is the dealie that is taking on the twist function because of its outer diameter. The inner diameter object, the bullet/boolit, does not see the destructive force of the lands, and also because of the lessor diameter itself it does not have the same outward thrust from the center. Need to treat the inner object as if it were shot in a barrel made for it, such as a 224 barrel. 22s as you know can rotate much faster for the same accuracy as 30s because of the diameter. ... felix

[45 2.1]

Dan, the sabbot is the dealie that is taking on the twist function because of its outer diameter. The inner diameter object, the bullet/boolit, does not see the destructive force of the lands, and also because of the lessor diameter itself it does not have the same outward thrust from the center. Need to treat the inner object as if it were shot in a barrel made for it, such as a 224 barrel. 22s as you know can rotate much faster for the same accuracy as 30s because of the diameter. ... felix

I've been reading this thread and LMAO. Everything said is a symtom of something else. Bullshop has a boolit going so far over the RPM theory that its ridiculous. Now the theory is that its destructive forces that cause the problem. Either define what the theory says or quit trying to evolve it. Its the material strength folks. Introduce a stronger boolit or bullet and the parameters change.

[joeb33050]

The M14/Palma business seems to show that accuracy was good, velocity increased and accuracy decreased, twist was reduced and accuracy came back at the higher velocity.

The results with BASS bullets seem to show that as velocity increases beyond some point, accuracy decreases.

There's a jump here to making the RPM the cause of the accuracy decrease.

Some talk about "over stabilization", yet I've never believed it because of the good results with short bullets in 30/06 10" twist. And others.

Most of us believe that cast bullets don't shoot as accurately at high velocities. Doesn't mean that it can't be done, means that very hard bullets and tricks are required that are beyond the reach of we mere mortals.

I'm trying to think of a test that would look at RPM.
Let's say we captured one of those experimenters and got him to develop a short hard bullet
accurate load in a slow twist rifle chambered for a certain cartridge.
Then we tortured him until he made a fast twist rifle chambered for the same cartridge and shot the same load in that fast twist rifle.
If the accuracy fell apart in the fast twist rifle it would seem reasonable to suspect the RPM as the culprit.
The immediate situation is that we all have fast twist rifles in which we could try the same load, but those pesky experimenters chamber their rifles for the 30 X 47 or 30BR. How do we get the same MV ? It would be best to deal with one action, one chambering reamer/throating reamer and two barrels.
What we have is our rifles and Quickload, that could give us the charge required to duplicate the experimenter's MV.
It would also be good to make the experimenter supply us with some bullets, so that that variable is out of the equation.
I would think that if three of you tried the experimenter's bullets with a Quickload charge at ~ the same MV, a couple of five shot 100 yard groups would tell the story.

Say we could find an experimenter with, for example, a 30 X 47 rifle with a , say, 17" twist. Imagine that he used 155 grain HTWW (+1% tin) bullets lubed with LBT Blue at 2650 fps and shoot accurately.
His bullets are turning 112,235 RPM.
Say we could force him to send 25 bullets to each of three willing participants, who had rifles of 10" and 12" twist, and that we got Quickload loads for their rifles giving the same MV.
Now say that those three participants shot three five shot groups each with those loads.
2650 fps 10" twist = 190800 RPM
2650 fps 12" twist = 159000 RPM
I think that 2650 fps with a 155 grain bullet is going to require a strong 308 or 30/06 rifle.

So, does this make sense?
Where oh where could we find such an experimenter?
joe b.

[Bass Ackward]

We see here that the slower burning RL19 gave slightly better accuracy through 2526 fps. But the point is; the best accuracy with that bullet in a 10" twist barrel was down at 1790-1950 fps. That is right smack dab in the RPM threshold.

That is why I'm only looking for the RPM when accuracy goes south.

Larry Gibson

Larry,

Thank You very much. Great point, I now see your problem. You just got the diagnosis wrong cause you can't let the RPM thing go out of your head. That is my other point.

The RL19 showed you that the problem was pressure or harmonics, not RPM. Do you know how I come to that conclusion?

Because if it were RPMs, ALL powders would produce the same size groups at the same RPMs. If the lead was flying apart, then flying apart is flying apart. All bullet designs would shoot the same size groups. All weight bullets would have the same HV potential.

There it is. RPM is twist related and NOT powder dependent, but pressure is.

What that experiment showed you is EXACTLY what I told you a while back. Now maybe the RL19 would have still worked had you sized differently. So your problem can be alignment OR pressure. I don't know the gun. If you throat was vastly larger than .3095, you might have needed a larger bullet diameter. If your throat peaked at say .311, then you would be able to handle a .308 diameter and your results would have been different again. Or maybe you needed a slightly harder bullet. Or maybe dropping down to RL22 / 7828 and you would have been in high heaven. Or just changing your tip pressure. Or it could be any of the above.

But you don't get lucky very often with just throwing a charge and shooting the same diameter and hardness that you did down below when accuracy was easy.

If the problem really WAS RPMs, then no load related factor, no gun related factor would change the end result and all groups would be the same at the same velocity level. Make sence?

Take that 311466 and get you a can of 7828, start at 48 grains and come on up with it if you can size that BIG thing down and still have lube grooves. Other wise, I can mold you up some bullets after the holiday and you can try again. If you have to size bigger for what ever reason, your top velocity ceiling will be lower than it otherwise would be.

Or I can send you my mold if you promise to give it back in a timely manner. I am worried that you get this working and I will never see it again. I never trust SFCs cause they are always sceeming to make E-8.

[felix]

What you said, Bob, is right on! It is the total material strength and, naturally, its consistency throughout. Nothing to laugh about except our gyrations to create accurate boolits. The whole process is a joke, and that is why casting is such a great hobby. ... felix

[charger 1]

I think if mold and person doing the casting, sizing and lubing know their bees wax, which includes throats etc we have limited ourselves to lower revs than need be. Uncle Veral says so to When Douglas sent me a 24 twist rather than a 34 I told Mr Smith as the mold was being made that my twist might be to quick. His response was."If you got the correct boolit there is such a thing as to quick a twist?"

[45 2.1]

What you said, Bob, is right on! It is the total material strength and, naturally, its consistency throughout. Nothing to laugh about except our gyrations to create accurate boolits. The whole process is a joke, and that is why casting is such a great hobby. ... felix

While there are some trying this, it would be nice to remember the basic rules. We have a statement from Larry Gibson:

Woke up this morning and since the wife was sleeping in I turned on the Mag 20, put some 60/40 WW/Lino alloy in it, degreased two Lyman 2 cavity moulds (311291 and 311466), put a pot of coffee on, turned on some smooth jazz on the radio and commenced to cast 16 lbs worth of bullets.

He planned on running these from 1800 fps up to about 2600 fps. there is quite a difference in pressure range from start to end, but do we see any alloy difference or heat treatment. None that he stated. The idea that previous experimenters was to match alloy strength to pressure. NOT being done here and he expects results, (or to prove his point). This is why i'm laughing. If he is serious about proving RPM theory to a reasonable conclusion, he shouldn't break the basic rules. From an alloy thats strong enough to one that is seriously deficient (in strength and alloy constituents) for the pressure level involved. Veral Smith wrote some books on this for the uninformed. I'm sure that something will come back about this.

[Bullshop]

Felix
OK I see its not just RPM then its also caliber related. So then the smaller the caliber the higher the rpm limit, and the larger the caliber the lower the rpm limit, if the boolit material is the same, is that it?
In that case the 6.5 cals should handle a higher rpm than the 30 cals. I am remembering when the cruise missle came out for the 6.5's and everybody was having fitts trying to get it over about 1700 fps and boolits were wizzing off to never never land.
Seems looking back thats what started the first big debate about rpm threshold. My friend Starmetal was in the thick of it and defending a position against it. He had me send him some 72gn .225" boolits to try in 7" twist. The boolits were acww but should have been harder even so he did perty good with them. If he had had the mold the play with there may have been a more interesting conclusion but at that time the discusion fizzled out. Perty good stuff here anyway. With all the stuff from both sides of the fence we are all learning and forming new idea's of things we just gotta try.
Blessings
BIC/BS

[felix]

That is all correct what you said, Dan! Keep in mind, though, and, as Starmetal Joe knows, the grooves have to be cleaner the further up the RPM ladder we go. The boolit has to have a firm grip on the lands throughout all speed ranges for any RPM "proof/spoof" to be genuine. This is why your plastic sabot does so well when compared to lead from shot to shot. So, keeping the barrel clean is a major problem here, but it has to be taken into account for realistic measurements as well. Nobody, but nobody, is going to clean a gun between shots. Nobody would shoot for fun otherwise. ... felix

[Bass Ackward]

You guys have got me wondering how all this applies to a boolit in a sabot. My tuned in load with the 06 has a 55 gn boolit doing 4100 fps from a 1/10" twist. How many rpm's is that? In developing the load it went to 4300 fps max velocity according to pressure indications but best accuracy was at 4100 fps comming up from the mid 30's. Thats about what you see from most any bullet load where the best is at just a bit under max, but this is still with a boolit.
So is this rpm over the threshold, and if so why does it work?
BIC/BS

Dan,

The RPMs of your sabot are 295,000.

It works because the sabot feels all the pressure and not the lead. You bullet is not deformed from pressure so it flies well and you thus have the flexibility to match accuracy to the harmonic sweet spot at 4100 fps.

And what kind of accuracy are you getting at this point?

[felix]

All true. ... felix

[Larry Gibson]

Bass

"Because if it were RPMs, ALL powders would produce the same size groups at the same RPMs. If the lead was flying apart, then flying apart is flying apart. All bullet designs would shoot the same size groups. All weight bullets would have the same HV potential."

You miss the point. First of all lets take a regular cast bullet that you or I cast. Is it perfectly balanced ? No it is not. If it were perfect and was not further damaged in the barrel from the rifling and obturation due to accelleration then every bullet would fly straight and true into the same hole. It is the imperfections in balance of the bullet that cause inaccuracy. I will grant you that barrel harmonics plays a part also but I talking strickly the bullet here; perfect bullets would fly true, imbalanced bullets do not. It is RPM during flight that causes these imbalances to adversely effects accuracy. Yes, different pressures and time/pressure curves cause barrel harmonics. The example given with your bullet and load is not the only combination that this effect of RPM adversely effecting accuracy occurs in. It occurs with EVERY medium and slow burning powder bullet combination that I've ever shot. And that sir has been a lot of combinations over the years.

Now that being said let us move on. If you notice my test is set up for 3 different powders, one of which is your previously stated powder to use. Also notice I am using 3 different rifles with barrels of different manufacture and contour. I surmise the harmonics for each is going to be different. Wouldn't you agree? However, I will do as you suggest and get some 7828 (a different powder than previously suggested as best) and start at 48 gr (is that for a .308 or '06?) and work up with 311466. There is still plenty of lube groove and lube left after sizing to .311 and .309 (.311 is on the left, .309 on the right - picture is of culled out bullets to be used for sighters). No need to send me the mold, at this time anyway.

Speaking of your LBT bullet; in both the .308 and the '06 your LBT bullet shot it's best accuracy down in the RPM threshold with 3 different powders. There was no doubt some different barrel harmonics between the different powders and the 2 different barrels there. But the results were the same; best accuracy was ALWAYS within the RPM threshold.

Now so what if the barrel harmonics are different. What do I mean? Lets say that we load different powders over a broad spectrum of pressure from 1700 fps to 2600 fps in the 10" twist '06. Using 4895 best accuracy is 1" with RL19 it is 1.3", with H4831SC it is 2.2" and with 7828 it is 1.9" moa at 100 yards. If we were comparing "best accuracies" we would assume the 4895 load to be the "most accurate". This is all regardless of velocity differences or RPM but just comparing the "accuracy" of the different loads. But what if we only compare the "best accuracy" velocity of one powder in one barrel to the velocity levels of the lessor accurate loads in that barrel with that powder. Would that not then be a valid comparison?

Thus we are not going to compare "best accuracy" between different powders but to determine the velocity (RPM) at which best accuracy is in each barrel with each powder. The different harmonics created by different powders in different barrels is then negated as the size of the group is not important, only how that size relates to the other groups fired with that one powder in that one barrel. In other words; if all the powders produce the "best accuracy" (regardless of the "best acuracy" group size disparity for the individual rifles) in or under the RPM threshold then the RPM threshold will be substantiated. Again, since each powder in each rifle is only being evaluted against itself, barrel harmonics are negated. The size of the group is imaterial as it is the trend in the sizes of the groups that is being measured.

If all of that is to confusing then let's wait and see what the test results are.

Larry Gibson

[Larry Gibson]

45 2.1

Laugh all you want but it is you who fail to understand the question. The question is this as previously stated;

"The question here is rather straight forward;

Why is it the average reloader when he gets a regular mould from Lyman, RCBS, Lee or Saeco for his rifle can get accuracy at a certain velocity yet loses accuracy above that velocity?

With the 30-06 for expample he gets good accuracy in the 1700-1900 fps range but above that he gets poor accuracy. He casts the bullets of WWs or an alloy like #2 as mentioned in Lyman's or RCBS cast bullet manuals and uses normal loading techniques as per the manuals. "

I'm not breaking any basic rules, I'm answering the question. Besides, many here say to shoot them soft at HV and many say to shoot them hard at HV. So what is the "rule". Lyman shows 2800+ fps in the .308 with 311466 and makes no mention of using a harder bullet than #2 alloy for that. Our intreped reloader reads that and thinks he should be able to do it accurately. But you bring up a point; so perhaps chapter two of the test will be to use WQ's WSS and linotype cast bullets and shoot them with the loads where accuracy falls off to see if accuracy can be sustained at/to a higher velocity/RPM.

I say that the best accuracy with cast bullets will come in or below the RPM threshold (regardless of alloy) and am conducting a comprehesive test to demonstrate that. Now within the context of your experience show me one of your loads that shoots its best accuracy above the RPM threshold. Kindly don't tell me what I should do (as Bass did with the gauntlet) but show me what you CAN do. The gauntlet has been passed to you.

Larry Gibson