Cast boolits pressure limits

[gumbo333]

I'm not a caster but I do shoot lots of purchased boolits. My question is how do you fellows figure the max pressure limits? Such as: in the 'Lee loading manuals in the Brinell Hardness & Max Pressure Chart' (chapter 10) it lists the various brinell hardness ratings and the max pressure that it should be loaded/ shot at, stated in PSI's. For example a boolit with a BHN of 15 would be a tick over 19000 psi, or boolits with a BHN of 10 would be just a tick over 14000 psi. Now put that in real life, using the Lyman Cast Bullet Handbook (4th Edition) for let's say or a 30/30 for all the various boolits weights of 150, 170 & 173, all the pressure ratings are given in CUP. I've heard there is no direct conversion from PSI to CUP, or visa-versa, so how do you fellows determine if your loading for a given powder like IMR 3031 or IMR 4198 (or any other powder) is under, near or exceeds that max pressure limit? I certainly hope my phrasing of this question is not so convoluted you can't understand it!

[bmortell]

I haven't found any of that pressure stuff usefull. I find 10 hardness works fine for everything I do, which is always 40k or less but nothing like what charts say

[Winger Ed.]

Interesting question.

Since there isn't a direct formula,
they'd probably have to use lab. equipment to arrive at the answer by measuring both separately.

I never thought about it much myself.
I just go by trusted books, and let it go at that for working up loads.

[dbosman]

Pressure limits and maximum velocity are two issues I never pay any attention to. If the bullet gets where I want it to go and do what I want it to do, I'm happy.

[JBinMN]

I have provided an answer to this before & links to where such things have been posted for some time & have had at least one member, Larry Gibson, post that the numbers for such pressure "limits" are incorrect due to testing that he has done with cast boolits & the pressure testing equipment he has, like an Oehler M43.

I would suggest contacting him thru PM if you are in a hurry, or perhaps wait a bit & maybe he will see this topic & reply on his own.

( Here is a PM link to L. Gibson here in the forum to save you from searching for it if you wish to contact him thru PM:
http://castboolits.gunloads.com/priv...do=newpm&u=696

I will refrain from posting anything further until he either chimes in here, or you say you have contacted him about such things. I am still researching such info & cannot sayfor certain any thing I personally have found that would help any more than what I have offered before in other topics.

There are other topis here at CB.GL forum where this has been discussed & I do not wish to engage in any dispute until I have gathered more info than I have to date.

While L. Gibson is not the only one who might have opinions & post them here, so far , I think L. Gibson is the one who has provided the most info about documentation on pressure testing that has been offered here in this forum.

G'Luck!

[charlie b]

It sounds like you are worried about the pressure limits of the rifle. I don't think there is anything to worry about in that arena. If I understand the context of the pressure information it is about proper upsetting of the bullet in the bore.

You might also want to read this stuff:
http://www.lasc.us/Fryxell_Book_Contents.htm

[JonB_in_Glencoe]

I am unfamiliar with Lee's pressure chart, but the examples you give appear to be Minimum (for proper obturation) and NOT Maximum.

I prefer the info at this link because it is referencing the alloy and not just BHN. BHN alone doesn't really define the strength of a alloy. Scroll half way down the page to "Approximate "Maximum" Chamber Pressure For Lead Alloys (PSI)"
http://www.lasc.us/CastBulletNotes.htm

For our purposes, there is a conversion formula that gets us 'close enough'.
I only use these formulas for converting pressures higher than 25Kpsi or 30Kcup.

PSI = -17902 + (1.516 x CUP)
or
CUP = (PSI + 17902) / 1.516

https://www.shootingsoftware.com/ftp/psicuparticle2.pdf

[agentwolf]

It sounds like you are worried about the pressure limits of the rifle. I don't think there is anything to worry about in that arena. If I understand the context of the pressure information it is about proper upsetting of the bullet in the bore.

You might also want to read this stuff:
http://www.lasc.us/Fryxell_Book_Contents.htm

Not sure if you were aware it was available for download in .pdf format here. It was referenced in another thread...
From Ingot to Target: A Cast Bullet Guide for Handgunners

[Larry Gibson]

There are serious flaws with the conversion formula posted above. It is not reliable. Denton made too many incorrect assumptions and used data from different sources to come up with that formula. There is no reliable conversion formula between CUP and psi. One can make a close presumption IF one has the CUP and psi data of one load taken at the same time. That does not mean the pressure data printed by "close" loads in different manuals or from different sources will give you reliable information/data for any sort of conversion of CUP to psi or visa versa. One can quickly get into dangerous pressure range using that formula interpolating data from various sources. I suggest it not be used. My comments here are derived from what most all ballisticians say who actually measure pressure and my own experience having measured the pressure of thousands of different loads in a multitude of cartridges.

The basic problem with Lee's formula is two fold. First he was seeking to answer the age old question of why accuracy went south at a certain level of velocity/pressure. The solution he came up with is only partially correct in a round about way. What he missed in his theory was/is the adverse affect of RPM at a certain level, especially with cast bullets. That is evidenced in his manual where Lee misinterprets the results "John" got in a test with a 10" twist 30-06 and one of Lee's cast bullets. The reason accuracy in the test was lost where it was was not because of too much psi but because of too much RPM.

Secondly the "constant" in his formula is based on the elastic limit/yield strength/tensile strength of pure lead for simplicity because every binary/ternary alloy, depending on the percentage of metals, will have a different "constant". Since he/we don't exactly know what the elastic limit/yield strength/tensile strength is of the alloy we are using Lee used the one for lead by default. Thus using that constant for pure lead gives the lowest psi where the bullet will supposedly "fail" and accuracy is lost. Since binary and ternary alloys are "harder (they basically have better grain structure which strengthens the alloy) they can stand a much higher psi depending on the strength of the alloy. That is the primary reason Lee's formula does not work most often; the constant is incorrect as binary and ternary alloys stand up to much higher psi than the formula indicates. Like many things though, the formula, does seem to work once in a while which gives some a false sense of reliability in it.

As a quick example; I regularly shoot bullets cast of #2 alloy to 2900 fps in my 30x60 XCB holding consistent moa accuracy to 600 yards and to 2700 fps with 1 1/2" moa accuracy in my 308W Palma rifle. The loads run a measured 50,000 psi +/- in each rifle as measured with the Oehler M43 PBL. According to Lee's formula that bullet cast of #2 alloy is supposed to "fail" with accuracy lost at 30 some thousand psi yet that does not happen. So how is it possible I'm shooting them with excellent accuracy at high velocity with 50,000 psi? The reason is both barrels have slower twists which keep the RPM under 140,000...... that's where Lee went wrong, he didn't understand that many cast bullets are much more adversely affected by RPM than they are of psi. I can give you many, many more examples that disprove Lee's formula not only my own but what many others have also found.

With that being said, don't get me wrong. I like Lee's products and certainly respect the man. I have and use quite a bit of Lee's products. However, Lee's max pressure formula is a dog that just doesn't hunt.

[gumbo333]

Thank you all and thank you Larry Gibson. I am not worried about pressure in any of my loads but I was certainly curious as to why accuracy went south as velocity increased with certain purchased cast boolits after I figured out size. Mostly I wanted a better understanding as to why. Also if there really was a way to corelate PSI to CUP pressure data. I see there really is not. Thanks so much Larry about a better understanding of twist/ rpm using softer to harder BHN boolits. I have lots of data manuals, my hobby of reading them just because I like to, but many are limited on cast data.You guys have a wealth of great knowledge here. Just wish I had discovered shooting cast lead at a much younger age. I'm having a ton of fun with mouse fart loads in great 45/70 and the great 30/30. Thanks again everyone.

[MT Gianni]

Assume that most of the box of bought bullets might be the same alloy. Do not assume that all other bullets are in any other design.

[gumbo333]

Thank you. No I do not assume that. Usually the places I buy boolits from state the alloy mix and BHN. Several offer a few different alloy mixes and bhn's for different applications. It is sometimes a bit difficult to find a softer alloy/ BHN suitable for hunting. Lately I have been using mostly Hytek and PC'd boolits just because! The information I've been given on this thread far exceeds anything that I thought I might be given. This bunch of people on this sight is simply amazing, so knowledgeable and so willing to share and help others. My question about pressure limits and BHN isn't anything I will possibly ever encounter, but something I just wanted to know, just because. I found it odd that most cast boolits data in manuals was in CUP and Lee's pressure limits were in PSI, (and I figured his pressure data was likely pretty correct) and how you could change from one to another. I had read many times for J bullets that there was no way change CUP to PSI's. Now I'm much much more informed. I had never given a moments thought about RPM's and different alloys elasticity. Larry Gibson should get an award of some sort! For me the information I received just today from so many unselfish shooters was/ is simply outstanding. Again, thank you all. gumbo

[44magLeo]

I the Lee book page 136, they show an example of there testing.
As it starts wit a low pressure load and work up you see the accuracy gets better. Then at a point just below the psi of the bullet is the best group. It then opens up once you get above that pressure.
If you use this as a guide line I think you will get good results.
As in start low, work up. And as in the example the group gets better then opens up. Then go back to the best group.
This is pretty much how we do it any way.
I think Lee made some good observations on bullet performance. It may not agree with what others say or not. The strength of a bullet is how we determine what velocity we can achieve. It has been this way from the start.
I think the rate of spin may play a role in cast accuracy. Just as it did with early jacketed bullets.
On an unbalanced bullet the rate of spin does have a dramatic effect on accuracy,
The faster the unbalanced bullet spins the less the accuracy.
In the early jacketed bullets they often had balance issues. They found that keeping the rate of spin just fast enough to stabilize the bullets worked.
That's one reason light bullets for a caliber back then could not be driven as fast as modern bullets. The rate of spin caused them to over expand on impact with game.
Even with modern bullets this happens. Take 22 caliber bullets. Hornady makes bullets that look alike on the out side but have different internal structure.
They make one for low velocity cartridges like the 22 Hornet. They make a bullet built a bit tougher for the higher velocity cartridges like the 22-250 and 220 Swift.
Load them wrong you won't get what you think you should.
Same with cast.
Load them wrong and it don't work so well.
Yes I have read through the links listed. I even have them bookmarked for easy reference.
I think they just get to the same place as Lee but by different routes.
Leo

[Larry Gibson]

"I(n) the Lee book page 136, they show an example of there testing.
As it starts wit a low pressure load and work up you see the accuracy gets better. Then at a point just below the psi of the bullet is the best group. It then opens up once you get above that pressure."

Lee's friend John (who conducted that test according to Lee) used Linotype alloyed bullets and achieved the best accuracy at 24,838 psi (assuming that to be a calculated psi as CUP, transducers nor strain gauge measurements are not that finite). Yet I, and numerous others, push softer alloyed cast bullets to 3,000 +/- fps at 50,000 +/- psi (actually measured, not calculated). Lee's formula and book says that should not be possible as the bullet should have "failed" because of Lee's calculated "compressive strength" of the softer alloy we use.

Again, as stated earlier, Lee failed to consider the adverse affect of RPM and the fact that a constant for pure lead gives a much lower value to the actual psi a ternary alloy can withstand. Those of us who do shoot ternary alloyed cast bullets at high velocity in the 2400 - 3000+ fps range understand that controlling the RPM via slower twist barrels is the secret along with properly designed cast bullets cast of excellent quality. The level of psi we push to far exceeds what the formula says is possible or Lee's further explanation on page 136.

Fact is Lee doesn't get to the same place because the psi of the load was only partially responsible for the loss of accuracy at the velocity attained in John's test. John pushed the RPM threshold by using a well designed bullet and probably a medium or slower burning powder. Lee doesn't say which of his bullets John used but we do know that the C309-160-R and the C312-160-2R shoot very well at higher velocity as they have attributes of design for that level. Those two bullets along with the C312-185-2R have successfully been pushed much higher in 12 - 14" twist barrels also at much higher psi than at the point of "compressive strength" that Lee's formula says where they should fail.

Lee's formula follows the old assumptions as to why accuracy is lost at a point with cast bullets. We now know those assumptions were not correct.

[charlie b]

Larry, is your ternary alloy the copper-tin-zinc or is it something else?

As an engineer I looked into this kind of thing years (decades) ago with a couple of bench rest shooters. Part of the investigation was on lube, alloy hardness and leading as related to velocity. It seemed to us that leading was more related to how hard the bullet was accelerated in the radial direction by the twist rather than outright muzzle velocity. When the bullets could be pushed faster we also discovered the problems of too fast a twist with respect to lead tensile strength. With 'normal' lead alloys there was a 'sweet spot' where the twist was fast enough to stabilize the bullet but not too fast to make the bullet come apart. They ended up settling on 200gn bullets with 14 twist barrels at around 2200fps (if I remember this all right). I left the area as one was going to try a twist rate in the 1-20 range, a lighter bullet, and much higher velocity but don't know if he ever did anything like that.

[Larry Gibson]

My ternary alloy is Lyman #2 [90-5-5] alloy. After considerable testing of different alloys in several different bullet designs and in 10, 12 and 14" twists barrels (all 308W) I found Lyman #2 alloy to give the best in casting quality and accuracy at HV. I also conducted an extensive test of many lubes (commercial and home made....supplied by others) and found Javelina and White Label 2500+ gave the best accuracy at high velocity....2600+ fps. After success with the 14" twist I had the 30x60 XCB rifle built with a 1-16" twist to attain 3000+ fps with a 164 gr 30 XCB bullet.

The "sweet spot" I found and others found is to keep the RPM under 140,000. I have never lost accuracy to any cast bullet coming apart from too much spin because accuracy is lost well before that when to cast bullet exceeds it's RPM Threshold. I have many times demonstrated and explained how accuracy is lost when the bullet exceeds the RPM Threshold. Using large targets at 200 and 300 yards to capture the non-linear expansion (read that loss of accuracy) it is found the bullets are still stable and have not come apart. That is because the bullet holes were all observed/accounted for so all bullets made it to the target and all the bullet holes were perfectly round of caliber size demonstrating the bullets were perfectly stable.

Granted "tensile strength" or "compressive strength" of the alloy does come into play somewhat and helps establish the RPM Threshold for the bullet/alloy and load used. However, what was proven conclusively was that when the RPM was kept under 140,000 the bullets "tensile strength" or "compressive strength" was never an issue even upwards of 50,000+ psi. I have demonstrated that by shooting many 10 shot moa groups out to 300 yards and even out to 500 and 600 yards with the 14" twist rifle at 2700 fps and the 16" twist rifle at 2900+ fps.

Also the original test objective was to see how fast a ternary alloyed cast bullet could shot and maintain 2 moa or less accuracy with linear group dispersion to 300 yards. That accuracy goal was exceeded with moa accuracy being consistent with 10. 12, 14 and 16" twist rifles. All loaded to 48 - 50,000 psi. Of course the highest velocity with 1 - 1/12 moa accuracy was dependent on keeping the RPM under control. I have also demonstrated with a 10" twist 308W how the RPM Threshold can be pushed up and still maintain 2 moa accuracy at 500 yards. None of those accomplishments would have been possible had we adhered to Lee's formula.

[charlie b]

So the tensile strength, which is what is dictating the 140,000 rpm limit, seems to be the sweet spot limit. And your results seem to mimic what we were seeing.

And, no, the bullet does not come apart at that point, it just stretches a little, which is also supported with your data. This is where the alloy was critical as a bit too much antimony and the bullets did come apart.

I also agree that chamber pressure had little to do with all this. We did discuss what effect a larger pressure 'spike' would have. Conjecture was a too large pressure spike would upset a bore riding bullet too much, but, we never explored it.

Thanks, that helps expand my 'database' a lot.

[Larry Gibson]

"So the tensile strength, which is what is dictating the 140,000 rpm limit, seems to be the sweet spot limit. And your results seem to mimic what we were seeing."

That is not a correct assumption as there is absolutely no evidence of the bullet stretching a little in inspecting recovered bullets or by careful measurement of the diameter of bullet holes. I think it would be a difficult leap to assume the "stretch is going to occur absolutely evenly around the circumference of the bullet. I f the stretch occurs in one location on the bullet then the bullet becomes even more imbalanced. If the bullet did stretch or swell, even a little, then the accuracy would be lost anytime a cast bullet goes above your presumed 140,000 RPM "limit".

While the centrifugal force of RPM can and does spin some bullets apart spinning bullets apart has nothing to do with the RPM threshold which occurs at a much lower RPM than is required to spin the bullet apart or even upset/stretch the bullet. The RPM threshold occurs at a point when;

the bullet is unbalanced or becomes unbalanced due to obturation in the bore during acceleration. The unbalanced bullet is forced to conform while in the barrel and its center of mass is revolving around its geometric center. When the bullet is free of the barrel's constraint, it will move in the direction that its mass center had at the point of release. After exiting the muzzle, the geometric center will begin to revolve about the center of mass and it will depart at an angle to the bore (line of departure). At 54,000 RPM to 250,000 RPM, depending on velocity and twist, the centrifugal force acting upon any imbalance in the bullet can be tremendous. It will result in an outward or radial acceleration from the intended flight path (line of departure) and will try to get the bullet to rotate in a constantly growing helix.

"Try to" are the key words here as there are things we do (slower burning powders, harder alloys, better designed bullets, perfect fit, etc.) that we do to push the RPM threshold upward. Conversely, using a faster powder, softer alloy, no GC, etc) lowers the RPM threshold. When the bullet goes beyond the RPM threshold it does not lose "stability”. It still is flying point forward. Its flight path simply becomes a larger expanding helical one. This is why when the RPM threshold is exceeded the groups expansion as range increases is non linear.

With cast bullets the RPM threshold will be exceeded long before the centrifugal force is enough to "spin the bullets apart". Exceeding the RPM threshold becomes apparent by the decrease in accuracy as velocity increases and the non linear expansion of group size as range increases

Thus if the RPM threshold (normally in the area of 120 - 140,000 RPM....where is dependent on numerous factors) is crossed accuracy suffers, particularly in a linear expansion of the group size as the range increases. However, as previously discussed in numerous other threads on the topic the RPM threshold can be moved up or down by varying several factors. I have thoroughly demonstrated this numerous times using a 10" twist .308W rifle, with 3 different bullet designs suitable for the purpose using #2 alloy and using slow burning powders. Pushing those bullets to 2200 - 2300 fps at 45 - 48,000 psi) at 167 - 178,000 RPM they have held 2 moa accuracy (just about the accuracy level the rifle is capable of) or less to 500 yards.

When using a correct powder for the velocity level desired there is not any evidence of any "pressure spikes". That is based on measuring and observing the time/pressure curve of thousands of test rounds at such velocity/pressure levels.

[charlie b]

I basically agree with all. 140,000 is a number dependent on several things including alloy and bullet shape. I would expect it to vary a lot.

As to bullets stretching, of course it could be happening and might or might not affect accuracy, but, probably will and not in a linear manner since the expansion probably will not be 'exact'. It would only take a few thousandths to cause an off balance condition. Bullets 'off center' (CG not on the axis) will spiral, depending on how far off the CG is and how much the aerodynamics of the bullet is 'fighting' the issue, hence the 'non-linear' part of this. Similar to the old issue of bullets seeming not to be stable at close range but remaining stable at longer ranges.

One of these days I'll get the 'bug' and calculate the tensile stresses due to rotation. Until then I'll just go shooting

[gumbo333]

Wow, I'll say it again, their is a bunch of really, really smart fellows on this forum. Really smart. I have learned a ton, hope many others have also. I thought Lee probably knew what he was putting in his manual. Found out otherwise. I wanted to know if there was a way to get from PSI to CUP, knowing there likely wasn't. And that turned into this fantastic discussion and learning curve, yep, about lead! I think you are all great. Thanks again for being so unselfish. gumbo