PSI or C.P.U.

[ColColt]

Is there a correlation between C.U.P. and psi? The reason I asked is that Speer's manual doesn't indicate pressure in psi but rather C.U.P. and if your trying to see what a given load will yield, pressure wise, you know what BHN to match that pressure. I haven't been able to fix a formula or table to cross reference psi and C.P.U.

One of my best groups has been 4.5 gr of 231 with a 158 gr SWC. Speer's manual indicates a velocity of 935 fps with a max load of 4.7 gr so, I figure with a four inch GP100 I'm right about 900 fps. Unfortunately, Speer doesn't give pressure figures in their manual so, I went to the Lyman Cast Bullet Handbook which doesn't give many 38+P loads. The closest I found was 4.0 gr of 231 with a velocity of 837 fps and pressure of 15,900 C.U.P. I don't know how that translates into psi but figure it must be maybe around 13,000 psi...just a guess. Therefore, I can only conclude that cast bullets that will probably do their best with this load/velocity would have a BHN around 10-11. Am I way off base here? Lyman's data was taken using a 4" barrel and Speer's a 6" barrel.

[357shooter]

I don't believe there is a reliable way to translate CUP and PSI. While they both measure pressure, they do it in a completly different way. That's why there's not bunch of tables available to do the conversion.

There is a calculation (found it via Google, but don't have it anymore) that is supposedly an approximation. It didn't seem to work very well.

Sounds like your attempting pressure matching for accuracy in a revolver. The best I can tell, it isn't helpful and doesn't work. I know this for sure in 357 magnums, and speculate it's true in other handgun calibers too.

I was going to point you to some test results that I posted, however looks like they are a mess...

My preferred alloy for 357 Magnum is BHN 8 of softer. Inlcuding Magnum loads. It would be convenient if an algorithm worked, matching alloy with pressure doesn't work. Again, 357 Magnums is what I'm referring to.

Added later: Tweaked the web page with some test results: http://357shooter.blogspot.com/2010/12/handgun-only-pressure-and-alloy.html

[Larry Gibson]

No, there is no direct correlation between psi as measured with piezo-eltronic sensors and CUP as measured with a copper crusher. Some have tried to make a case for correlation but across the wide range of pressures of cartridges it is not valid and can lead to trouble very quickly if used.

Also not in times passed many CUP measurements were also listed as "psi". Some where in most manuals it will tell what they use. SAAMI and CIP now list both figures for most of the older cartridges and mostly just psi for the newer cartridges.

Larry Gibson

[Wayne Smith]

Nope. No consistent or predictable relationship, or this would have been done years ago.

[danski26]

I hate to disagree, however there is a correlation between the two systems. It is complicated and is not all that practicle, however it does exist. Last months handloader magazine made mention of it. The "word" on the street is that they do not correlate, I believe in an effort to keep people from believing ther is a DIRECT correlation, however that is just my speculation. It is actually very difficult, statisticly to NOT correlate in anything. It is just the degree of "accurate" correlation.

This is the best explanation of the formula that I have seen.

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

[danski26]

This is the formula:

ANSI PSI = -17902 + 1.51586 x ANSI CUP

[357shooter]

That's the same article I found a couple of years ago, and the same calculation that didn't prove to be useful or accurate.

It doesn't matter much anyway as pressure matching a alloy via the Lee/style formula doesn't work in 357 magnum anyway.

[danski26]

You are correct shooter. It warns not to use it for handguns however does not explain the problem with the formula.

[462]

I, too, tried that formula and, in one instance, the result was a negative number. I would caution against using it, and forget that it even exists.

Col. Colt, a Google search should come up with formulas for boolit BHN using PSI and CUP, and Lee's hardness tester kit gives pressures for each reading. I've found them all to be next to worthless and do not use them. My guns tell me what they like.

Others may have experienced different results.

[Larry Gibson]

This is the formula:

ANSI PSI = -17902 + 1.51586 x ANSI CUP

That formula will work only for a norrow band of cartridges in the middle of the psi scale of useful pressures. At the bottom and top ends it is very troublesome resulting in negative numbers as mentioned on the bottom end and to low psi figures on the high end. Since most cast bullet load psi's are on the low end I stand by "the word on the street" to not use it. My measurement of psi's via an Oehler M43 verify this. No correlation that is reliable, hence don't use this formula.

Larry Gibson

[HangFireW8]

Here's how I've figured it so far: for 45/70, CUP and PSI are the same thing. That is not a surprise, as the US CUP system was first calculated and devised for the 45/70, and as far as that goes, they got it exactly right.

Later they simply started using the same pressure testing hardware on more modern chambered barrels (30 US Army, 30'03, 30'06), and still called it PSI- for a while. Around WWII they figured out that it just wasn't so, CUP no longer equaled PSI on modern bottleneck cartridges. So they changed the labeling. That doesn't change the fact that it's still the same for 45/70.

I haven't got an exact formula, but the short version is: the more bottleneck and/or overbore the cartridge, and the slower the powder, the more CUP diverges from PSI. Why? It's a matter of the difference between peak pressure and sustained pressure above the deformation pressure for the copper pellet.

If a high pressure cartridge sustains a pressure above the deformation pressure of the copper pellet, it continues to crush it, leaving the same height as a more brief peak pressure. Different peak pressures, different sustained pressures, no way to tell them apart.

Bottleneck cartridges with slow powders are more likely to sustain a high pressure while avoiding extreme peak pressures. Thus, when measured with more modern PSI equipment they diverge more from CUP peak ratings.

[runfiverun]

p.s.i. is done with a transducer.
and the c.u.p. is done with a copper pellet.
when the transducer come along it measured pressure much more accuraely, and many of the loads in the manuals changed because of it.

[mpmarty]

This confusion between CUP, LUP and PSI is what causes most arguments about 5.56 and 7.62 Nato vs. .223 and .308. Most military rounds still refer to pressure in Copper Units of Pressure and commercial SAAMI standards are usually in PSI.

[Doc Highwall]

The CUP fails at low pressure and the lower you go the more inaccurate it is because it has to not only crush the copper cylinder but it also has to pierce the brass cartridge case. For shotguns that operate at lower pressures then handguns they have to use lead because it will deform with greater accuracy at lower pressures. The newer systems that use a transducer I am sure will be the mainstay for newer loadings and cartridges but would be prohibitively expensive to redo the last 70+ years of reloading manuals. There is enough problems with using different components from different lots and in different guns with chamber and barrel dimensions that pressure cannot be correlated from one gun to another and are unique especially at max pressures for which it was obtained from. The reloading manuals are guides at best and indicate trends of what the pressures might be.

[onondaga]

One system measures average pressure and the other system measures peak pressure, that is why the conversion formulas are only accurate to plus or minus 3,000 units for safety. You can't win here, there is no direct relation. Disagreement is futile, Physics is Physics and not an opinion. The formulas are designed to provide a margin of safety in conversions because there is no direct conversion

Considering the relatively large numbers in pressure values, the +_ 3,000 is a small amount. I personally use +_ 4,000 units as the error factor with the formulas and never use them anywhere near maximum load calculations.

I'd really recommend only using data that has pressure listed in PSI for cast bullet loads because with plain base boolits, plus 3000 units is a big number when you are within 10% of your alloy ultimate compressive strength and that plus 3000 most likely will cause gas jetting and trash your accuracy hopes with plain base cast boolits.

The Lee system of matching alloy strength to load pressure for predicting an accuracy sweet spot with plain base cast boolits has a much larger flaw than comparing PSI to CPU . It is the skills of the caster and loader to get a quality bullet to fit the firearm bore correctly. Lee assumes good boolits with no plus minus error factor. This leaves a significant amount of shooters out of the fold and complaining that the Lee system doesn't work and it is nonsense.



Gary

[dale2242]

Check this link......dale

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

[357shooter]

One system measures average pressure and the other system measures peak pressure, that is why the conversion formulas are only accurate to plus or minus 3,000 units for safety. You can't win here, there is no direct relation. Disagreement is futile, Physics is Physics and not an opinion. The formulas are designed to provide a margin of safety in conversions because there is no direct conversion

Considering the relatively large numbers in pressure values, the +_ 3,000 is a small amount. I personally use +_ 4,000 units as the error factor with the formulas and never use them anywhere near maximum load calculations.

I'd really recommend only using data that has pressure listed in PSI for cast bullet loads because with plain base boolits, plus 3000 units is a big number when you are within 10% of your alloy ultimate compressive strength and that plus 3000 most likely will cause gas jetting and trash your accuracy hopes with plain base cast boolits.

The Lee system of matching alloy strength to load pressure for predicting an accuracy sweet spot with plain base cast boolits has a much larger flaw than comparing PSI to CPU . It is the skills of the caster and loader to get a quality bullet to fit the firearm bore correctly. Lee assumes good boolits with no plus minus error factor. This leaves a significant amount of shooters out of the fold and complaining that the Lee system doesn't work and it is nonsense.



GaryFYI, the OP is about a 357 magnum revolver. The thread has drifted a bit into general discussion on CUP/PSI conversion and general feedback on alloy / pressure matching. Sometimes that gets confusing, at least to me.

Regarding you comment on the Lee system assuming good bullets, are you talking rifles or revolvers or both? I believe the Lee system was developed using rifles only, according to the Lee book. He suggested it may work for handguns and stopped with just that thought.

However:

The test results I've gotten in a 357 Magnum show it flat doesn't work, for 357 magnum. I qualify the caliber because I haven't tested other revolver / handgun calibers.

I'm trying to clarify the various posts that sometimes are specific to the OP as opposed to feedback that is much more general and addresses more than the GP100/357. It's all good, but confusing.

Does anyone disagree that CUP to PSI conversion for the OP / 357 Magnum doesn't work?

Does anyone disagree that alloy / pressure matching in the OP / 357 is a waste of time?

[44MAG#1]

My lastest Speer book show PSI for the 38 special and the 357 loads. What Speer book are you talking about. 17000 psi for standard 38 Special, 20000 psi for +P 38 Special loads and 35000 psi for 357 Mag.
Speer also uses CUP for some cartridges like the 357 sillouette (sp) loads at 45000 cup.

[462]

Check this link......dale

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

That is the formula that can produce negative numbers, as stated in a number of previous replies.

[Rocky Raab]

Most if not all labs are abandoning the copper crusher system. Crusher results are not only limited and misleading, but using the system is more time-consuming and subject to subjective interpretation. Piezo and strain gauge system are simply better in all respects.

When you see CUP numbers in load manuals, it is due to one or more of several reasons: The lab is simply reprinting old data. (Not every load in a new manual is re-tested, you know.) Or, the lab has a copper crusher barrel for that cartridge and it is not economically feasible to upgrade it because the round isn't that popular. Or, they simply haven't replaced that barrel yet even though they intend to.

Or, in the case of the Lee manual, they do NO testing whatever and simply copy data from other sources - including both CUP and PSI numbers. (Typos, too! But that's another thing altogether.)

[onondaga]

Rifles only with the Lee system with the caveat you mentioned by Lee on handguns is correct for the pressure to alloy matching theory of Lee. Sorry about the subject drift.

Gary

[onondaga]

Good job on the homework---that regression analysis study is where the comparison comes from.

Gary

[357shooter]

Rifles only with the Lee system with the caveat you mentioned by Lee on handguns is correct for the pressure to alloy matching theory of Lee. Sorry about the subject drift.

GaryThanks for the help, sometimes I get confused by the large body of knowlege and information from all the folks here.

[ColColt]

What I had in mind was primarily for the 38/357 caliber but also, 45ACP since these are the three I shoot most. I do know that if you alloy a bullet too soft there can be leading/pressure problems but conversely, the same can be said of hard bullets. So, I was making an attempt to pit bullet weight and hardness(softness) with velocity and pressure to assure a good seal, great accuracy and safe pressures in whatever velocity range. that's it in a nutshell.

Hence, my dilemma about the CUP VS psi calculations. I've sen in various reloading manuals to include the Lyman Cast Bullet Handbook. There has to be a certain pressure best for a given bullet weight and hardness that will perform best regardless of caliber...IMHO. The 38 and 38+P have pressure limits of 17,000 and 20,000 psi respectively while the 357 is more akin to 35,000. Knowing the pressure limitations of each there has to be a bullet weight, velocity range and hardness that will give that caliber it's best accuracy. This is what I was endeavoring to find out. The problem is, I wasn't sure about where I'd be pressure wise with a certain combination of the above. In other words, if the bullet hardness matches the chamber pressure a good seal will exist between the bore and bullet which is what you want for less leading and better accuracy. I wanted to know where I'd be with less than maximum pressures/velocity. I suppose with a 38+p load maximum pressure is 17,000(about). So, a bullet with a BHN of 12 should prove the best but, what if I'm not loading a maximum velocity load with that bullet? Same for the 357.

BTW-the Speer manual I had in reference to is the #14 manual.

[mpmarty]

And the speer manual is primarily for jacketed projectiles so your concerns are moot as far as the speer book goes. Since you don't appear to have any means to actually measure the pressure of your reloads I fail to see how you can hope for an answer.

[357shooter]

What I had in mind was primarily for the 38/357 caliber but also, 45ACP since these are the three I shoot most. I do know that if you alloy a bullet too soft there can be leading/pressure problems but conversely, the same can be said of hard bullets. So, I was making an attempt to pit bullet weight and hardness(softness) with velocity and pressure to assure a good seal, great accuracy and safe pressures in whatever velocity range. that's it in a nutshell.

Hence, my dilemma about the CUP VS psi calculations. I've sen in various reloading manuals to include the Lyman Cast Bullet Handbook. There has to be a certain pressure best for a given bullet weight and hardness that will perform best regardless of caliber...IMHO. The 38 and 38+P have pressure limits of 17,000 and 20,000 psi respectively while the 357 is more akin to 35,000. Knowing the pressure limitations of each there has to be a bullet weight, velocity range and hardness that will give that caliber it's best accuracy. This is what I was endeavoring to find out. The problem is, I wasn't sure about where I'd be pressure wise with a certain combination of the above. In other words, if the bullet hardness matches the chamber pressure a good seal will exist between the bore and bullet which is what you want for less leading and better accuracy. I wanted to know where I'd be with less than maximum pressures/velocity. I suppose with a 38+p load maximum pressure is 17,000(about). So, a bullet with a BHN of 12 should prove the best but, what if I'm not loading a maximum velocity load with that bullet? Same for the 357.

BTW-the Speer manual I had in reference to is the #14 manual.I prefer BHN 6-8 for 357 magnums and 38 specials. From plinker loads all the way to 34,000PSI loads. I can shoot them all day long with no leading. With BHN 6. It does take cast-friendly-powder. But even something like 700X, a fast burner, works great. Others that work well are HP-38, Unique, 2400, H110.

BHN 12 will shoot fine too, but you don't need it.

My points are:

-The calculations for maximum pressure are useless

-The calculations for the ideal match and best accuracy are useless

Not trying to be blunt, but after all the posts that stated the formulas don't work you still seem to be thinking you need to match your alloy and pressure. It's a waste of time, take it from someone that tried it. Just cast some bullets and test various loads and see what works best for you.

In your 357 and 45acp you can shoot any lead hardness if the bullets are sized to fit your guns. In 357 magnum you may get the best accuracy from soft bullets, only trying it will tell. 45acp may work out the same.

Undersized bullets can cause problems, with the softies being the most forgiving and the hardcast the least forgiving. The solution is to find the size your gun likes and forget the matching exercise, it becomes frustrating when it doesn't work.

[USSR]

There has to be a certain pressure best for a given bullet weight and hardness that will perform best regardless of caliber...

Seems like a lot of folks are getting all wrapped up about this pressure/hardness relationship. It is not a narrow band, where if you are a thou or two over/under, then things go all to hell.

Don

[462]

"-The calculations for maximum pressure are useless

-The calculations for the ideal match and best accuracy are useless"

That about sums it up...it's all a waste of time.

[ColColt]

And the speer manual is primarily for jacketed projectiles so your concerns are moot as far as the speer book goes. Since you don't appear to have any means to actually measure the pressure of your reloads I fail to see how you can hope for an answer.

The Speer manual has loads listed for the 158 gr LSWC which was what I really had in reference to. If I had a means to measure pressure this thread wouldn't have been started, would it?

Sorry I even brought this up-won't happen again.

[Rocky Raab]

Not entirely your fault. Some of these "experts" just love to argue their pet ideas -- and will do so at the drop of a wheelweight.

[357shooter]

The Speer manual has loads listed for the 158 gr LSWC which was what I really had in reference to. If I had a means to measure pressure this thread wouldn't have been started, would it?

Sorry I even brought this up-won't happen again.It's a good question, one that comes up often. There's enough literature out there that eveyone, at some point, pursues alloy & pressure matching. At least to some degree.

In rifle's some folks say it has proven useful, many say otherwise. In handguns, it isn't useful.

It is true that sometimes increasing a load will resolve a leading issue. Sometimes not. Adjusting to resolve a sizing or other problem doesn't validate the pressure / alloy matching approach for either 1) obtaining accuracy or to 2)determine the maxiumum load a alloy can handle.