Is there a conversion factor to convert from CUP to psi? What is the max pressure in psi for the 270, 30-06, and 300 Winchester Magnum?

short answer is no. art alphin devotes a whole chapter to this topic in his A-Squared reloading manual. going off the top of my head (i.e. memory which ain't that great any more) in lower pressure loads cup will be higher than psi, there'll come a point at which they're the same and then the trend will go the other direction. again, going off memory, is a function of pressure, pressure curve, burn rate, etc. too many variables for a simple multiplier. which means that crossover point floats per cartridge, load, powder, ad nauseum. by the time it gets to the high end, it "SEEMS" that 50cup loads run around 60ksi when you can find corresponding data. but even then, usually from 2 different sources, 2 different pressure bbls, different components to some extent, you see the problem.

i know what you're getting at as i hate that too. and reloading manual creators depending on the age of their equipment will mix and match throughout the range of ctgs for which they furnish data. makes it real tough to compare say 300winmag in psi w/ 30-338 in cup.

long answer is no...

And the medium answe is that, if you take CUP maxes and PSI maxes for SAAMI cartridges, there's a strong corrolation to the number 86%.

The Accurate Powder website lists the 30-06 as having a saami max of 60 kpsi and 50,000 C.U.P. The Win Mag Saami max is 64 kpsi and 54,000 C.U.P. The 270 (which we know is a deritive of the 30-06 case) is 65 kpsi and 52,000 C.U.P. This is what is posted on the website, if I am wrong, somebody please correct me.

no, w/o looking i'm sure you're not wrong. and at the top end that trend is pretty consistent w/ in a few %. the problem at least as explained in a-squared manual - and explained very well - is that trend doesn't hold for different pressure levels. drop to 45ksi or 40 or 35 or what have you and you get totally different relationships. for a normal modern saami spec load the 50-60 or 10ksi difference is likely going to be pretty similar. but that's coincidence and is a function of similar circumstances and won't be repeatable or proportionate thru-out the full pressure range of all the different ctgs.

Good God!! Here we go again, eh??

This ol' hoss is DEAD! MUERTO!! TODT! KAPUT!!

Thanks for the information.

http://stevespages.com/saamipressure.html

and

http://stevespages.com/crusher.html

No! There are "conversions" made but they are simply rough estimates.

The CUP system operates in the plastic range of material deformation. The PSI system in the elastic. A difficult problem to solve to begin with. Now add to that the fact that the CUP system is operating in the plastic range of copper while the PSI is operating in the elastic range of steel, and you have a NEARLY impossible problem to solve!

You can use the maximum allowable CUP and PSI values to get a ratio, IF your loads are operating at those pressure levels.

Basically, the difference gets bigger and bigger as the pressures increase. With PSI reading higher (and more accurately to reality I might add). CUP is outdated and really, IMHO, should be scrapped. When CUP and LUP were created when there were no such things as strain gages.


ASS_CLOWN

[sigh]

Yes, there is a conversion, and it works rather well.

Dr. Lloyd Brownell, University of Michigan, and author of Firearms Pressure Factors, published the curve on page 35, figure 14. He couldn't do that if there were no conversion.

The new RCBS software will let you switch between PSI and CUP. You can't do that if there is no conversion factor.

The European equivalent of ANSI/SAAMI, CIP, publishes both CUP and PSI numbers, but they actually measure only PSI, and calculate CUP from it. You can't do that if there is no conversion factor.

You can read my own humble analysis of actual data at http://www.shootingsoftware.com/tech.htm. Before some of you start bellyaching that it can't be so, or that it's not a perfect conversion, or that SAAMI says otherwise, read the dang article.

The bad news is that the measurement systems used to measure PSI and CUP aren't very good. With CUP, the same tech, on the same day, with the same barrel and lot of ammunition does well to repeat a 10-shot average measurement within 1,000 PSI. Because of the inherent random error in both systems, NO MEASUREMENT WILL REPEAT EXACTLY, except by very good luck. Why do you think no two reloading manuals agree? The conversion formula is as accurate as the mediocre measurement systems used will allow it to be. If the systems are good enough to produce reloading data, the conversion is good enough to convert the measurements.

If you have instrumented a barrel with both a copper crusher, and a piezoelectric transducer, and want to predict one reading from the other, you will reliably end up within 6,000 PSI. If you want to convert a spec from one system to another, you get the advantage of averaging down the random error, and you'll be within 1-2,000 PSI. And that is about as good as the measurement systems are, anyway.

Results apply only to rifles, over the range 28 KPSI to 65 KPSI.

I think Denton's argument rings true. For what it's worth, I estimate psi (P) from CUP (C) using

P = C * ( 1 + C*C / 14000 )

On my to-do list for this month is to find a better fit to the (limited) data I have for CUP and psi. The above conversion was done by hand with a calculator, and I think I can do a bit better with my computer.

quote:

Originally posted by elkhunter:
Is there a conversion factor to convert from CUP to psi? ...

Hey Elkhunter, If there was a conversion that was reliable, it would be in the various Reloading Manuals.

quote:

Hey Elkhunter, If there was a conversion that was reliable, it would be in the various Reloading Manuals.

Not necessarily.

For decades, the reloading manuals incorrectly called PSI and CUP the same thing. It wasn't until the early '60's that they fixed that blooper.

For those who don't want to read the article, the conversion is

ANSI/SAAMI PSI = -17,902 + 1.51 x ANSI/SAAMI CUP

There is good reason to believe that the true conversion has some curvature, and this is a straight-line approximation. The approximation holds pretty well from 28,000 PSI to 65,000 PSI.

asdf's approximation may also work pretty well.

Thanks, everyone, for the answers. I learned something here.

quote:

Originally posted by gixxer:
The Accurate Powder website lists the 30-06 as having a saami max of 60 kpsi and 50,000 C.U.P. The Win Mag Saami max is 64 kpsi and 54,000 C.U.P. The 270 (which we know is a deritive of the 30-06 case) is 65 kpsi and 52,000 C.U.P. This is what is posted on the website, if I am wrong, somebody please correct me.

I would like to know, if 30/06 is 60Kpsi, how
come the 270 can be 65kpsi? Same case and rifles almost. Surely this saami man and the
cart. manuf. don't get togeather and the manuf.
say "we would like to get some more performance for our new wizz-bang cart. how about we up the pressure"??
JL.

JAL, the story I hear on that issue is that the 30-06 still has some older, weaker actions out there. The 270 does not.

This thread prompted me to read Denton's article on CUP vs. PSI. The article made perfect sense and is impossible to argue with. In the article, Denton emphasizes that his correlation may only be valid for rifles, and above 28,000 psi.

Then I got out my old Lyman CB manual which lists pressures in CUP. If I try to apply the correlation to low pressure handgun cartridges like the 38 special -- and Denton has warned that the correlation may not be valid in this range -- then the linear correlation falls apart and gives really goofy numbers, like 16,000 CUP = 6000 psi... NOT!

So even though there SHOULD be a good correlation between CUP and PSI, and there MAY in fact be a linear correlation for most cartridges, and perhaps with enough data one could find a reliable correlation for all cartridges, I still have to conclude that it is better to put CUP behind us and use modern PSI data, unless you are stuck on a deserted island with only a 50 year old reloading manual to go by.

The problem with a straight line fit becomes obvious at the low pressures, and Denton has warned of that. His article, along with some numbers from a book by a Lloyd Brownell (which were posted here about a year ago) lead me to try a curved line to fit the data. It works pretty well over the whole range, but it is off at the upper end of the data from Brownell. Maybe I'll work on this tonight.

Karl

quote:

Originally posted by popenmann:
This thread prompted me to read Denton's article on CUP vs. PSI. The article made perfect sense and is impossible to argue with. ...

Ah yes, written by the same person who recommended to a Beginning Reloader that he should remove "live seated primers" by inserting a sharp pointed tool into the case mouth and hammer on the anvil.

Not exactly what I said, and what I did say, I later said was wrong.

Irrelevant.