Cup to PSI conversion

[gamestalker]

One of the current loads published by Hogdon for .357 mag indicates a load using 296 / H110 for 158 gr. XTP as having a range of 28,600 to 40,800 cup, or some where there about. I was unable to locate any other current data that has been retested using a transducer, that is published using both formats to compare with. This is more out of curiosity than for any other purpose, as my loads with 296 / H110 have been well established and current.

GS

 

[KansasSasquatch]

Are you wondering what the CUP pressure is for that load? If I had to guess, based on Hodgson's data I would say it ranges from 23,300PSI to 33,100 PSI. Max PSI for .357 mag is listed as 35,000PSI and I believe max CUP pressure for it is 43-44,000CUP. Again, just a guess.

 

[Jesse Heywood]

There is no conversion. There is too much difference in data collected. Best you can do is guestimate, which will bite you in the rear. Always consider the PSI load to be superior.

 

[Bart B.]

SAAMI's document:

http://www.saami.org/specifications_and_information/publications/download/205.pdf

... lists CUP and PSI numbers for handgun cartridges. Look up the data for the .357 Mag then check them out.

Further reading explains and shows how both are measured. Which may help explain why all the different means of doing it don't always end up with the same numbers.

 

[Clark]

What appears to be the case is heavy duty 38 Specials were over loaded to the point of sticky cases, and then backed off a margin to keep cases from sticking. They called it 357 magnum.
So they measured the pressure, and they registered that pressure.
Years went by.
Thin walled 357 mag revolvers were built.
The stretchy steel allowed the brass to get sticky at lower pressures.
The 357 mag pressures were then lowered so cases will not stick.
I have posted this before:
http://www.thehighroad.org/showthread.php?t=691185&page=2
It comes from a 1993 post by John Berkovitz
https://groups.google.com/forum/?hl=en&fromgroups#!msg/rec.guns/S_dalM1NJe0/cBSU4bR2jz8J

I own a 357 mag that gets sticky cases below the SAAMI registered pressures, so I have to down load it.
I also own a 38 special rifle with strong extractor [not a star extractor that extracts 6 cases at once with finger tip force]. I have shot double the powder charge of a 357 mag max load in that rifle.

 

[gamestalker]

Thanks Bart B., that's exactly what I wanted to know.

GS

 

[Hondo 60]

There is NO cup->PSI formula (or vice versa).
One does not equate to the other.

 

[kelbro]

CUP pressures on a revolver are quite accurate.

 

[Bart B.]

Kelbro, I don't know how accurate a revolver cylinder would measure a given load by a CUP system because of the difficulty of putting a hole in one and mounting all the extra hardware to do so. Nor do I know of any such system available.

That's not the standard SAAMI's set in place for testing ammo used in revolvers. Read the following starting with page 58 on velocity and pressure testing:

http://www.saami.org/specifications_and_information/publications/download/205.pdf

Such equipment shown in SAAMI's document, CUP or PSI systems, is not possible for use on revolver cylinders nor semiauto pistol barrels, either, in my opinion.

For everyone, here's a link to someone's belief and proof CUP numbers can be converted to PSI ones:

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

Ain't math just plain wonderful???????