[rob42049]

Here's a thumbnail guide to SAAMI max average pressure for most of the chamberings in common use. Make a copy of this for your reference. If you have additions, please add them below. Thanks. Safety first!

 

[bgavin]

There is no 1:1 correlation between PSI and CUP...
However, the SAAMI publications often cite both measurement values.

For each specific cartridge, the ratio varies.
For example, the 243 Winchester is 60,000 PSI and 52,000 CUP.
The PSI/CUP ratio is 1.153846

For this cartridge, those interested in converting CUP to PSI can multiply CUP * 1.153846 to find PSI.
This is approximate, but probably much closer than a SWAG.

SAAMI does not publish both numbers for all cartridges.

 

[rob42049]

There is no 1:1 correlation between PSI and CUP...
However, the SAAMI publications often cite both measurement values.

For each specific cartridge, the ratio varies.
For example, the 243 Winchester is 60,000 PSI and 52,000 CUP.
The PSI/CUP ratio is 1.153846

For this cartridge, those interested in converting CUP to PSI can multiply CUP * 1.153846 to find PSI.
This is approximate, but probably much closer than a SWAG.

SAAMI does not publish both numbers for all cartridges.

There is absolutely no correlation between CUP and PSI. None at all. People have tried and failed. It just doesn't work, there's too many variables with case volume and barrel volume. The reason you see CUP listings is because the cartridge in question is not popular enough, does not generate enough sales to justify the expense of testing with a piezo-electric setup. Each cartridge tested on piezo-electric equipment costs quite a lot, to where a lab won't do it if not worth the cost or laboratory time. DO NOT, repeat DO NOT try to interpret PSI pressure by looking at the CUP reading, you may get in trouble. Trust lab info that has been tested.

Way back when there was LUP, lead units of pressure. They used a lead cylinder in the crusher fitting. This was replaced by copper crush fittings early in the 20th Century as the new smokeless powder became standard.

 

[Lever Jac]

Rob thanks for the info.

 

[bgavin]

Walk through this a minute.

SAAMI posts two values: a max pressure for CUP, a max pressure for PSI.
Both of these values reflect a maximum pressure from different measurement systems.

Any load value under either maximum will not be in violation of being over pressure.
As long as your load is under either max, it won't blow up your gun.

Nobody is telling anybody to go rogue and push their loads above either limit.
Using the SAAMI numbers for the 243 Winchester, 60k PSI and 52k CUP are the maximums.

Let's work a load from Lyman #50
Sierra #1530 85gr, 38 grains of IMR 4350, CUP stated at 36,900.
This CUP is a long way under the 52,000 CUP maximum SAAMI pressure.

Say the loader is curious and wants to know the ballpark PSI..
Using the SAAMI max pressure numbers, the PSI/CUP ratio is 1.15384600
36,900 * 1.15384600 = 42,577 PSI... well under the SAAMI 60k maximum.

At no time is this published load going to be dangerous because the guy converted CUP to PSI.
Both are far under the SAAMI limits.

 

[Mike Britton]

so this chart implies 28,000# for .45/70. Is that within reasonable operating pressures for the last group of trapdoors? I believe the 1886 models?

 

[kingstrider]

That chart makes me want to buy a 454 Blackhawk I have been eyeing.

 

[Rollingblock]

so this chart implies 28,000# for .45/70. Is that within reasonable operating pressures for the last group of trapdoors? I believe the 1886 models?

No one can answer that because no one agrees. Loading Manual maximums for Trapdoors:
Lyman - 18,000 cup
Speer - 21,000 cup
Hornady - 25,000 cup
Hodgdon - 28,000 cup
Factory loads - 18,000 cup (reported)


Note that for the .45-70, 28,000 cup = 28,000 psi. Perfect correlation, the .45-70 is special!

 

[35remington]

It is also a mistake to assume the 52K CUP figure for the 17 Rem is he same pressure level the 307/356 Winchester’s are loaded to despite the fact they are both also listed as 52 K CUP.

In actual use they are more in the vicinity of 15,000 PSI apart.....despite having the same figure for both. The 17 runs at higher pressure.

What CUP means for many cartridges is that they are loading the cartridge to a safe level but it isn’t directly related to anything. Comparing identical CUP numbers is often a mistake.

And yes......forget conversion factors from one to the other. Try applying it to the 35 Remington or 45-70. The 35 Remington is backwards and the conversion factoring can’t account for it. Attempting to do so should have been recognized by those formulating a conversion factor as a waste of time from the get go.

A failure to look at information first caused that mistake.

 

[rob42049]

I posted that chart for reference. Everyone should make a copy for their files. The inventors of Piezo-electric measuring systems, the makers of the systems, powder distributors laboratories and bullet makers laboratories caution against trying to correlate CUP with PSI. DO NOT DO IT.

If somebody on the Internet wants to multiply/divide/add/subtract numbers, more power to him. It's not even safe to suggest it, new reloaders may take the misguided effort for fact and get in trouble. I'll stick with ballistic labs' and manufacturers' recommendations. As always, safety first.