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Discussion Starter #1
How do you correlate CUP to PSI? I have reloading data from Lyman that lists 40-65 Winchester loads in CUP. But, it lists 45-70 Gov't loads in PSI.

Why?

Both listings are for smokeless powder.
 

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Don, if what I think I know is correct and what I think I remember reading, there is no consistent formula for conversion or comparison chart. I would suspect the 45-70 has had more pressure work done at a later date than the 40-65. The 45-70 is vastly more popular and has a lot more components to choose from and 3 load levels. The 40-65 is best described as having a niche' market.

I think the changeover was over a period of time and not by a specific date. Well, I guess it had to be. There's a pile of cartridges for which the pressure data would have to be reworked and a lot of them have never been....and probably never will be reworked.
 

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Yeah, look up “CUP vs PSI,” start reading and your head will spin even IF you are an engineer! Bottom line there is no usable “conversion” like Fahrenheit to Celsius.

The problem is there is no standard, it all depends on what equipment the ammo or gun manufacturer used to measure, and they don’t actually measure the exact same thing!

I have no idea why Saami and others sometimes use one or the other, and it’s annoying as heck when the reloading chart sometimes from the same component manufacturer switches back and forth in the same chart!

I think it’s a conspiracy to force us to “start with 10% reduced load and work up” etc, etc. just to get us to waste primers powder and buddets getting to where we want to go because we can’t decipher their posted pressures🤨
 

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I think SAAMI and CIP use what they have or can readily get. For them to accurately change from CUP to PSI they would have to have a pressure barrel chambered in every cartridge in their book(s). That's a lot of money, plus the labor and materials to do and record the testing. The less common cartridges are going to get the short end of the stick simply for that reason...they're less popular. Don mentioned the 40-65. Unless your a BPCR shooter, how many people do you know who shoot that cartridge? I have an old manual that has pressure data, in CUP, for the 33 Winchester, for powders no longer available, except for IMR-3031. I doubt Hornady, Lyman, Sierra or any of the others are going to jump right on getting new PSI data. Especially since none of them have made a suitable bullet for that cartridge in 30 or more years and even then, only Hornady made one.

polish has a valid point in that there is no standard, no government proof house, thank the Lord, as in Europe, Britain and elsewhere. We do have SAAMI, as mentioned and they do set the standards in the US as CIP does in Europe. But I'm afraid the disparity between CUP and PSI is something we're just going to have to live with, certainly for the remainder of my life and probably forever with older, obsolete, less popular cartridges.
 

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Discussion Starter #5
The Lyman manual lists the 40-65 pressure at ~18000 cup. I'm guessing psi could be anywhere from 20000-26000 psi. What do yall think?
 

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If I remember right, that's in the Trapdoor, BP range, depending on the bullet and, I think the Trapdoor max's out at 28K CUP. Your number seems close to me, depending on the bullet weight...and I'm definitely guessing.

As best I remember the 40-65 was first chambered in the '86 Winchester. So it stands to reason that whatever that rifle is rated for, the 40-65 isn't any higher. I think it was also chambered in the 1885 Winchester but that's a different animal.
 
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Discussion Starter #7
The 18000 cup range from the manual was with smokeless loads.
 

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Yes sir, but CUP is CUP, whether black or smokeless, same as PSI is PSI, no matter how it was derived. CUP, as with PSI, does not take into consideration the pressure curve.
 
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Discussion Starter #9
Let's take this discussion here to see my concerns
 

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If you look at the old Lee manuals you may see 25 loads for one cartridge and one under the other some in CUP, some in PSI, it is confusing. In some calibers they are close to the same in others not. I think you just have to blow it off and just start below max and work up. I have always preferred PSI because that is what I memorized starting in 1970s when I started loading.

The only people I know who got into trouble did so because of basic errors, wrong powder, double loads, or some really dumb stuff like experimental dual power loads. In high school my buddy tried to make a 22 Mag from a 22 LR. He simply changed out the powder and fired it in a single shot rifle. It took off part of his thumb which was good, because the thumb stopped it from hitting his eye.

He also modified some 12 gauge loads. I recall shooting a jack rabbit at 80 yards with one. Later we learned the equivalent loads would be about 5 drams of powder. Surprising either of us is alive. Actually, I have not checked on him in years, so maybe not.

To me PSI CUP simply does not matter except when I develop loads, I look at the charts and try to use the lower pressure loads if they give similar velocities.
 

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Discussion Starter #11
But pressure does matter whether measured in cup or psi. When trying to determine whether an action can handle this or that much pressure it is important to recognize how much pressure a load will generate without showing signs of excessive pressure.
 

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I looked it up years ago but lost my info so I checked again and shootingsoftware.com pretty much agrees with you. The formula they put forward is: PSI = -17,902 + 1.516 x CUP . If you work it out by the rules, multiply then addition of the negative you come up with 27,288 PSI. So 20,000 - 26,000 is right where they place it.
 
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