measuring pressure


February 14, 2013, 09:20 AM
Is there a formula for this? Is the pressure measured for each load? I have 2 manuals and check out the web page for the powders I use, and there is not consensus. My Speer book from 1986 is usually much more agressive, and considers anything under18,500 cup to be standard pressure for .38 special and I think 22,500 cup to be the upper +p limit. My Lyman 49th edition I don't think exceeds 18,700 C.U.P. for +p. BUT there are some standard pressure .38 special loads that exceed the top +p load that the Speer book lists. And the web page is even more conservative with the pressures.

How is this stuff measured? How significant is bullet weight in its impact on pressure? My Lyman manual lists as standard pressure max loads for 155, 158, and 160 grain cast bullets as 4.7, 4.0, and 4.9 grains of W231 respectively. Why would the 158 grain load be so much lighter? Should I weigh the slug dry, or lubed? What about a gas check design, does it change max load if used or not?

I'm asking because the 4.8 grains of W231 behind a Lee 90322 (which is advertised as 158 but weighs in at 160+) is very accurate our of my revolvers, but one of them is a Colt Cobra, is that too stiff a load for practice, or should it be ok in the aluminum framed revolver? Or should I use lighter less accurate loads as a precaution? Or is there a formula I can use to determine how much pressure I'm creating, and if so, is there a reasonable guide for use for alloy framed revolvers?

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February 14, 2013, 11:55 AM
No, there is no formula to measure pressure.
Thats what your reloading manuals are for.
They have ballistics labs and pressure test equipment to measure it to insure the load data they publish is acceptable..

SAAMI Pressure limits have been reduced again since your 1986 Speer book was published.
Current pressure limits are 17,000 PSI for .38 Spl, and 18,500 PSI for +P.

The difference in charges you mentioned is due to bullet design.
The 158 RNFP has much more actual bearing surface contacting the bore then the 155 SWC or 160 RN. That translates to more bore friction with the 158 RNFP and a lower max charge.

All Lyman's bullet weights are shown as cast from the alloy stated, without lube.
And all three in my manual are cast from Linotype, which will produce a lighter bullet then if they were cast from a softer alloy.

The three bullets when cast from wheel weights or Lyman #2 alloy might weigh 158, 161, and 163 or so cast from softer lead.

And a little more after sizing & lube.

Gas checks should never be necessary for use at .38 Spl pressure & velocity.

All of the standard pressure loads shown are standard pressure.
Thus they are all safe in your Colt.

It goes without saying if you go to the max load shown giving 16,800 PSI, they will be harder on the gun then the Starting loads giving 9,800 - 11,000 PSI.
You be the judge of that I guess.


February 14, 2013, 12:18 PM
Thanks for the reply.

Does that mean I need specific loading info for the mold design and material I use for casting, and if so, where would I find it?

February 14, 2013, 12:56 PM
If you have a Lyman manual that is as good as you are going to find.
Nobody else offers as much cast bullet data with the specific bullet shapes shown.

Just match your bullet design & weight as close as possible to their bullet designs and use the data they published for it.


February 14, 2013, 01:40 PM
Since you seem to know about this stuff, why does Lyman list the pressure in C.U.P. instead of PSI? Does C.U.P. correspond to PSI, or is it something else?

February 14, 2013, 01:47 PM
C.U.P = Copper Units of Pressure.
It was measured in a pressure barrel with a hole in the chamber.
A copper slug container was screwed in it, and then the copper slug was measured after firing to see how much shorter is was.

It was a pressure measurement system formerly used before computers and pressure transducers became widespread.

Newer PSI is pressure transducer direct measurement in Pounds per Square Inch.

There is no relationship between CUP & PSI, and no way to convert one to the other.

There is still a lot of CUP data out there, as the ballistics labs have neither the time or the money to re-shoot all the old data that has been safely used for many years.


Jesse Heywood
February 14, 2013, 02:39 PM
RC is correct on every point. The relationship between the two is a point of controversy. Some think there is a way to convert. Scientifically, the best one can do is to guesstimate. The reason is the two are completely different measuring systems.

The CUP system was only capable of giving a reading of maximum average pressure.
The PSI system provides pressure measurement on a timeline and can show the pressure at any time from when the primer is lit until the bullet is exiting the barrel. Using this data has shown pressure spikes that are beyond the CUP system made possible.

Currently the labs are busy, and with that testing cost is at a premium, so there is not a move to retest the older rounds. But that is changing, systems are now being developed that run on a PC.

February 14, 2013, 02:47 PM
That's interesting. I wish I knew more about this stuff. I see threads about this issue on this board from time to time, and I'm so poorly informed that often I am unable to follow them. Both manuals use C.U.P numbers, the new limits are based on psi (which don't correspond to C.U.P.), so I assume that it is possible that a 22,000 C.U.P. could fall within current psi limits for +p ammo. But at the same time it would be unwise to expect that to be the case. However Lyman lists pressures that don't correlate to current standards, so the level of safety still has an element of faith. It is too bad that the powder web pages (which list in psi) don't have upper or lower limit loads that correspond to the manuals to give an idea of when you might be bumping against the psi limits, and how much actual stress you are placing on the gun according to reccomended limits.

Again, thanks.

February 14, 2013, 02:50 PM
Jesse Heywood, are you saying that there will/could be systems that a handloader could hook up to a laptop to check loads in our personal guns? That would be an awesome way to measure what you are doing.

February 14, 2013, 03:10 PM
No, I don't think thats what he is saying.

At a minimum, you still need a pressure transducer attached to the firearm chamber to measure the pressure distortation. Then the PC interface & software to do anything with the reading.

The cheapest system I know of is the Oehler Research Model 83.

And it costs a whole bunch of big bills!

But it seems to me you are worrying too much about nothing.

If you follow the data in the current reloading manuals, there is no way you will exceed acceptable .38 Spl pressure limits. Whether it is measured in CUP or PSI.


February 14, 2013, 03:25 PM
there will/could be systems that a handloader could hook up to a laptop to check loads in our personal guns?

Oehler had one available a while ago.

It uses a strain gauge mounted on the barrel/receiver to measure the expansion of the metal under the pressure of the cartridge.

The strain gauge is sensitive to the micro inch change in size of the steel.

It suffers from the same calibration issues as piezo gauges though.

The main issue is how to make the same stress vs. time relationship that are carefully calibrated to a known value.

The last time I worked on the issue the labs still used a 'standard cartridge' they made up using the same lot of powder, same lot of bullets, same lot of brass, and then compared the measured pressure to what they measured before.
Without any further changes they then fire the round to be tested.

The CUP method was never sensitive to very short pressure variations, unlike the strain gauge or piezo systems (there limit is often the electronics (bandwidth) and electrical 'noise' in the system).

February 14, 2013, 03:31 PM
I probably am. But what got me questioning this was using a cast bullet design not listed in the Lyman manual. Should I go by weight? Measured or what the mold says? So I weighed it, they come in over 160 grains dry in a swc design. 158 and 160 have a .7 grain difference in max load according to Lyman. Plus I've found the 4.8 grain load is very accurate, and I like the idea of a deeper penetrating hard-cast swc for a winter self-defense round out of a .38 snub. But I don't want to exceed max load pressure levels.

Regardless these are issues that I enjoy considering, and thinking about this stuff makes the hobby more fun. Well, getting answers to my questions and learning a little is more fun. Just thinking about it and getting confused isn't much fun.

February 14, 2013, 03:44 PM
Like I said in post #4:
Just match your bullet design & weight as close as possible to their bullet designs and use the data they published for it.


Jesse Heywood
February 14, 2013, 03:48 PM
One of the rules of thumb when loading is you can use load data from the next heavier bullet. It's in one of the manuals, I can't recall which. With lead, the bullet shape doesn't make a big difference in load data. So you can use data listed for a SWC if you are loading round nose. In all cases, start at the lower end and work up.

It sounds like you are doing things correctly. The only recommendation I can make is to purchase some different manuals when possible. They all have different ways of explaining things. One that I love is Ken Waters Pet Loads. He spent his life developing loads for all kinds cartridges, both rifle and handgun.

February 14, 2013, 04:14 PM
Many thanks guys.

Well, I certainly didn't get into reloading to get every scintilla of performance out of a cartridge or gun. Max loads aren't really what I'm interested in. I do like W2976/H110 in a .357, so there isn't tons of flexibility there. I just wanted to make sure the load I like isn't running into the realm of inadvisable for frequent use in the Cobra. Which it likely is closer to than I want for practice.

Again, thanks. And, yes I do need more manuals, and components, and some different molds, and some steel targets, and some more holsters, and a .357 carbine, and on and on...

Places like this are a big help though, in making sure the limited time and money I do have get focused in a worthwhile direction.

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