View Full Version : Chamber pressure chart?

05-15-2010, 03:30 PM
I just bought Lee's lead hardness tester, and it works great. In the instructions it has the recommended maximum pressures for each hardness, but none of my reloading manuals has any pressure info for the loads. Can anyone steer me in the right direction, as to a chart showing pressure levels of different calibers/loads? Thanks a million!

05-15-2010, 03:43 PM
I just bought Lee's lead hardness tester, and it works great. In the instructions it has the recommended maximum pressures for each hardness, but none of my reloading manuals has any pressure info for the loads. Can anyone steer me in the right direction, as to a chart showing pressure levels of different calibers/loads? Thanks a million!

IMR, Hodgdon's free loads have pressures, and they cover Winchester powder as well, as do the old Midway LoadMap books. Richard Lee's Modern Reloading II summarizes most available freely available loads with pressures.


05-15-2010, 03:44 PM
Chamber pressure values are shown in the Lee Loading Manual, and in the Lyman Manuals.

05-15-2010, 03:55 PM
As usual, you not only have answers, they are given very quickly! Thanks-

05-15-2010, 05:06 PM
Ok, one more thing. How do you convert CUP pressure to PSI?

05-15-2010, 05:23 PM
Now you're going to start an argument! Here's one man's idea: http://www.shootingsoftware.com/ftp/psicuparticle2.pdf Google it and you'll find others.

05-15-2010, 05:37 PM
You don't convert CUP to PSI. They really measure pressure in different ways that are not linearly correlated.

05-15-2010, 06:21 PM
You don't convert CUP to PSI. They really measure pressure in different ways that are not linearly correlated.

+1, some claim you can convert one to the other but that doesn't make it true. Can't be done.

05-15-2010, 06:42 PM
Actually, they are both units of the same thing. The attempt is to measure the maximum amount of force delivered to some object. The amount of copper bent is one DIRECT measurement. The amount of Piezo material bent is another. Copper is less sensitive than most materials having a linear/predictable piezo effect (electrical voltage production). ... felix

05-15-2010, 09:29 PM
The amount of copper bent is one DIRECT measurement.Ummm .... not quite - almost. It tells us the same thing but the copper undergoes plastic flow which is time and pressure influenced, making it a little more complex than the piezo strain guage which is very fast and linear. With copper, a short sharp pressure spike can look like a long slow but low pressure curve. That can be deceptive as it then also ignores the shock effect of a suddenly applied load which can be many times more destructive than a gradually applied load of the same magnitude. Just splitting hairs!:mrgreen:

05-16-2010, 12:18 AM
Ok, I need to know PSI for each load. If a manual lists pressure in CUPs, I'm just SOL? Thats just **, because AFAIK, I need this ASAP. JK!

05-16-2010, 06:08 AM
A chart (or a way to produc an accurate chart) that lists the powders all in PSI doesn't exist as far as I know.

What are you trying to do? Maybe there's another approach that someone can help with.

05-16-2010, 08:21 AM
I would like a general pressure range for each of the cartridges/loads that I load for, so I know what hardness the bullets should be at, (according to Lee). Although I think accuracy is good now, with my cast loads, I'm always open to suggestions (by Lee or anyone else), on how to make them more accurate.
I see nobody thought my abbreviating post above was funny. I guess thats why I'd never made it in comedy!

Larry Gibson
05-16-2010, 08:48 AM

The posts that tel you there is not a direct conversion of CUP to PSI of visa versa are correct. Those who measure pressures (ballisticians mostly) will tell you that. The formula in the article to convert CUP to PSI is correct only at a very narrow band in the middle range of acceptable cartridge pressures. I also measure pressures but am no a ballistician though I do study the science. I most every instance measuring pressures of cast bullet loads there is absolutely little to none whatsoever correlation between the two.

Also, contrary to what Lee says, the amount of presure a specific alloyed bullet can withstand during accelleration is entriely dependent on the time/pressure curve of that accelleration. In otherwords; an alloy cast bullet will stand up to more pressure when a slower burning powder is used. Best advice is to work up loads as one should. When accuracy goes south then you are pushing that particular alloy with that bullet using that powder with too much pressure. I would also advise keeping within published data.

Also keep in mind that the pressure the factories get in their test barrels is not the same pressures you get in your production gun. The pressures one factory gets in their pressure barrel is different from what another company gets in their test barrel with the same cartridge. That is why there is 'reference ammunition within the industry. No one can say that such an such load will produce exactly a certain PSI.

Larry Gibson

Bass Ackward
05-16-2010, 09:34 AM
I would like a general pressure range for each of the cartridges/loads that I load for, so I know what hardness the bullets should be at, (according to Lee). Although I think accuracy is good now, with my cast loads, I'm always open to suggestions (by Lee or anyone else), on how to make them more accurate.
I see nobody thought my abbreviating post above was funny. I guess thats why I'd never made it in comedy!

Then you need a computer program called Quickload. Just chrono the load and adjust your factors so that the program predicts the velocity that you registered on your chrono and you have the pressure for that load. Those are expensive.

That's why the lazy man's way is so popular. Use a GC design, mold with what you got, use a good lube, start low and come up, and watch Professor Target for the correct place to stop. Clean when necessary. :grin:

05-16-2010, 11:56 AM
I would like a general pressure range for each of the cartridges/loads that I load for, so I know what hardness the bullets should be at, (according to Lee). Although I think accuracy is good now, with my cast loads, I'm always open to suggestions (by Lee or anyone else), on how to make them more accurate.
I see nobody thought my abbreviating post above was funny. I guess thats why I'd never made it in comedy!Since you are open to suggestions: I went down the path of buying powders to get the PSI at 90% pressure and to match the pressures documented by Lee. Maybe it works well in rifles, mabye it works for others, but in my 38 special and 357 magnum it's not close to the most accurate load. As a general guideline the advice such as this (from http://www.lasc.us/CastBulletNotes.htm ) has been much more helpful:

Bullet BHN / "Minimum" Chamber Pressure For Lead Alloys (PSI)

The formula (from the pages of HandLoader Magazine) to determine at what pressure an alloy of given BHN will obturate the base of the bullet and seal the bore. If the bullet is too hard to obturate, gas cutting usually occurs on the base band on the non-driving side of the rifling and barrel leading is likely. Simply multiply the alloy BHN by 1,422.
Example: Alloy BHN of 12 multiplied by 1422 = 17,064. An alloy of 12 BHN should be used with a load that develops a "minimum" of 17,000 psi. Need more info on minimum / maximum alloy BHN? These Glen E. Fryxell articles explain alloy BHN in easy to understand language.
Cast Bullet Alloys And Obturation

I did find that testing 10 rounds (per Lee) at the range working lighter to heavier does help see what loads group the best, what load shoots to point of aim the best.

Maybe others found the pressures that Lee recommends to work for them. It just cost me a bunch of time to figure it out. YMMV

05-18-2010, 11:12 PM
Ok, one more thing. How do you convert CUP pressure to PSI?

Very carefully! :)

Seriously, the answer is "depends". For standard loads in the 45/70, the conversion is using an equals sign. This is not surprising as the U.S. CUP system was developed with and for the 45/70, and the engineers, even back then, got it pretty much exactly right.

The problem is when the industry began using that same system and calibration for other cartridges.

The more overbore the cartridge (that is, the less it looks like the straight-walled 45/70 and the more it looks like the 220 Swift), the greater the difference between CUP and PSI.

Why is that? Overbore cartridges usually use slower, more progressive powders. These powders have longer sustained pressures, which crush the copper gauges longer and make it appear like a higher peak pressure has occurred- which hasn't.

This measurement differential has allowed the industry to safely developed higher sustained pressure cartridges with the newer measurement systems then they would have dared with the old copper crusher system. (Not to mention other advances such as in powder).

Likewise, fast powders in a small case (think Bullseye in a 9mm) often have a peak pressure measured in PSI that is much higher than the CUP measurement would indicate. This is because the peak pressure isn't sustained long enough to fully deform the copper.

This has resulted in some old fast powder loads to be removed from modern loading books, even though time and experience has proven them safe.

I have worked with barrels equipped with Copper Crusher, Piezoelectric, and both, and PE are much faster to deal with and give detailed information about the sustained pressure that crusher gauges simply can't. Still it is nice to be able to correlate your data with older stuff and use both on occasion. We had gauges that were loaded in with the propellant as well, but of course we aren't talking Small Arms here. You could only load them up so high in the charge or you had to go walkies with the metal detector.


cajun shooter
05-19-2010, 10:50 AM
If you try to compare the two you will have more gray than when you started. I also agree that many good loads of the past that had cup's were considered wrong and removed from print. The safest way to do any load is the old way of stages of trial firings. Buy yourself a good CED and read all the classic signs. Remember that the hottest loads were not always the most accurate nor the best for the job intended. A animal be it fur bearing or human will not know the difference in the pressure of a 41 or 44 mag.

05-21-2010, 11:51 PM
Sorry about posting this late, but took me a while to find what I was looking for. This is a pretty good article on measuring pressures by Allan Jones who in my opinion a super ballistician. He writes for Shooting Times.

http://www.shootingtimes.com/s.gif CUP, psi & Reloading Data by Allan Jones

Here is the piezo-electric conformal transducer. The threaded cylinder with the contoured tip is the sensor.My 20-year tour as a reloading-manual developer began during the transition from copper crusher pressure testing to modern piezo-electric testing, and what good timing that was. Learning both let me understand the effects of each on published load data and gain confidence in the loads derived by each method.

Copper Crusher Basics
Crusher testing is a very old technology and is strictly mechanical. It is so simple and low-tech that you could measure cartridge pressures on a desert island with no electricity or computers as long as you had a decent micrometer and some place to write the results.

A crusher-type test barrel is pierced or ported at or near the chamber in a spot precisely called out in the cartridge's standards. Once a cartridge is loaded in the barrel, a soft copper gascheck is pushed into the port against the case, and a special steel piston is inserted and snugged against the gascheck. The gascheck prevents gas escape that could erode the expensive barrel.

A copper crusher is placed on the upper end of the piston. Crushers are made to high material and dimensional standards, and each lot is calibrated to ensure consistent deformation. To provide the resistance that ensures the "crush" and captures the crusher, the pressure barrel has a heavy yoke--basically an inverted "U." The yoke attaches to the massive frame of the universal receiver above the barrel. A large anvil screw in the top of the yoke lets the operator secure the crusher.

Pressure against the piston shortens the crusher. We measure the crusher and consult a table of pressure values provided with each crusher lot. Starting at 0.499 inch, the table lists decreasing lengths in thousandths of an inch and, beside each length, the pressure in copper units of pressure (CUP) that it takes to shorten that much. The system must be reset after each shot, requiring that you remove the piston, knock the gascheck into the fired case (which now has a hole in it), extract the case, and start over. It is a tedious procedure, but it served well for years.

Piezo-Electric Basics
When modern electronics and sensing methods became more prevalent in the last 30 years of the 20th century, ballisticians found a way to improve the speed and accuracy of pressure measurement. The heart of the system is a little gadget called a piezo-electric transducer. Each one contains a tiny crystal that emits a small electrical signal when compressed. The signal is directly proportional to the force applied and is linear, making it an excellent method for testing pressure.

If you replace the crusher piston with an electronic sensor, you avoid the need to reset the barrel for every shot. But you need potent electronics. The tiny signal output from the crystal must be amplified and then fed into a computer that applies all the initial settings derived for that barrel and transducer to yield an accurate result.

In Sporting Arms and Ammunition Manufacturers Institute (SAAMI) protocols, the transducer's sensing surface is contoured to match the chamber's radius and taper. The device "conforms" to the case wall, giving this type of sensor its name--the conformal transducer. A small metal bracket ensures the sensor is installed with the taper in the proper directions. When properly installed in a pressure barrel, the transducer is almost invisible to the casual observer. The transducer can leave a slight mark on a fired case but is otherwise like using any firearm in that you do not have to reset anything other than the breech between shots.

A PCB transducer is fitted to a Krieger pressure barrel. The bracket ensures the contoured sensor is in the proper orientation.

Being strictly mechanical, the crusher system records only peak pressure--nothing else. It's like a black and white snapshot taken with an old box camera. By comparison, transducer testing is a high-definition, full-color video. The new system has a time basis and records from the point the fired case contacts the transducer to the point where the bullet exists the test barrel.
Just as you can derive more information about an event from a video than a single still photo, modern electronic testing opened new doors for those of us who develop reloading data. We can see when the peak pressure occurs (time-to-peak), how long it is maintained, and even analyze primer performance--something hard to detect with a crusher system.

In addition, transducer testing gives higher resolution. The nature of crusher calibration creates large increments of pressure up to several hundred CUP per increment, but a transducer technically can read to 1 pound per square inch (psi). I found this higher resolution let me confidently produce a more consistent data set for each cartridge and better judge which propellants were best suited to that cartridge.

It's About The Units
People seem to think there is some absolute relation between copper units of pressure and the pounds per square inch results of transducer testing. There isn't. CUP is a special unit of measure defined only for ballistic testing; psi is a standard unit of measure for all pressure testing in the English system. Whether you are testing radiator caps or valves for a nuclear submarine, psi is the same. CUP has no other use.

There is about a 96 percent correlation between assigned psi and CUP for rifle cartridges, with the psi value being higher. However, that means that 4 percent of the time, you can't make it work. That's because of some rifle cartridges whose psi assignment is lower than their CUP assignment. It's even more prevalent with handgun cartridges. When you convert inches to feet, dividing by 12 always works. Not so for ballistic pressure units.

Location, Location, Location
The location of the pressure sensor is a major factor in the relation between psi and CUP values. The reason that 96 percent of rifle cartridges have a reasonable correlation between the two systems is that the sensor location is similar. The sensor locations can be very different for handgun cartridges, especially the short ones. Because the yoke and anvil assembly on the universal receiver must be a minimum distance from the breech end to create a safe and secure contact, many handgun cartridges were set up with the crusher sensor at the case mouth. The shortest centerfire cartridge, the .25 Auto, has a max case length of 0.615 inch, but the anvil assembly means the crusher port is 0.698 inch from the breechface. The accompanying chart shows how much the sensor position varies for the .25 Auto, 9mm Luger, and .38 S&W Special.

The transducer is secured to the barrel, so it can be positioned much closer to the breech, usually mid-body. This means a transducer can detect pressure events that occur very early in the pressure cycle--things a crusher at the case mouth may fail to detect.

Effects On Load Data
First, let me repeat that when the sensing ports for the two systems are similarly located, the ballistician will not see much difference in final charge-weight assignments. The newest cartridges no longer have crusher standards, so this is a moot point for them.

For me, the two handgun cartridges that highlighted the effects of sensor location were the 9mm Luger and the 10mm Auto. When we started on Speer Reloading Manual #12 (the first Speer manual to incorporate transducer testing), we were using crusher data from older Speer manuals as starting points. Most of the loads compared well within the safe zone, but some propellants that "shot spec" on crusher were exceeding the transducer limit of 35,000 psi by up to 7,000 psi. Retesting on crusher showed them still normal on the older system.

CUP, psi & Reloading Data

http://www.shootingtimes.com/ammunition/ST_cuppsireloaddata_200905-C.jpg After firing, there is a faint trace from a conformal transducer on this .44 Magnum case fired in a modern pressure barrel.

Before we used this barrel, we received the complete SAAMI technical data packet for the 10mm, and there was only a transducer standard. We built another barrel, this time a transducer version. Because we had inquiring minds, we did parallel testing with each system. Most loads compared well, but again, we found some propellants that posted 36,000 CUP shot nearly 45,000 psi, well over the 10mm's 37,500 psi limit.

In both cartridges, the propellants that did not "line up" between the two systems were the fast-burning handgun propellants. By the time we adjusted their loads to read in-spec on transducer, velocities were falling off.

My immediate thought was, "Are crusher loads unsafe?" We faced one big, obvious fact: The "old loads" had not damaged any cartridge cases or firearms when used as prescribed. Why not?

The engineering director nailed the answer: Some potentially dangerous things are of such short duration that they do not have time to produce damage.

Look at fire walkers. They routinely walk on coals hot enough to burn skin, but their technique avoids staying in contact with the heat long enough to suffer burns.

Fast-burning propellants can create pressure effects early in the firing cycle that a crusher at the case mouth will never detect. They last such a short time that even the soft brass case suffers no ill effects. It is a credit to the sensitivity of the conformal transducer that it picks up these early effects and includes them in the total picture of the event.

We seldom saw these effects in rifle cartridges. Most standards for bottleneck cartridge chambers place crusher and transducer ports much closer, at least as a percentage of total chamber length. We shot transducer pressures for .30-06 loads with the various styles of Speer 180-grain bullets, data for which had previously been developed on crusher only.

The crusher loads were set to max between 47,000 and 48,500 CUP, just under the SAAMI max of 50,000 CUP. Transducer testing showed that the same loads fell between 56,700 and 58,500 psi, just under the 60,000 psi limit for the same cartridge. No changes in published data were needed. Across the range of modern rifle cartridges, we did not find any surprises when we added transducer data to our existing files of crusher data.

So the handgun reloader stands to be most affected, but is this a serious problem? No, it's not. The lesson I see in this is for the reloader to take advantage of the wide range of handgun propellants we enjoy today. Don't try getting high-velocity loads with the quickest burning propellants. In spite of some wonderful mid-rate handgun propellants on the market, some reloaders are still trying to get top velocities from Bullseye, W231, AA No. 2, and other very quick-burning handgun propellants. Save those fine quick-burners for your target loads.

Transducer testing provides a direct payoff to the hobby reloader: volume of data. We had to budget 50 percent more time when developing loads that still used crusher standards. Transducer testing let my team greatly expand the coverage in the latest Speer manual.

I'm a real traditionalist--blue steel, case-coloring, and fine walnut still speed up my pulse. However, for pressure testing, give me the high-tech conformal transducer.