Reloading pressure

[shepherm]

Can someone tell me why hodgdon reloading data has such low pressure maximums? I am loading for a 22-250 and was looking at the data and noticed how low the pressure look compared to other reloading manuals.
http://www.hodgdon.com/data/rifle/22250rem.php
http://www.imrpowder.com/data/rifle/22-250rem-2005apr03.php
Thanks for any info
Mark

[quickdtoo]

Apples to oranges, CUP to PSI? :wink:

[shepherm]

Ok now I feel stupid. But this may make me look worst. What does cup stand for?
Thanks
Mark

[quickdtoo]

Copper Units of Pressure....

Tim

http://www.reloadbench.com/gloss/cuppsi.html

[Iowegan]

To expand on quickdtoo's correct but short answer: ammunition was tested with a "crusher" method. A precisly measured copper pellet was inserted in a special fixture. When the round was fired, it compressed the pellet. The pelet was then remeasured and, based on it's length,  the chamber pressure could be calculated. Copper units of pressure (CUP) was the unit of measurement. This crude method was in use for many years. Now technology has improved and uses piezo transducers to measure actual pressure in pounds per square inch (PSI). Not only is the measurement more accurate, the transducer produces a pressure curve signal that can be viewed on special test equipment. We now know a lot more about what goes on inside a chamber and can see pressure spikes that went unnoticed before. Some loads are still rated with CUP but as time goes on, they will all be retested with the piezo transducers and restated with PSI.

[quickdtoo]

To expand on quickdtoo's correct but short answer..

I thought the included Reload Bench link explained it rather well without me trying to explain it! :wink:

Tim

[james]

Thanks for your posts Iowegan. Physics was never my strong suit but I am enlightened occasionally from this site. I printed the post regarding sectional density.  You should write a book..... if you haven't already.
james

[Iowegan]

Thanks James!

quickdtoo, The referenced link is pretty dated. It didn't even mention the newer piezo transducer methods. In fact, the article stated, "PSI is an erroneous term". The author did do a decent job explaining the crusher method though.

To add a bit; many reloaders think they can convert CUP to PSI with some simple formula. I have seen a few of these formulas in print. Obviously the authors didn't have a clue because the two (PSI and CUP) measurements aren't compatible at all.

Imagine a pressure curve that starts at zero then increases to a peak pressure and decreases back to zero over a period of time. That's what a crusher "sees" and it only “records” a peak long enough in duration to crush the pellet. A Piezo device draws a chart that starts with primer detonation. It graphs the total powder burn until the bullet leaves the barrel. Included in the chart are spikes that are short in time but very high in pressure, much like teeth on a saw blade.  A crusher pellet won't respond fast enough to see these damaging spikes. Piezo charts also show secondary pressure peaks that happen after the initial peak. Because these secondary peaks are lower than the initial peak, it won't show up on a crusher pellet.  Secondary peaks happen in the barrel whereas primary peaks happen in the chamber.

What we are seeing in the newer reloading manuals is often a reduction in powder charges or elimination of loads that were printed in previous manuals. Many think this is just some lawyer issue when it is really is a “smarter” load based on more and better information from piezo transducers and their associated computerized test equipment. When pressures are stated in PSI in modern reloading manuals, it means they were tested with the piezo transducers and indeed are pounds per square inch.

It's just as quickdtoo stated .... Apples to oranges, CUP to PSI?

[quickdtoo]

Thanks for the detail Iowegan, my only point was I'd rather rely on data and info from reliable sources than try to explain in my words and muddy the water, so to speak. :oops:

Here's another source that's not so dated by Denton Bramwell on the PSI vs Cup correlation.

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

Personally, I just use data from current reloading manuals and leave the scientific stuff to the pros, it's less complicated that way. It would be nice if SAAMI would use one or the other for all their MAPs instead of some of each method.

http://www.leverguns.com/articles/saami_pressures.htm

Tim

[victorcharlie]

Does that mean that the thousands of rounds that I shot over the years that were 2 grains over current published figures weren't safe?

[Siskiyou]

Does that mean that the thousands of rounds that I shot over the years that were 2 grains over current published figures weren't safe?

Darn, I always thought older manuals were meant for tougher older guns. :-)

[Iowegan]

quickdtoo, The shootingsoftware reference is a pretty good one. This guy has come up with a left handed way to approximate PSI from CUP. Better than any other I've seen. Because these two types of measurements are taken at totally different datum points, the best you can do is approximate and I think he did a good job of it.

The other reference (from leverguns) shows a nice trend. Nearly all the popular cartridges now have a PSI rating, leaving the more obsolete stuff with the older CUP rating. Someday, SAAMI will go exclusively to PSI and the rest of the shooting industry will follow.

victorcharlie, Yes, it is very possible your old manuals have loads where pressures are too high. Keep in mind, the newer methods of testing show a lot more than just max pressure limits. They now can "see" (through the magic of piezo transducers, electronics, and metallurgy) what effect long term excessive pressure spikes will do. We normally think of damage as being catastrophic, meaning the gun blows up. The new test methods can predict long term effects such as metal fatigue, throat erosion, and bore wear.

Here's a totally hypothetical example: Let's say a tire manufacture recommends you carry 40 psi in your car tires. With new testing techniques, the manufacture finds your tires will last 10,000 miles longer and are less likely to blow out if you run 35 psi. Would you continue to run 40 psi because you did before? Or, would you drop the pressure to get longer use and less chance of a catastrophic failure?

And here's another major issue: Let’s take the most popular cartridge in the US, a 30-'06. Just about every rifle manufacture has made a gun in 30-'06; pumps, levers, bolts, semi-autos, and even full auto machine guns. The metallurgy from brand to brand and even between models can be night and day. Some guns may be brand new while others are pushing the century mark. A load that might be perfectly safe in one gun might blow the next gun apart. So what SAAMI has done is establish max operating pressures that should be safe in every 30-'06 ever made. This is a difficult task so what really happens is they lower the pressure limits so a load will be safe in all guns. So here you sit with a modern gun with a strong design. Does it mean your gun is going to blow up with an older and hotter load? No. But it does mean your gun will last longer and is less likely to have a catastrophic failure with lighter loads.

Here’s another good example: For several years, S&W petitioned SAAMI to lower the pressure standards on 357 Mag ammo. Finally SAAMI adopted a second standard that dropped the pressure from 46,000 CUP to 35,000 PSI (note: this accounts for a loss of at least 100 ft lbs of energy at the muzzle). With this lighter load, a S&W Mod 19 or 66 will live a much longer life. If you have a Ruger Blackhawk, you aren’t going to see any difference in life expectancy, just lower velocity. Note: not long after the SAAMI specs changed, S&W discontinued their K-frame 357 Mags … go figure?

[EsoxLucius]

Obviously the authors didn't have a clue because the two (PSI and CUP) measurements aren't compatible at all.

Denton Bramwell's article is only showing that PSI and CUP are highly correlated, not that the formulas are infallible.  As he states, the copper crusher and piezo-transducer methods are measuring the same thing, peak pressure, hence there has to be a conversion between them.  What limits the formula are the limitations and error in each of the measuring systems.

[PaulS]

PSI and CUP get a lot of press every time someone sees a figure in a book and then tries to understand what it means compared to what is in their own manual. Don't do it! They don't measure the same thing.

Yes, the guns are the same, the ammo is the same but the machines don't measure the same thing.

In the old(er) crusher machine here is what happens after the calibrations are made and the differences resolved:

The gun is fired and pressure starts to build. The copper slug that is going to compress has not yet seen its yield pressure yet. That little copper slug won't begin to deform until it is hit with over 5000 psi for a period of time. As the pressure goes up the piston starts to compress the crusher and we are beginning to measure what is going on in the chamber. and then the pressure starts to drop and the deformation of the crusher slows down and then we dip below 10000 psi and the slug stops deforming. It is measured - the calculations are made and the value is determined to be safe.
If you put that same copper pellet in a hydraulic press you could smash it to a coin with less than 40000 PSI (at the crusher) the only reason it stays in a cylindrical shape is because the pressure is only applied for such a short period of time. What that copper pellet is measuring is impulse. It isn't measuring PSI, pressure or force. It can only measure the impulse of the force. The time X pressure EFFECT and not the actual pressure. You could load a very fast pistol powder in a rifle case and measure a "safe" load in that 308 case (one is as good as another) that actually exceeds the safe pressure by twice. The 100000 psi pulse would be so quick that the copper crusher would not have time to show the over-pressure that existed. It takes time, and a lot of it comparitively speaking, to move the metal in that slug to show the pressure and if there isn't enough time it won't show it. So, to clarify what the crusher does and doesn't measure here it is again. It doesn't measure peak preasure, average presure, force or psi. It does measure the impulse of a load.

Now, the piezo-electric machine that they use to measure PSI measures force over time in small enough increments that it can be rendered to PSI.
What is a piezo-whatchacallit - and how does it work?

The piezo-electric sensor is a piece of quartz crystal that is placed into a precision presure device. A ring or strain tube that measures the force exerted from the inside that compresses or from the outside that stretches that crystal. When the crystal is stretched or compressed it generates an electrical current that is proportionate to the amount of stretch or compression. When properly calibrated it can measure the amount of force it "feels" as the firing pin hits the end of its throw on an empty chamber. It can actually measure the firing pin hit, bounce and hit again and display it on the computer screen or print it out on paper.
When you use a calibrated piezo-electric device to measure chamber pressure the chamber pressure is measured every couple of millionths of a second. It measures the pressure exerted on the walls of the chamber about 3000 times as the bullet travels down the barrel and you can see the pressure drop as the bullet leaves the muzzle. Is there error? Sure there is! and there can be up to 2% error. Now at 50000 psi that is about 1000 psi error - at 60000 psi that is 1200 psi and at 65000 its 1300 psi.
I can live with 1300 psi error for a cartridge that produces 65000 psi because I will just load it to 63700 and call it good!

In sumary the copper crusher method has more error related to burn time and the length of time that the burn is above its yield strength than it does from actual pressures involved. The error in the copper crusher just in the yield strength is more than you get from a piezo-electric device. Does this mean that all that old (and some of the new) data is worthless? no, it just means that we can measure pressures more accuratly now and we have been shooting some ammo a lot hotter than we might have if we had known better. Is it unsafe in our guns? Maybe. It might cause small mirco-fractures that will eventually lead to catastrophic failure but it might not. Those loads might be just fine to shoot but if I have new data that tells me that they ae hotter than I should use I will probably change to the lower pressure loads. I understand that Hodgdon is going to continue to use their old crusher method to measure loads so it must have some value. SAAMI is changing all of their new data to the psi standard as it is approved so it will become the standard whether Hodgdon likes it or not.

anyway - that is the diference between the two - so keep loading your proven loads and get used to seeing the PSI where it used to be CUP.