Pressure Testing

The sole purpose for conducting chamber pressure tests with RSI's Pressuretrace II Hardware and Software was to create a pressure timeline, comparing actual current manufactured and handload pressures of today with those of yesteryear. Below is a virtual work in progress chart that shows such work

DEFINITION AND PURPOSE  SAAMI Website

SAAMI Definitive Proof cartridges are cartridges commercially loaded by SAAMI member companies which develop pressure substantially exceeding those developed by normal service loads. The pressure levels are designed to assure gun safety when using ammunition loaded to service pressures in accordance with accepted American practices. 

Proof cartridges are designed to stress firearms components which contain the cartridge in order to assure safety in the recommended use of the firearm during its service life. 

It is important from the safety standpoint that Definitive Proof cartridges be used only for the proof of firearms. Adequate precaution must be taken to protect personnel performing firearms proof testing. 

The supply of Definitive Proof cartridges will be the responsibility of the company that first introduced that particular caliber to the Institute. Definitive Proof Cartridges should be loaded with the heaviest bullet used at the time of introduction and the slowest powder which will meet the pressure values indicated for that particular cartridge to maintain effective pressure-distance relationship. Once established, the bullet weight for the proof load does not change unless the bullet becomes obsolete. All changes in Definitive Proof cartridges bullet weight must be approved by the Joint Technical Committee. 

NOTE: The heat treatment of cartridge cases for definitive proof loads may be different from the treatment of service cases, at the option of the manufacturer. Other case modifications to minimize firing casualties such as gas leakage around primers are also permissible. The use of such options must not affect the stressing of the firearm’s components.

 PROOF PRESSURE INTERPRETATION 

The following specifications define the proof loads based on tests fired in standard test barrels with the ammunition at a temperature of 60°-80°F (15.6°-26.7°C). Tests shall be in accordance with the procedures and equipment shown in Sections II and III of this Standard. 

Pressure values are given on the following pages in terms of minimum and maximum averages and extreme variations for 10-round tests in standard test barrels. 

The Standard Deviations for Definitive Proof Cartridges are derived from the Standard Deviations for service loads. A multiplication factor of 1.25 is used to obtain Definitive Proof Load Standard Deviations from Service Load Standard Deviations. 

The minimum and maximum average Definitive Proof Pressures are computed as follows: 

• The Minimum Average Definitive Proof Pressure is calculated by multiplying the Maximum Probable Lot Mean (MPLM) service pressure by a factor of 1.3 (i.e., 130%) and rounding UP to the nearest multiple of 500 psi. 

• The Maximum Average Definitive Proof Pressure is calculated by multiplying the Maximum Probable Lot Mean (MPLM) service pressure by a factor of 1.4 (i.e., 140%) and rounding DOWN to the nearest multiple of 500 psi. 

• The Proof Maximum Extreme Variation (EV) is calculated by multiplying the Proof Standard Deviation (which in the case of Centerfire Rifle is equal to the Service Standard Deviation multiplied by 1.25) by the constant 5.16(14) ) and rounding UP to the next 100 psi. 

• The Minimum Proof Individual (MPI) pressure is positioned three standard deviations (proof) below the Minimum Average Definitive Proof Pressure, with the calculated value being rounded DOWN to the next multiple of 100 psi. 

Example (using transducer data): 

Cartridge: 22 Hornet                        MPLM Pressure = 50,200 psi                   SERVICE = 1,960 psi 

1. Definitive Proof Load Standard Deviation (PROOF) = Service Load Standard Deviation (SERVICE) x 1.25 i.e.: 1,960 psi x 1.25 = 2,450 psi 

2. Min. Avg. Proof Pressure = Maximum Probable Lot Mean Pressure x 1.30 i.e.: 50,200 psi x 1.30 = 65,260 psi rounded up to nearest 500 psi = 65,500 psi 

3. Max. Avg Proof Pressure = Maximum Probable Lot Mean Pressure x 1.40 i.e.; 50,200 psi x 1.40 = 70,280 psi rounded down to nearest 500 psi = 70,000 psi 

4. Max. Proof E.V. = Proof Standard Deviation ( (PROOF)) x 5.16 i.e.: 2,450 psi x 5.16 = 12,642 psi rounded up to next 100 psi = 12,700 psi. 

5. Minimum Proof Individual = Min. Avg. Proof Pressure – (3 x  (PROOF)) i.e., 65,500 psi – (3 x 2,450 psi) = 58,150 psi rounded down to next 100 psi = 58,100 psi. 

"Pressure Spikes", or the lack thereof

I took the best shot, from each recent thirteen 10 shot groups, that best represents the average pressure of that group and made a gif. The only "pressure spikes" observed with any of the 96 total tests over-all were the high power loads of fast burning pistol powders. There were no "pressure spikes" for any normal loads regardless of using pistol powders or rifle powder. Early black powder loads had the same pressure curves as several of the pistol powder loads used.

There are slower burning loads which have a shallow initial burn which the pressures drop by 1 millisecond. The loads that dropped pressure faster were typically the fast burning pistol powders near the Bullseye/Titewad/Red Dot/Clays/700X/Titegroup/WST/Trail Boss powder burn rates that drop down faster, giving the "pressure spike" appearance. Even then, the peak pressures from those particular charges exceed or would exceed 13,000cup/11,000psi MAP set for Winchester 73' and revolver use, only to be used in the Model 92' or Marlin type actions 

Pressure Testing

Figuring Out Today's Ballistics...continued!

"Using the Pressuretrace II"

from Recreational Software, Inc

All of my PressuretraceII results are published with Jim Ristow's permission.

I forgot just how long it has been but let's just say since about 2008. I saw this program back then but never thought I would ever be able to use it. Just like everything else out there, it costs money.

​As you may already know, there is no correlation between CUP and PSI. Each is a separate test with separate results. There is no magic formula to convert one to the other, even though each method has to be calibrated with some sort of data. The same is true for the PT-II software. The PT-II software uses a strain gauge attached over the chamber, chamber and cartridge dimensions and measures the expansion of the barrel when fired. The problem with using this with the 44-40 is that the only "control" we have is SAAMI's MAP of 11,000psi. What makes this difficult to use is the only trusted handloading information manuals use CUP. Even Lyman's max and high pressure loads are in CUP. The only thing we can do is use factory loads, understanding that they may be loaded to 5% to 25% below SAAMI MAPs or try MAX loads published in Handloading manuals to see how close they are to max pressures. For example, if Lyman says a given load produces 13,000 max cup, then that load should produce 11,000psi from using the strain gauge. This itself makes it hard to make a pressure correction to be accurate. Lyman's published loads should be close in order to develop a "control" BUT I feel a factory loaded known pressure would be better...achieved from a Universal test barrel. 

A handloader must make a decision, create his own limitations, use the limitations he feels comfortable with.

With that I need to back up a little bit and share some information from a shooter, aka Unclenick;

"44-40 pressures,

 SAAMI barrels have minimum chambers and nominal bore dimensions. To mimic one, you have to get a barrel blank made to their bore cross-sectional area dimensions and then chamber it with a special reamer ground to produce the SAAMI minimum chamber diameters and not run it in past half a thousandth beyond what produces SAAMI minimum headspace. That's ±¼ of a thousandth, which can take some skill to do. Standard reamers are commonly made a little wider so they can be resharpened without losing dimensional precision. Denton Bramwell points out in the article [Link] most chambers made with standard reamers can expect to produce lower numbers.

SAAMI test barrels are fired with the cartridge manipulated to ensure the powder has fallen back over the flash hole. They are not tapped against anything to do this, as that packs the powder which can lower its burn rate a little. They want worst case highest pressure, so they just turn the case around to let the powder fall back to the rear, then load it carefully so as to avoid shifting the powder position. If you don't do that, you will get lower numbers than they do.

For rifle cartridges, SAAMI expects no more than 4% standard deviation in pressure. If your readings produce a higher SD than that, you are probably using a sample too small to be relied on to produce a good SD number. SAAMI uses ten rounds. Our own board member, Statshooter, who is a professor of statistics, says 30 rounds are needed. Denton Bramwell feels 15 rounds give him what he needs to know. So, why does SAAMI just use 10? They accept the SD results produced by 10 can be off on the high side by two standard errors (about another 2.5%), so they have a second number in addition to the MAP called the Maximum Probable Lot Mean, or MPLM, that allows the next sample of 10 from the same lot could produce a standard deviation that was different by that much. So, while the MAP is 11,000 psi, they allow they might get an average as high as 11,300 psi from another random sample of 10 from the same lot, and that is acceptable. They have a still higher number, called the Maximum Probable Sample Mean (MPSM) that allows that bullet bonding with the case and other factors can produce an increase in pressure as a lot of ammunition ages, and for the 44-40 that number is 11,700 psi. So, in effect, they are saying 11,700 psi is the safe limit, but, wait, there's more: All those numbers, the MAP, the MPLM, and the MPSM are averages produced by 10 round samples. The individual rounds making up that average will go above and below that average. SAAMI controls that by allowing what they call the Maximum Extreme Variation (MEV) which is the maximum spread between pressure readings for any of the 10 rounds in any sample taken at any time in the life of the lot. For the 44-40, the MEV is 2300 psi (this has to be calculated per the standard's instructions; it is not in a table). Theoretically, you could have aged loads centered on 11,700 psi whose 10 shots had one round as high as 13770 psi that would still be acceptable to SAAMI (though it is extremely improbable as it requires all 9 other shots to be exactly 11,470 psi). Mostly, though, you don't see individual rounds more than about ten percent above the MAP.

Even with all of that care, the fact is the absolute pressures are not exactly knowable. SAAMI reference cartridges are made in lots that are sent around to member's test facilities every two years to watch for reading changes. The members use SAAMI standard calibration methods (see the SAAMI standard) and report their results for both pressure and velocity from SAAMI standard test barrels. Interestingly, these vary more than the SAAMI standard deviation limit. The old 1992 standard gave an example for each method firing the same lots of reference loads, and the copper crushers, using targe tables for calibration, produced over 23% difference in average pressure for 10 shots among 9 labs. The transducers, using hydraulic pressure for calibration, produced over 11% difference in 10-shot averages among 7 labs. The reported results for each set of tests from each lab were, in turn, averaged and the final average was declared to be the pressure value that reference lot produced. This way, in effect, the reference lot pressure in the example was evaluated by looking at 90 and 70 total sample shots for the crusher and transducer, respectively. It compensates for average measurement calibration and operation errors, combined, to give a practical expected correct reading.

​I decided to use Buffalo Bore's "Heavy" 44-40 factory loads as my "Control" because they claim these loads are loaded to SAAMI MAP of 11,000 psi....certainly closer to SAAMI than Winchester is.  I do not have their exact testing results but setting their loads with a correction factor to the SAAMI max gets me the results I need. Why is this important? Because most all factory loads are loaded to between 5%-25% BELOW SAAMI MAP for safety reasons. Buffalo Bore warns using these loads in old weak actions or weapons with tight chambers or small bores (.425 etc. rather than .429-.430). I feel Winchester Super-X factory hunting loads are loaded well below SAAMI MAP as I will explain shortly as well as are evident from the test results.

​Using the PressureTrace software I loaded up some Winchester Super-X ammunition and fired a few shots. The pressures were lower than expected. After making some adjustments I then fired Buffalo Bore's factory ammunition and concluded that indeed BB loads are much closer to SAAMI MAP although an exact measurement is unknown to me. Thus I elected to add a correction pressure and use BB's loads to equal SAAMI MAP of 11,000 psi as my "control" loads.

6-22-2022

WARNING: DO NOT SHOOT OLD AMMO

1873 to 1884
Black Powder Replication

1884 thru 1910's
Black Powder and Smokeless Powder
Replication Loads

1910's thru 1970's
Replication and Factory Loads

1970's thru 1990's
Factory Loads

1990's thru 2010's

Factory Loads

Custom Handloads

Test 92 Notes: Shots 1 thru 3 are loaded into the chamber normal. Shot 5, powder positioned to the back of the case, while shot 6 powder was positioned to the front of the case. Although the powder was at the front of the case in shot 6, it had a normal ignition while shots 1 and 2 did not.