Cartridge Pressure Standards

In the U.S., voluntary pressure standards for rifle cartridges are set by SAAMI, a member of ANSI. Most other countries in the world follow the standards of Europe's CIP (Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives or Permanent International Commission for the Proof of Small-arms, sometimes referred to as the International Proof Commission).

The pressures in the table below are rounded up to the nearest ksi, ie.1000 psi. Cartridges are arranged, approximately, in order of bullet diameter and then by case volume. Use the copper crusher numbers with the Powley Computer.

                     piezo                crusher

                   SAAMI  CIP         SAAMI  CIP  other

.17 Hornet          50
.17 Mach 4                60*                52
.17 Fireb           55
.17 Rem             63    62           52    53
.204 Ruger          58    59
.22 Short           21
.22 Long            24
.22 LR              24
.22 WMRF            24
 5.6x35R                  39                 35
.22 Hornet          49    44           43    41?
.218 Bee                  46           40    41
.22 Rem Jet               37           40
.221 Rem                  46           52
 5.6x50R                  49                 44
.222                50    54           46    46
.222 Mag.                 59           50    51
.223                55    62           52    54
.219 Zip.                 41                 37
.225 Win.                 57           50    49
.22-250             65    59           53    51
.220 Swift          62    62           54    54
.224 Wea            64
.223 WSSM           65    65
 5.6x61R v.H.             55
 5.6x52R                  48                 42
.22 Sav.                  48                 42    49
 6x70R                    38
 6x45               55
 6x50R                    64
 6x52R Bret.              48
.243 WSSM           65    65
.243 Win.           60    60           52    52
 6 Rem.             65    62           52    54
.240 Wea.                 64                 55               
 6x62                     62                 54
 6x62R                    62                 54
.240 Fl.NE                46                 41
.25-20                    39           28    35    21/29
.25-20 S.S.                                        22
.256 Win.                 51           43    44
.25-36 Marlin                                      31
.25-35                    44           37    39    33
.25 Rem.                  36                       35
.250 Sav.                 53           45    46    52?
.25 WSSM            65
.257 Rob.           54    51           45    45
  " +P              58                 50
.25-06              63    65           53    56
.257 Wea                  64                 55
 6.5 Grendel        52
 6.5x52R                  36                 32
 6.5 Jap.                 43                 38
 6.5x50R                  53                 46
 6.5x70R                  41                 36
 6.5x52 Car.              41
 6.5x53R                  46*                41
 6.5x54 M-S               53                 46
 6.5x54 Mauser            44                 39
.260 Rem            60    60
 6.5x55             51    55           46
 6.5x58R                  41                 36
 6.5x57                   57                 49
 6.5x58 Mauser            51                 45
 6.5x57R                  48                 42
 6.5 Creedm         52    63
 6.5x70R                  41
 6.5x72R                  55*                48
 6.5-06 A-S         65
 6.5 Rem Mag              63           53    54
.264 Win Mag        64    62           54    54
 6.8 SPC            55    59
.270                65    62           52    54
.270 WSM            65    65
.270 Wea            63    64
 7x50R                    53
 7-30 Wat.          45    49           40    
 7x72R                    41                 36
 7-08               61    60           52
 7x57               51    57           46    49
 7x57R                    49                 44
 7 Fl.Mag.                48                 42
 7x65R                    55                 48
 7x75R v.H.               60                 52
.284 Win            56    64           54    55
.280 Rem            60    59           50    51
 7 WSM                    65
 7 Rem Mag          61    62           52    54
 7 Wea              65    64
 7 STW              65
.280 Fl.NE                43                 38
.30 Carb.           40    46           40
.30-357 AeT               44
.30 Rem AR          55
.30-30              42    46           38    41    38
.30 Rem                   41           35    36
.303 Sav                  39           34    35    43?
.300 Sav            47    53           46    46
.308 Marlin         48
.30 Fl.Purdey             46                 41
.30-40                    47           40    41    38
.307 Win                  60           52    52
.308 Win            62    60           52    52
.30-06              60    59           50    51    
.30 R Blaser              59                 51
.300 WSM                  65
.30 Super Belt            53
.300 RCM            65
.300 H&H            58    62           54    54
.300 Win            64    62           54    54
.300 WSM            64
.30 Fl.Mag.               46                 41
.308 Norma                64
.300 Wea            65    64           55    55
.300 RUM            65    65
.300 Lap.Mag.             68
.30-378 Wea               64
 7.62x39            45
.303 Brit           49    53           45    46
 7.62x53R                 57
 7.62x54R                 57                 46
.32-20                    30           16    28    18/26
 8x72R                    41                 36
.32-40                    34           30    30    18/35
.32 Spl.            42    44           38    39
.32 Rem                   43           37    38
 8x50R                    51!                45!
 8 Lebel                                     51!
 8x57 JRS                 48                 42
 8x57 JS            35    57           37    49
 8x60RS                   49                 44
 8x64S                    59                 51
 8x65RS                   59                 51
 8x75RS                   55                 48
 8 Rem Mag          65    67           54    57
.318                      48                 42
.333 Riml.NE              48                 42
.338 Fed            62
.338 Marlin         46
.33 WCF                   44                 39    35
.338-06 A-Sq.       63                 52
.338 RCM            65
.338 Win            64    62           54    54
.340 Wea            63
.338 RUM            65
.338-378 Wea              64
.348                      46           40    41
.351 Win SL               53           45
 9 Luger            35    34           33
 9x57                     41                 36
 9x57R                    41                 36
.38 Spl             17    22           17
  " +P              19                 20
.357 Mag            35    44           45    
.357 Max            40    45           48
 400/350                  41                 36
.350 No.2                 48                 42
.350 Mag Rigby            45
.35 Rem             34    40           35    36    35
.35 WCF                   44           39    39    40
.356 Win                  60           52    52
.358 Win                  59           52    51
.350 Rem Mag              62           53    54
.35 Whelen          62    58           52
.358 STA            65
 9x53R                    49
.360 NE 2¼                36                 32
 9.3x72R                  29                 26                  
 9.3x57                   44                 38
 9.3x62                   57                 49
 9.3x64                   64                 55
 9.3x72R                  29
 9.3x74R                  49                 44
 9.3x65R                  55*                48
.375 Win                  55           52    55
.375 NE 2½                32                 29
 9.5x57 M-S               44
.376 Steyr          62    62
.375 H&H            62    62           53    54
.375 Ruger          62
.375 Wea                  64
.375 Fl.Mag.              47                 41
.369 NE                   44                 39
.378 Wea                  64                 55
.38-55                    35           30    31    20/32
.38-40                    17           14    15    16/22
.40-82 Win                24                 22
.41 Rem Mag         36    44           40
.400 Jef.                 41                 36
 450/400 3¼               43                 38
.400 H&H                  64
.405 WCF            46    36                 32    44
.416 Taylor         65    
.416 Ruger          62
.416 Rem.           65    62           54    54
.416 Rigby          52    47                 41
.404 Jef.                 53                 46
.44-40              11    16           13    15    14/19
.44 Spl             16    15           14
.44 Rem Mag         36    41           40    
.444                42    51           44    45
.45 ACP             21    19           18
.45 Colt            14    16           14
.454 Casull               57
.45-70              28    32           28    29    25
.450 Mar            44
.458 Win.           60    62           53    54
.458 Lott           63    62                       54~
.450 NE 3¼                44                 39
.450 No.2 NE              41                       36~
.460 Wea                  64                 55
.465 Belted               62
 500/465                  36                 32
.480 Ruger          48    48
.475 Linebaugh            50
.475 Turnbull       42
.470 NE             41    39           35    35
.475 No.2 NE              40                       36~
.505 Gibbs                39                 35
.500 Jef.                 48                       41~
.500 NE 3"                41                 36
.50 BMG                   54
.577 3" NE                36                 32
.600 NE                   36                 32
.700 NE                   40
 4 Bore Rifle             36


   ^   estimated from load book data (see below)
   *   estimated from CIP crusher value (see below)
   ?   possible typo in original
   !   may reflect later military loadings
   ~   A-Square standard (see below)

The CIP listings are more extensive. SAAMI standards are voluntary, and a small custom gun maker need not use an official proof load before selling a gun chambered for some obscure cartridge. European laws probably require every gun be proofed to some official standard.

There is a slight difference in definitions between the CIP and SAAMI, and this may explain why the CIP numbers are generally a bit higher; see the section on Statistics, below.

Pressures for shotshells are not included above. At SAAMI they run from 11 ksi for the 10 ga to 12.5 ksi for the .410 in the standard case lengths, but a few of the longest shotshells are as high as 14 ksi.

References
Purpose

This page was first written when neither SAAMI nor CIP provided on the web any of their specifications. After obtaining written copies for my own use, I chose to summarize the information here. Also included is historical information on cartridge pressure measurement I had come across. The table above still provides a quick comparison between the two standards, where some interesting inconsistencies exist, for example the .444 Marlin, .405 Winchester, .223 Rem, and 8 Mauser.

Conversion Formulas

No accurate conversion between crusher and true pressure exists, but approximations can be made. In all the conversions here, pressures are in ksi. Expect errors of several ksi, or about 15%, with such formulas. Many factors determine how much the indicated pressure reading from a crusher misses the true pressure, and the error varies among cartridges and even among different loads for one cartridge. The following conversions might be accurate enough for many practical purposes.

In Denton Bramwell's article, he offers a formula he derived using a basic statistical analysis of SAAMI's ratings, covering only pressures between 28,000 and 54,000 CUP :

piezo = 1.52 * crusher  - 18
He also demonstrates that within this pressure range, the CIP appears to have generally used a simple conversion between their crusher and piezo ratings, roughly equal to:
piezo = 1.21 * crusher  - 2.8
CIP pressures are multiples of 50 bar (about 700 psi), probably rounded after the conversion. (Please note that CIP crusher readings should not be equated with SAAMI CUP crusher readings; see below.)

In the 09/1968 issue of Handloader, Lloyd Brownell presents test data (crusher, but not necessarily CUP) which suggests to me a linear conversion formula is not the best choice, and in my Powley Computer I use:

piezo = crusher * ( 1 + ( crusher^2.2 )/30000 )
From 0 to about 60 ksi crusher, it fits both SAAMI's ratings and Brownell's data well, but it is low at the high end of Brownell's data. Brownell's data shows little to no error below 20 ksi, and a curve fit to only his data between 20 and 67 ksi crusher is:
piezo = crusher +  ( (crusher - 20) ^ 2.3 ) / 210
Reference Ammunition

Under SAAMI specifications, reference ammunition is required only for the qualification of new pressure barrels. A new pressure barrel must demonstrate it generates nearly the same pressure and velocity as existing SAAMI spec. pressure barrels. Reference ammunition is as uniform as possible, and ideally all pressure barrels will show the same indicated pressure and velocity. If one barrel is found to be different, something is off in either the barrel or its sensors.

Reference ammunition is not used to calibrate pressure sensors. Piezo systems are calibrated hydraulically. Crushers are calibrated by the manufacturer. (The use of reference ammunition to try to correct crusher measurements is listed as "optional.") To quote SAAMI: "Reference Ammunition cannot guarantee the absolute accuracy of any test system."

CIP procedures permit the use of reference ammunition to correct pressure readings taken at any one lab. The reference ammo has been fired at several trusted labs, and the average of these readings is the reference value. Reference cartridge pressures measured at any other lab are compared to the reference value, and if the difference is less than 10%, the offset can be used as the correction in subsequent tests at that lab.

Proof Loads

For rifles, SAAMI recommends a proof load between 33 and 44 percent over the nominal rating, and the CIP today requires 25 percent over their rating (an older standard called for 30% over). While SAAMI requires only a single proof firing, the CIP wants two firings, except in long guns designed for low pressure cartridges (under 26 ksi), where only a single proof cartridge need be fired.

For handguns, the CIP uses 30 percent over, while SAAMI varies the proof load with the rated pressure. For cartridges rated over 20 ksi, SAAMI uses the same overloads as with rifles, but low pressure cartridges have a higher overload, with those rated under 15 ksi having a minimum of 44% over.

To conduct a proper proof, one would ideally need precise gauges to verify no stressed part has yielded (ie., taken a permanent deformation) in the slightest. If no yielding occurs at the proof pressure, then the gun should have an adequate fatigue life at normal operating pressures. I've read that in practice, visual inspections are permitted on production guns, so I suspect properly instrumented proofs are only conducted on prototypes.

Interestingly, the same percentage overload is used with both piezo and crusher ratings at SAAMI. Above, it was noted there is evidence that crusher's underestimation of pressure grows ever worse as the true pressure rises. One curious side effect is that rifles proofed with crushers may well be proofed to a higher standard than those proofed with piezo.

Under the British base crusher standards described below, proof loads ran 30 to 45% above normal. To maximize breech thrust, proof cartridges were oiled before firing.

Statistics

(Sorry, this section is necessarily heavy in jargon.)

More than with many physical measurements, that of chamber pressure displays a large scatter. For this reason, SAAMI defines pressure ratings in statistical terms.

In the table above, SAAMI's Maximum Average Pressure (MAP) is listed. This is the number often quoted as the SAAMI "pressure rating," and SAAMI states the MAP "is the recommended maximum pressure level for loading commercial sporting ammunition." When loads are worked up either for production or for presentation in load books, they will be limited to the MAP, and for most practical purposes, this is the cartridge's rating.

The MAP can be a bit lower than the average in a large lot of ammunition. To determine pressure, SAAMI recommends 10 rounds be tested and averaged. With such a sampling size, there is a chance this average pressure could be below that of the larger lot. Basic statistical considerations would place the average for large lots to likely be within 2 "standard errors" of this smaller sampling. For ammunition testing, SAAMI suggests 2 standard errors will be about 2.5% of the MAP, and adding this to the MAP gives SAAMI's Maximum Probable Lot Mean (MPLM).

This MPLM is the pressure for which a gun should be designed since it is possible large runs of ammunition could average this pressure. The MPLM is, then, the actual pressure rating of the cartridge, and the SAAMI proof loads are defined relative to MPLM, specifically between 30 and 40 percent over MPLM. (Since MPLM is 2.5% over the 10 shot MAP, the compounded result is proof loads are between 33 and 44 percent of the MAP shown in the table.)

An average over 10 rounds could be below the MPLM (as reflected in the lower MAP rating), or it could be above the MPLM. When testing a lot of production ammunition, SAAMI recommends no 10 round average exceed the Maximum Probable Sample Mean (MPSM). Based again on statistics, the MPSM is taken to be 6.3% over the MAP. (While a load developed to MAP could be over the average for the production lot, the proof load is developed on the assumption the MAP was below the MPLM, therefore it's safe to assume the MPSM is above MPLM as well.)

Lastly, there is the Maximum Extreme Variation (MEV). There is a small chance that in a very large lot of ammunition, a single sample might test much higher than the averages. From statistics, SAAMI recommends an MEV no more than 20.6% above the MAP. Keep in mind that though a single cartridge might approach MEV, averages over 10 cartridges must continue to fall near MPLM (and below MPSM), so it is unlikely any significant number of cartridges approaching MEV will pass through.

To summarize, SAAMI pressure ratings reflect load development done with pressures limited to the MAP. In large production lots, the average could be a bit higher but likely will be below the MPLM. The worst case sampling of 10 in the production lot shouldn't exceed the MPSM, and the worst case single cartridge shouldn't exceed the MEV.

The CIP ratings are equivalent to SAAMI's MPLM, and this difference in definitions for the pressure rating likely explains why the CIP numbers are generally a bit higher than SAAMI's (again, MPLM is 2.5% over MAP). The CIP equivalent to SAAMI's MEV is 15% over the rating (relative to MPLM, SAAMI's MEV is 17.7% over). CIP proofs are 25% over the rating (SAAMI wants at least 30% over MPLM).

Measuring Cartridge Pressures

By 1861, the US Ordnance Dept. was using an early pressure gauge for cartridges. A hole was drilled in the cartridge case, and paper covered the hole to prevent spilling of powder while the cartridge was assembled. This hole was aligned with one in the barrel, and in the barrel's hole was a gas check, a piston, and a hardened steel knife. The knife pressed into a copper plate supported by a steel plate. After firing, pressure was applied to the knife elsewhere on the copper plate until the same cut was made, and the pressure presumed from this.

This method was soon replaced by "crushers," developed by Nobel in Europe at about the same time. Crushers are copper or lead cylinders deformed by the piston. The deformed length of the crusher is measured and compared to a table supplied by their maker with each lot of crushers. The pressure is read directly from this table. With older U.S. military tests—and perhaps with all other crusher standards as well—these tables make no allowance for dynamic effects, namely the fact the pressure peak is so brief that the crusher and its piston cannot track it. Instead, stable (usually called "static") pressures are applied to sample crushers, and the deformed lengths are recorded. After a gun firing, the length of the crusher is measured and compared to those found in the static tests.

Even in the 1800's, it was known that crushers did not accurately measure pressure. This was discovered by measuring the recoil acceleration of cannons, by marking a foil strip with a stylus on a tuning fork. Even though engraving and rotational forces on the projectile were not measured, the recoil clearly showed the pressure was higher than the crusher indicated.

Because the indicated pressure from crushers is known to be off, SAAMI many decades ago began referring to the indicated pressure from their tests as "Copper Units of Pressure," a clumsy name commonly shortened to "CUP." For low pressure cartridges such as shotshells, lead is used for the crusher, and SAAMI refers to these numbers as "LUP." Strictly speaking, such units should only be used to identify crusher readings taken in accordance with SAAMI procedures.

CUP is measured in psi. "CUP" effectively means "psi as measured by a system known to be inaccurate." The ballistician William Davis wrote he cared little for the CUP label. He preferred something like "psi(c)" to note the psi reading was taken from copper crushers.

Both SAAMI and the CIP have detailed specifications for the arrangement and dimensions of the crusher. Because these two systems are not identical, the two crusher standards can not always agree. Further, as explained above, CIP crusher ratings are generally a bit higher than SAAMI's due to differences in definitions. Also, SAAMI is generally more conservative with older military rounds, such as the 8mm Mauser.

With SAAMI's arrangement, the piston is over the brass case, and the case will rupture somewhere below 20 ksi. The resulting sudden jump in pressure under the piston magnifies problems with piston inertia, and this makes the reading more sensitive to parameters such as burning rate, case strength, and true peak pressure. The CIP arrangement requires the piston case be drilled at the sensor location, and one benefit is that crusher and piezo ratios are much more consistent from cartridge to cartridge, allowing them to reasonably use a conversion formula (above).

In Britain, a third set of crusher standards were developed, using a "base" crusher. The crusher was a short, thick tube placed behind a piston at the base of the cartridge, and the firing pin passed through the center. The cartridge case was well oiled before firing, to minimize cling to the chamber walls (if not oiled, the indicated pressures were about 25% lower). To prevent case rupture on set back of the base, the crusher was first deformed in a press to a pressure a bit lower than that expected in firing. The units were generally stated in British long tons per square inch, or tsi. Pressures indicated by this method run 10 to 20% below those indicated by radial crushers. Kynamco in England still rates their production cartridges with this method.

Piezo systems have been available since the 1920s and today are the accepted standard, but other systems to produce a pressure trace were tried. Vieille in France, who developed the first military smokeless powder, also "invented a rotating recording crusher gauge with which pressure could be measured as it varies with the time." I have no details on Vieille's crusher, but I've read he was the first to detect pressure waves inside the chamber. The Russian Serebriakov employed in 1923 "a conical crusher" to investigate burn rates in a calorimeter—again, no further details. In the U.K., J. J. Thomson began experimenting with piezo systems during WW-I. A system demonstrated to the Springfield Armory in 1921 had the chamber piston press against "a very stout stiff steel bar, which had a polished end forming a reflecting surface to act as a mirror." A light beam was reflected onto a revolving drum of film to record the pressure trace.

Piezo systems measure the displacement of electric charge in a crystal as it is compressed. For SAAMI tests, a piston in the side of the chamber is cut to conform to the case. This leaves the brass cartridge case between the gas and the sensor, but the sensor is calibrated by hydraulically pressurizing a test chamber with a case in it, and in this way the effect of the case is known.

It is possible to measure the pressure more directly, without the conformal piston. Older VihtaVouri test data (2nd ed.) was taken with the piezo sensor just in front of the case mouth, and NATO does this as well. However, the pressure is slightly lower there, and one cannot sense the pressure until the bullet's base has passed, preventing one from seeing how smooth is the initial ignition of the charge. One can also cut a small hole in the case to align with a narrow pressure sensor port, and the CIP uses this method. Either way can expose the sensor to the hot gases, leading to shorter sensor life. However, neither a conformal piston nor calibration of the brass case is required, and these approaches will likely be less expensive than SAAMI's.

Other pressure transducers can be used in place of the piezo transducer. Once more common was one using a strain gauge. Such transducers fit where the piezo unit does in modern tests.

Strain gauges can also be used in an another fashion to measure pressure. The strain gauge is glued to the surface of the barrel, over the chamber, and the "hoop" strain is measured from the small changes in resistance in the gauge's wires. The test barrel ideally has somewhat thinner walls, allowing for greater strain during firing. One must also compute an offset to compensate for the pressure being contained by the brass of the case. In a lab, this might be done hydraulically, as with piezo, but I've read it is not. The properties of cartridge brass are fairly well known, and the measurement of the brass thickness is readily done, so it's also possible to compute the offset with fair accuracy. Strain gauge systems can be quite affordable; the Pressure Trace available from RSI is marketed to the handloader. Oehler Research also sells such systems for laboratory use.

Measuring Pressures in Handloads

In the U.S., H.P.White tests ammunition, and may be able to do so for home handloads. In recent years, Western Powders advertised a modestly priced service to test handloads in their piezo rigs, but as of late 2006 that service is not listed on their web site. In Europe, the Birmingham Proof House in the U.K. will test handloads as will DEVA in Germany, and other national labs might as well.


8/2005 - 9/2013