From another forum........
In the U.S., voluntary pressure standards for rifle cartridges are set by SAAMI, a member of ANSI.
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. The CIP equivalent to SAAMI's MEV is 15% over the rating (relative to MPLM, SAAMI's MEV is 17.7% over).
Guns are tested for proof at 33% to 44% over NOMINAL Cartridge ratings...so for the 30/06 that would be 90-100,000 PSI. If the gun passes this test, it is considered to have a Normal Lifespan ahead of it at NOMINAL cartridge pressures...
In the following link, look at pages 11 and 15 for popular pressure specs from SAAMI
http://www.saami.org/Publications/205.pdf
