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Old August 6, 2012, 05:09 PM   #53
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 9,997
CAUTION: The following post discusses loading beyond currently published maximums for cartridges. Rely on the information AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.

62,000 is correct, but it's for a standard SAAMI test that averages the peak pressures of just 10 shots. They allow 4% standard deviation and 20.6% increase as the extreme variation with 99% confidence. In other words, you could run over 74,800 psi for one shot in 100 and still be keeping SAAMI happy. These are all below the 83,000-89,000 psi proof load range.

Most handloaders can hold down pressure standard deviation below 4% pretty easily, allowing them to aim higher than SAAMI MAP without physical danger. Figure that 4% SD is equivalent to close to 2% powder charge error, or ±0.9 grains out of 45 grains in a .308). This is one reason many handloaders get away with higher average pressures than SAAMI specs would allow.

On the other hand, there is a price to be paid in wear and tear on the barrel and on bolt lugs and all other parts subjected to the forces of pressure and friction. Figure a barrel at 67,000 psi might have half the throat life of one fired at 57,000 psi, and one at 47,000 psi may last half again longer than one at 57,000 psi. For a .308 we might be looking at 47kpsi, 57kpsi, and 67kpsi as having around 5000, 3500 and <2000 round life, respectively (and depending on powder choices and barrel cooling between shots and whatnot), before fliers started appearing on targets. For match rifles these are possible kinds of numbers.
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