38 Special 17,000 PSI
38 Special +P 20,000 PSI (Peak pressure)

For +P, the allowable PEAK pressure is 3,000 PSI higher.
Since the speed is determined by the total force for the time the bullet is in the barrel, you can't predict MV with peak pressure alone.

I can load a +P with very fast powder that has LESS MV than a standard load with a medium powder.

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