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Every gun manufacturer will warn against using +P+ ammunition because there are no standards that define "+P+".

SAAMI defines pressure limits and other aspects of ammunition standards for regular ammunition. SAAMI also defines "+P" standards for certain calibers (9 mm +P, .38 SP +P, .38 Super +P, and .45 AUTO +P).

For what it is worth, SAAMI .357 Mag allows 37,800 PSI maximum probable sample mean pressure and does not even define a +P, much less +P+.

http://www.saami.org/specifications_and ... ndex.cfm#1" onclick="window.open(this.href);return false;

However, SAAMI does not define +P+.

Thus, a homegrown idiot ammunition manufacturer could conceivably load ammo into a .357 Mag case to 75,000 psi and call it "+P+". Blow up your gun or your hand, but there is no SAAMI limit. Thus no manufacturer will ever approve +P+.

This gets to the second question, why in the world would you want a .357 to be loaded that heavy anyway? Are you going to hunt Alaska Brown Bear with a .357 Mag? Loads that heavy are certainly not helpful or useful for self defense anyway, since the main worry with .357 Mag is overpenetration, not underpenetration.