+P ammo information per Wikipedia.
Commercially available +P cartridges
Cartridges that are commonly boosted with +P pressures are the 9 mm Luger, .45 ACP, and .38 Special, which are all cartridges that date from the dawn of the 20th century. There has been significant improvement in metallurgy and quality since the first guns in those calibers have been made, with the result that higher pressures are now safe in modern firearms. Many models will specify to the degree they can use +P ammunition; for example, many aluminum alloy framed .38 Special revolvers should not regularly be used with +P ammunition, for while the cylinder is capable of withstanding the pressures, the added force will increase wear and reduce the service life of the gun.
SAAMI specifications for common +P cartridges are as follows:
Cartridge Standard pressure +P pressure Notes
9 mm Luger 35,000................ 38,500 10% increase
.38 Special 17,000................ 18,500 9% increase
.45 ACP 21,000................ 23,000 9.5% increase
.38 Auto 26,500................ 36,500 38% increase to make .38 Super
.45 Colt 14,000................ 27,500 96.4% increase, For use in certain modern revolvers and lever rifles
The +P+ designation is not currently used by the SAAMI, but is used by some manufacturers to designate loads that exceed the +P SAAMI specification. One source lists the 9 × 19 mm +P+ loading as having a pressure of 42,000 psi, an 18% increase over the standard pressure of 35,000 psi, and the .38 Special +P+ as 22,000, a 29% increase over the standard pressure.
Small ammunition makers and reloading guides will often include special loads for specific purposes, such as the above listed .45 Colt load from Buffalo Bore Ammunition. These loads are generally designed to provide maximum performance from older cartridges, when used in newer, stronger firearms. The 14,000 psi limit for .45 Colt, for example, reflects the black powder performance of the round, and is safe even in firearms built in 1873, when the cartridge was introduced. Using modern, solid head brass in a Ruger Blackhawk revolver, a similar design originally chambered in the high pressure .44 Magnum, the pressure can be pushed far higher with no ill effects. However, since these loads, with nearly double the pressure, could destroy a firearm intended for black powder level loads, they are less commonly encountered.
38Spl +P+ Box