SAAMI, the Sporting Arms and Ammunition Manufacturers' Institute, sets standards for chamber and ammunition dimensions and ammunition pressures in the United States.
Voluntary adherence by makers assures the shooter that all ammo will fit and safely fire in all firearms for which it is intended. SAAMI lists a few handgun cartridges at higher-than-normal standard pressure, called “+P.” Note that “higher pressure” does not mean “high pressure.” These +P loads in 9mm, .38 Special and .45 ACP are only nine to 10 percent above standard SAAMI pressure—nowhere near approaching magnum loads that might be twice that of the parent design. Note that these cartridges are all 100 or more years old.
Improvements in metallurgy and handgun design make +P loads viable in modern firearms, but shooting +P in older guns could be disastrous, and a steady diet of +P in some modern guns may wear them out prematurely. If your handgun is not marked, “+P,” don’t shoot +P ammo in it until you contact the firearm manufacturer and learn whether it is safe to do so. SAAMI, incidentally, does not have a standard for +P+ ammunition, which various sources list as approximately 18 to 30 percent over SAAMI standard pressure.