Overpressure ammunition

Overpressure ammunition, commonly designated as +P or +P+, is small arms ammunition that has been loaded to produce a higher internal pressure when fired than is standard for ammunition of its caliber (see internal ballistics), but less than the pressures generated by a proof round. This is done typically to produce rounds with a higher muzzle velocity and stopping power, such as ammunition used for security, defensive or hunting purposes. Because of this, +P ammunition is typically found in handgun calibers which might be used for armed security and defensive purposes. Magnum cartridges, such as the .357 Magnum, are usually developed by greatly increasing the working pressure of an existing cartridge, and the resulting cartridges are typically different in some small manner to prevent them from being chambered in firearms not specifically designed for them. For example, the .357 Magnum is slightly longer than the .38 Special, which is the parent cartridge of lower pressure from which it was derived. +P ammunition, however, is externally identical to standard ammunition of its caliber. It is not an advisable practice to utilize +P ammunition in firearms of questionable quality or in a state of disrepair. In such cases, the margin of safety may be eroded to the extent that hazards or malfunctions will result. The burning characteristics of black powder used in early cartridges meant that these cartridges operated at low pressures, generally under 25,000 psi. These cartridges were limited by their case capacity, and the only way to get more power was to increase the case dimensions to hold more powder; this can be seen in firearms such as those made by Sharps Rifle Manufacturing Company, which made rifles with nominal powder capacities from 70 grains (.45-70) to 110 grains (.45-110). With the advent of smokeless powder, which has a far greater energy density than black powder, it was possible to generate far more power in the large cases of the older black-powder cartridges such as the .32-40 Winchester and .