+P Vs. Magnum Cartridges
Magnum cartridges, such as .357 Magnum or .44 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 Mag is slightly longer than the .38 spl, the lower powered round from which it was derived. +P ammunition, however, is externally identical to standard ammunition of its caliber. This was done when the ammunition manufacturers lowered the pressure of their standard rounds (1972). +P ammunition was designed to replicate the performance of what had been standard pressure cartridges. +P cartridges are not recommended to be used in firearms of questionable quality.
Read more about this topic: Overpressure Ammunition