Overpressure ammunition
From Wikipedia, the free encyclopedia
Overpressure ammunition, commonly designated as +P or +P+, is small arms ammunition that has been loaded to a higher internal pressure 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 defensive purposes. Because of this, +P ammunition is found in handgun calibers which might be used for defensive purposes.
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[edit] +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 slightly longer than the lower powered rounds from which they were derived (.38 Special and .44 Special, respectively). This prevents the magnum rounds from being chambered in firearms not specifically designed for them. +P ammunition, however, is externally identical to standard ammunition of its caliber. This is commonly done to update older cartridges to take advantage of newer firearms that can handle higher pressures, and +P cartridges should NOT be used in older or weaker firearms, but only in modern, high strength designs.
[edit] Standards
In the United States, standards related to arms and ammunition are maintained and published by the Small Arms and Ammunition Manufacturer's Institute (SAAMI), which publishes standard internal pressures of calibers measured in copper units of pressure. +P rounds are loaded to 10-15% greater internal pressure, while +P+ rounds are loaded up to 25% greater pressure than standard. In both cases this is below the pressure of proof test cartridge, which all firearms are required to withstand before they may be sold. By way of comparison, magnum calibers may be loaded to nearly twice the pressure of the rounds from which they were derived. Overpressure rounds are commonly defensive rounds and are loaded by police and others in need of defensive rounds. Accordingly, most overpressure rounds are hollowpoints or other types of expanding ammunition.
It must be noted that "higher pressure" is not the same as "high pressure". The +P standard is designed so that if a shooter was to accidentally use a +P cartridge in a non-+P-rated firearm, the chance of a one-time explosive failure was minimal as long as the gun was in good physical condition. Repeated firing of +P ammunition in a gun not rated for it will drastically speed mechanical failure of the gun and could very possibly cause a hazardous condition, such as catastrophic chamber or cylinder failure which usually results in severe injury to the user. +P cartridges are loaded to pressures far below those typically found in magnums.
[edit] Commercially available +P cartridges
Cartridges that are commonly boosted with +P pressures are the 9mm 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.
[edit] Custom and handloaded overpressure cartridges
There are some blackpowder cartridges, such as the .45 Colt and .45 -70 (both dating from 1873) that are capable of being loaded to far higher levels than originally possible. Because modern firearms are much stronger than the original black powder era firearms (for example, many .45 Colt guns are built on the same frame as .44 Magnum versions) a combination of modern firearm and specially loaded ammunition can provide performance to rival modern cartridges. However, these high-pressure loading can only be used in modern firearms. Because of the potential danger in shooting these "unofficial magnum" cartridges, they are generally only available if handloaded or purchased from low-volume specialty manufacturers. SAAMI specfications may or may not exist for these loads, so extra caution must be used. Generally, the manufacturer will specify exactly which makes and/or models of firearm can or cannot be safely used with a given load, such as "Only for use in Ruger and Thompson/Center Contenders" or "Only for use in modern single shot or bolt action designs, not for Marlin lever action or any black powder action".
[edit] +P ammunition use
Unless the firearm is explicitly marked as being +P rated or it is clearly stated in the gun's manual, +P ammunition should not be used. If in doubt, a check by a gunsmith or a phone call to the gun's manufacturer will verify the safety of +P ammunition in a particular firearm. Ammunition that is loaded to +P pressures is clearly marked on the headstamp as such, usually in the form "9mm Luger +P".
