Overpressure ammunition
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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] History
The burning characteristics of black powder used in early cartridges meant that these cartridges operated at low pressures, generally uder 25000 psi. These cartridges were limited by their case capacity, and the only way to get more power was to increase the case 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 (.40-70) to 110 grains (.45-110)[1]. With the advent of smokeless powder, whose burn rate increases as the pressure increases, pressure became a more significant concern. Some modern cartridges run at pressures over 60000 psi.
The first cartridge deliberately loaded by a major manufacturer to higher than standard pressure was the .38-44 HV load, which was a .38 Special cartridge loaded specifically for use in Smith & Wesson's large frame ".38-44 Heavy Duty" revolvers, and loaded to a much higher velocity than standard .38 Special ammunition. The .38-44 revolvers were made by using a .44 Special size gun but barrel and cylinder bored to .38 Special. Since the frame, cylinder, and barrel were much stronger than the standard .38 Special components, it was capable of withstanding much higher pressures. The .38-44 HV round, while no longer available, was in most cases the equal of the later .357 Magnum, which works at more than double the pressure of standard .38 Special[2].
[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, formerly measured in copper units of pressure and currently in psi based on piezoelectric instrumentation. Official +P pressures are established by the SAAMI for certain cartridges; in general the +P pressure is approximately 10% higher than the standard pressure (see chart below). SAAMI does not have a +P+ pressure standard, but this indicates a pressure higher than the +P loading. In both cases this is below the pressure of proof test cartridge, which all firearms are required to withstand before they may be sold. Proof pressure are established by the SAAMI, as a percentage of the working pressure, so this places an upper bound on the +P+ pressures of 30-40%[3]. 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.
SAAMI specifications for common +P cartridges are as follows:
Cartridge | Standard pressure | +P pressure | Notes |
---|---|---|---|
9 x 19 mm | 35,000 | 38,500 | 10% increase |
.38 Special | 17,000 | 18,500 | 9% increase |
.45 ACP | 21,000 | 23,500 | 9.5% increase |
.38 Auto | 26,500 | 33,500 | 25% increase to make .38 Super |
.38 Super | 33,000 | 36,500 | 12% increase |
.45 Colt | 14,000 | 25,000 | 79% increase, Ruger only load |
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 x 19 mm +P+ loading as having a pressure of 42,000 psi, an 18% increase over the standard pressure of 35,000 psi[4], 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 for Ruger revolvers. These loads are generally designed to provide maximum performance from older cartridges, when used in newer, stronger firearms. The 17,000 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 catridge 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.
[edit] Custom and handloaded overpressure cartridges
Some older cartridges, especially those that were originally 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".
In addition to the SAAMI recognized Ruger .45 Colt load, there are many other higher pressure specialized loads available. In many cases, these loads are not pressure tested, but are tested by firing in particular firearms, then checked for signs of excessive pressure. In some cases, high pressure ammunition is restricted to law enforcement sales, such as Federal's .38 Special and 9mm +P+ Hydra-Shok[5] cartridges. The following table lists some non-SAAMI +P loads for which the manufacturer publishes pressure information.
Cartridge | Standard pressure | +P pressure | Notes |
---|---|---|---|
.45-70 | 28,000 | 35,000 | 25% increase, Garrett Cartdidge |
.44 Magnum | 36,000 | 43,500 | 21% increase, Garrett Cartdidge |
[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".
In addition to questions of safety and durability are issues of reliability and useabilty. Since +P cartridges may generate a significantly different quality of recoil, this can impact firearm function. For example, recoil operated firearms may fail to function if the velocity of the recoiling parts is too high; in lightweight revolvers, the cartridge case may recoil away from the bullet with sufficient force to overcome the crimp, allowing the bullet to move forward in the cylinder and causing the cylinder to bind[6]. The increased velocities and pressures of a +P loading will increase muzzle blast and recoil, and may proove difficult to handle for many shooters; these problems are excacerbated by compact, lightweight guns with short barrels.
[edit] +P ammunition and velocity
In general, the purpose of a +P cartridge is to get a higher velocity for a given bullet weight than a standard pressure cartridge. However, the pressure rating used to determine if a round is +P is the peak pressure, which is not an accurate indication of the velocity, since it is the area under the pressure curve that determines the total energy imparted to the bullet (see internal ballistics). A large number of factors can impact the peak pressure of a load, such as:
- Bullet weight
- Bullet seating depth, which is a factor of
- Bullet material
- Bullet shape
- Bullet diameter
- Test barrel diameter
- Test barrel chamber shape
- Bullet hardness
- Friction in bore
- Crimp strength
- Gunpowder burn rate
- Primer strength
- Cartridge case volume
Because of these factors, it is possible two have two loads where each is propelling the same bullet weight at the same velocity, but one is a standard pressure load and one is a +P load. Even in the same firearm, with the same components, high pressure, low capacity cartridges such as the .40 S&W have been shown to have a 20% increase in pressure with as little as a 0.05 inch (1.2 mm) change in seating depth[7].
[edit] References
- Demystifying +P, Handguns Magazine
- SAAMI pressure chart