Overpressure ammunition, commonly designated as +P or +P+, is

small arms A firearm is any type of gun designed to be readily carried and used by an individual. The term is legally defined further in different countries (see Legal definitions). The first firearms originated in 10th-century China, when bamboo tubes c ...

ammunition Ammunition (informally ammo) is the material fired, scattered, dropped, or detonated from any weapon or weapon system. Ammunition is both expendable weapons (e.g., bombs, missiles, grenades, land mines) and the component parts of other weap ...

that has been loaded to produce a higher internal pressure when fired than is standard for ammunition of its

caliber In guns, particularly firearms, caliber (or calibre; sometimes abbreviated as "cal") is the specified nominal internal diameter of the gun barrel Gauge (firearms) , bore – regardless of how or where the bore is measured and whether the f ...

(see

internal ballistics Internal ballistics (also interior ballistics), a subfield of ballistics, is the study of the propulsion of a projectile. In guns, internal ballistics covers the time from the propellant's ignition until the projectile exits the gun barrel. The s ...

), but less than the pressures generated by a proof round. This is done typically to produce rounds with a higher

muzzle velocity Muzzle velocity is the speed of a projectile (bullet, pellet, slug, ball/shots or shell) with respect to the muzzle at the moment it leaves the end of a gun's barrel (i.e. the muzzle). Firearm muzzle velocities range from approximately to i ...

and

stopping power Stopping power is the ability of a weapon – typically a ranged weapon such as a firearm – to cause a target (human or animal) to be incapacitated or immobilized. Stopping power contrasts with lethality in that it pertains only to a weapon's ...

, such as ammunition used for

defensive Defense or defence may refer to: Tactical, martial, and political acts or groups * Defense (military), forces primarily intended for warfare * Civil defense, the organizing of civilians to deal with emergencies or enemy attacks * Defense indust ...

hunting Hunting is the human activity, human practice of seeking, pursuing, capturing, or killing wildlife or feral animals. The most common reasons for humans to hunt are to harvest food (i.e. meat) and useful animal products (fur/hide (skin), hide, ...

purposes. Because of this, +P ammunition is typically found in

handgun A handgun is a short- barrelled gun, typically a firearm, that is designed to be usable with only one hand. It is distinguished from a long gun (i.e. rifle, shotgun or machine gun, etc.), which needs to be held by both hands and also braced ...

calibers which might be used for defensive purposes.

+P vs. magnum cartridges

Magnum cartridges, such as the

.357 Magnum The .357 Smith & Wesson Magnum, .357 S&W Magnum, .357 Magnum, or 9×33mmR as it is known in unofficial metric designation, is a smokeless powder cartridge with a bullet diameter. It was created by Elmer Keith, Phillip B. Sharpe, and Douglas B. ...

, 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 The .38 Special, also commonly known as .38 S&W Special (not to be confused with .38 S&W), .38 Smith & Wesson Special, .38 Spl, .38 Spc, (pronounced "thirty-eight special"), or 9x29mmR is a rimmed, centerfire cartridge designed by Smith & ...

, 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. +P ammunition is not recommended to be used in firearms of questionable quality or in a state of disrepair.usually because the extra chamber pressure may cause hazards or malfunctions

History

The burning characteristics of

black powder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, carbon (in the form of charcoal) and potassium nitrate (saltpeter). Th ...

used in early cartridges meant that these cartridges operated at low pressures, generally under ~25,000

psi Psi, PSI or Ψ may refer to: Alphabetic letters * Psi (Greek) (Ψ, ψ), the 23rd letter of the Greek alphabet * Psi (Cyrillic) (Ѱ, ѱ), letter of the early Cyrillic alphabet, adopted from Greek Arts and entertainment * "Psi" as an abbreviatio ...

. 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 Sharps Rifle Manufacturing Company was the manufacturer of the Sharps Rifle. The company was organized by Samuel Robbins and Richard S. Lawrence as a holding company in Hartford, Connecticut, on October 9, 1851 with $100,000 in capital. Despite ...

, which made rifles with nominal powder capacities from 70 grains (.45-70) to 110 grains (.45-110). With the advent of

smokeless powder Finnish smokeless powderSmokeless powder is a type of propellant used in firearms and artillery that produces less smoke and less fouling when fired compared to gunpowder ("black powder"). The combustion products are mainly gaseous, compared to a ...

, 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. Cartridges such as the

.32-20 Winchester The .32-20 Winchester, also known as .32 WCF (Winchester center fire), was the first small-game lever-action cartridge that Winchester produced.

and

.44-40 Winchester The .44-40 Winchester, also known as .44 Winchester, .44 WCF (Winchester Center Fire), and .44 Largo (in Spanish-speaking countries) was introduced in 1873 by the Winchester Repeating Arms Company. It was the first metallic centerfire cartridge m ...

were chambered in both revolvers and lever action rifles, and the rifles' actions could handle much higher pressures. This led to "machine gun only" loads in these calibers, which provided far more velocity and energy in the rifles, but were not safe in the revolvers due to the higher pressures they generated. These loads were eventually dropped due to a combination of safety concerns, and new smokeless rifle cartridges that offered even higher velocities, such as the

.30-30 Winchester The .30-30 Winchester/.30 Winchester Center Fire cartridge was first marketed in 1895 for the Winchester Model 1894 lever-action rifle..38 ACP The .38 ACP ( Automatic Colt Pistol), also known as the .38 Auto or 9x23mmSR, is a semi-rimmed pistol cartridge that was introduced at the turn of the 20th century for the John Browning-designed Colt M1900. It was first used in Colt's Model ...

, originally introduced in 1900. This cartridge provided performance similar to other .38 caliber cartridges of the time. However, in 1929 the cartridge was redesigned as the .38 Super Automatic, or just

.38 Super The .38 Super, also known as .38 Super +P, .38 Super Auto, .38 Super Automatic, .38 Super Automatic +P, or 9×23mmSR, is a pistol cartridge that fires a bullet. It was introduced in the late 1920s as a higher pressure loading of the .38 ACP, a ...

, along with a significant increase in operating pressure and muzzle energy, making it the most powerful auto pistol cartridge available, in both energy and velocity, for many years. Like the "rifle only" loads, the .38 Super could still be chambered in the older .38 ACP guns, producing a dangerous combination. Capable of reaching a muzzle energy of 500 ft-lbs, the .38 Super remains a viable defensive cartridge, though it is most popular in shooting sports such as IPSC. A similar move to a high pressure loading was done on the

in 1930, producing the .38-44 HV loadings, and eventually leading to the development and production of the

in 1935.

Standards

In the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

, standards related to arms and ammunition are maintained and published by the

Sporting Arms and Ammunition Manufacturers' Institute The Sporting Arms and Ammunition Manufacturers' Institute (SAAMI, pronounced "Sammy") is an association of American manufacturers of firearms, ammunition, and components. SAAMI is an accredited standards developer that publishes several Americ ...

(SAAMI), which publishes standard internal pressures of calibers, formerly measured in

copper units of pressure Copper units of pressure or CUP, and the related lead units of pressure or LUP, are terms applied to pressure measurements used in the field of internal ballistics for the estimation of chamber pressures in firearms. These terms were adopted by c ...

and currently in

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, approximately 25% higher than the standard pressure. In both cases this is below the pressure of a proof test cartridge. Proof pressures 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%. 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 maximum power in a compact firearm. Accordingly, most overpressure rounds are hollow points or other types of expanding ammunition. "Higher pressure" is not the same as "high pressure"; +P cartridges are generally loaded to pressures far below those typically found in magnum cartridges. The +P standard is designed so that if a shooter were to accidentally use a +P cartridge in a non +P rated firearm, the chance of a one-time explosive failure is 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, however, and so it should only be used in firearms designated by the manufacturer as safe for +P use.

Commercially available +P cartridges

Cartridges that are commonly improved with +P pressures are the 9mm Parabellum,

.45 ACP The .45 ACP ( Automatic Colt Pistol) or .45 Auto (11.43×23mm) is a rimless straight-walled handgun cartridge designed by John Moses Browning in 1904, for use in his prototype Colt semi-automatic pistol. After successful military trials, it ...

.38 ACP The .38 ACP ( Automatic Colt Pistol), also known as the .38 Auto or 9x23mmSR, is a semi-rimmed pistol cartridge that was introduced at the turn of the 20th century for the John Browning-designed Colt M1900. It was first used in Colt's Model ...

, and

.257 Roberts The .257 Roberts, also known as .257 Bob, is a medium-powered .25 caliber rifle cartridge. It has been described as the best compromise between the low recoil and flat trajectory of smaller calibers such as the 5 mm (.22") and 6 mm (.24"), and ...

which are all cartridges that date from the late 19th and the early 20th century. There has been significant improvement in

metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the sc ...

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 the degree to which they can use +P ammunition; for example, many

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...

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: The +P+ designation is not currently used by SAAMI, but is used by some manufacturers to designate loads that exceed the +P SAAMI specifications. One source lists the 9×19mm +P+ loading as having a pressure of 42,000 psi, a 20% increase over the standard pressure of 35,000 psi, and the .38 Special +P+ as 22,000 psi, a 25.71% increase over the standard pressure of 17,500 psi. Small ammunition makers and reloading guides will often include special loads for specific purposes, such as the below listed

.45 Colt The .45 Colt (11.43×33mmR), is a rimmed, straight-walled, handgun cartridge dating to 1872. It was originally a black-powder revolver round developed for the Colt Single Action Army revolver. This cartridge was adopted by the U.S. Army in 1 ...

load from Buffalo Bore Ammunition. These loads are generally designed to provide maximum performance from older cartridges, when used in newer, stronger firearms. The 14,000 psi limit for .45 Colt, for example, reflects the

performance of the round, and is safe even in firearms built in 1873, when the cartridge was introduced. Using modern, solid head brass in a

Ruger Blackhawk The Ruger Blackhawk is a six-shot, single-action revolver manufactured by Sturm, Ruger & Co. It is produced in a variety of finishes, calibers, and barrel lengths. History In the early 1950s, Westerns were popular in movies and television. Colt ...

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.

Custom and handloaded overpressure cartridges

Some older cartridges, especially those that were originally black-powder cartridges, such as the

(1873) and .45-70 (1873), are capable of being loaded to far higher levels than was 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 Handloading, or reloading, is the practice of making firearm cartridges by assembling the individual components (case, primer, propellant, and projectile), rather than purchasing mass-assembled, factory-loaded ammunition. The term ''hand ...

or purchased from low-volume specialty manufacturers. SAAMI specifications may or may not exist for these loads, so extra caution must be used. Generally, the manufacturer or data publisher 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"'', ''"Use only in modern Marlin and Winchester repeating actions"'' or ''"Ruger No. 1 and No. 3"''. 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 +P+ and 9mm Parabellum +P+ Hydra-Shok cartridges. The following table lists some non-SAAMI +P loads for which the manufacturer publishes pressure information.

+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 contacting 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 A headstamp is the markings on the bottom of a cartridge case designed for a firearm. It usually tells who manufactured the case. If it is a civilian case it often also tells the caliber: if it is military, the year of manufacture is often added ...

as such, for example a 9mm would be marked "9mm Parabellum +P". The use of +P or +P+ ammunition does accelerate wear and reduces the service life on the component parts on any pistol. In addition to questions of safety and durability are issues of reliability and usability. Since +P cartridges may generate a significantly different quality of

recoil Recoil (often called knockback, kickback or simply kick) is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, as according to Newton's third law the force requ ...

, this can affect firearm function. For example,

recoil operated Recoil operation is an operating mechanism used to implement locked breech, autoloading firearms. Recoil operated firearms use the energy of recoil to cycle the action, as opposed to gas operation or blowback operation using the pressure of the p ...

firearms may fail to function if the velocity of the recoiling parts is too high; in lightweight

revolver A revolver (also called a wheel gun) is a repeating handgun that has at least one barrel and uses a revolving cylinder containing multiple chambers (each holding a single cartridge) for firing. Because most revolver models hold up to six roun ...

s, 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. The increased velocities and pressures of a +P loading will increase

muzzle blast A muzzle blast is an explosive shockwave created at the muzzle of a firearm during shooting. Before a projectile leaves the gun barrel, it obturates the bore and "plugs up" the pressurized gaseous products of the propellant combustion behind i ...

and recoil, and may prove difficult to handle for many shooters; these problems are exacerbated by compact, lightweight guns with short barrels.

+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

). 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 *

Smokeless powder Finnish smokeless powderSmokeless powder is a type of propellant used in firearms and artillery that produces less smoke and less fouling when fired compared to gunpowder ("black powder"). The combustion products are mainly gaseous, compared to a ...

burn rate *Primer strength *Cartridge case volume Because of these factors, it is possible to 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, cartridges with low powder capacity and high operating pressures, such as the .40 S&W, have been shown to have a significant increase in pressure with very minor differences in bullet seating depth. One example in .40 S&W demonstrated a 20% pressure increase with a 0.05 inch (1.2 mm) change in seating depth.

References

{{Reflist
Demystifying +P
Handguns Magazine

Ammunition