Overpressure ammunition
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 typically found in handgun
calibers which might be used for defensive purposes.
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 Special
, 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 that had been standard pressure cartridges. +P cartridges are not recommended to be used in firearms of questionable quality, but rather in proven manufacturers.
(~172,369 kPa
). 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 (.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. Cartridges such as the .32-20 Winchester
and .44-40 Winchester
were chambered in both revolvers and lever action rifles, and the rifles' actions could handle much higher pressures. This led to "rifle 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 first modern smokeless powder cartridge deliberately loaded by a major manufacturer to higher than standard pressure was the .38 ACP
, originally introduced in 1900. This cartridge provided performance similar to other .38 caliber cartridges of the time. However, in 1929 the cartridge was renamed the .38 Super Automatic, or just .38 Super
, 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. It was commonly found in the M1911 pistol, and was also available in the Thompson submachine gun
. 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 .38 Special
in 1930, producing the .38-44 HV loadings, and eventually leading to the development of the .357 Magnum
.
, standards related to arms and ammunition are maintained and published by the Sporting Arms and Ammunition Manufacturers' 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%. 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.
It must be noted that "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 was 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.
, .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:
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 × 19 mm +P+ loading as having a pressure of 42,000 psi, an 18% increase over the standard pressure of 35,000 psi, 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 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 black powder 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
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.
(both dating from 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
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 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 cartridges. The following table lists some non-SAAMI +P loads for which the manufacturer publishes pressure information.
as such, for example a 9mm would be marked "9mm Luger +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
, this can effect firearm function. For example, recoil operated
firearms may fail to function if the velocity of the recoiling parts is too high; in lightweight revolver
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
and recoil, and may prove difficult to handle for many shooters; these problems are exacerbated by compact, lightweight guns with short barrels.
). A large number of factors can impact the peak pressure of a load, such as:
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.
Small arms
Small arms is a term of art used by armed forces to denote infantry weapons an individual soldier may carry. The description is usually limited to revolvers, pistols, submachine guns, carbines, assault rifles, battle rifles, multiple barrel firearms, sniper rifles, squad automatic weapons, light...
ammunition
Ammunition
Ammunition is a generic term derived from the French language la munition which embraced all material used for war , but which in time came to refer specifically to gunpowder and artillery. The collective term for all types of ammunition is munitions...
that has been loaded to a higher internal pressure than is standard for ammunition of its caliber
Caliber
In guns including firearms, caliber or calibre is the approximate internal diameter of the barrel in relation to the diameter of the projectile used in it....
(see internal ballistics
Internal ballistics
Internal ballistics, a subfield of ballistics, is the study of a projectile's behavior from the time its propellant's igniter is initiated until it exits the gun barrel...
), but less than the pressures generated by a proof round. This is done typically to produce rounds with a higher muzzle velocity
Muzzle velocity
Muzzle velocity is the speed a projectile has at the moment it leaves the muzzle of the gun. Muzzle velocities range from approximately to in black powder muskets , to more than in modern rifles with high-performance cartridges such as the .220 Swift and .204 Ruger, all the way to for tank guns...
and stopping power
Stopping power
Stopping power is a colloquial term used to describe the ability of a firearm or other weapon to cause a penetrating ballistic injury to a target, human or animal, sufficient to incapacitate the target where it stands....
, such as ammunition used for defensive purposes. Because of this, +P ammunition is typically found in handgun
Handgun
A handgun is a firearm designed to be held and operated by one hand. This characteristic differentiates handguns as a general class of firearms from long guns such as rifles and shotguns ....
calibers which might be used for defensive purposes.
+P vs. magnum cartridges
Magnum cartridges, such as .357 Magnum.357 Magnum
The .357 S&W Magnum , or simply .357 Magnum, is a revolver cartridge created by Elmer Keith, Phillip B. Sharpe, Colonel D. B. Wesson of firearms manufacturer Smith & Wesson, and Winchester. It is based upon Smith & Wesson's earlier .38 Special cartridge. The .357 Magnum cartridge was introduced in...
or .44 Magnum
.44 Magnum
The .44 Remington Magnum, or simply .44 Magnum, is a large-bore cartridge originally designed for revolvers. After introduction, it was quickly adopted for carbines and rifles...
, 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 Special
.38 Special
The .38 Smith & Wesson Special is a rimmed, centerfire cartridge designed by Smith & Wesson. It is most commonly used in revolvers, although some semi-automatic pistols and carbines also use this round...
, 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 that had been standard pressure cartridges. +P cartridges are not recommended to be used in firearms of questionable quality, but rather in proven manufacturers.
History
The burning characteristics of black powder used in early cartridges meant that these cartridges operated at low pressures, generally under 25,000 psiPounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
(~172,369 kPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
). 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
Sharps Rifle Manufacturing Company
Sharps Rifle Manufacturing Company was the manufacturer of Sharps Rifle. It 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. John C. Palmer was president, Christian Sharps an engineer, and Richard S....
, which made rifles with nominal powder capacities from 70 grains (.45-70
.45-70
The .45-70 rifle cartridge, also known as .45-70 Government, was developed at the U.S. Army's Springfield Armory for use in the Springfield Model 1873...
) 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. Cartridges such as the .32-20 Winchester
.32-20 Winchester
The .32-20 Winchester, also known as the .32 WCF , was the first small-game lever-action cartridge thatWinchester produced. It was initially introduced as a blackpowder cartridge in 1882 for small-game, varmint hunting, and deer. Colt produced a single-action pistol chambered for this cartridge a...
and .44-40 Winchester
.44-40 Winchester
The .44-40 Winchester, also known as the .44 Winchester, the .44 WCF , and the .44 Largo was introduced in 1873 by the Winchester Repeating Arms Company. It was the first centerfire metallic cartridge offered by Winchester,and was brought out as the standard chambering for the new Winchester Model...
were chambered in both revolvers and lever action rifles, and the rifles' actions could handle much higher pressures. This led to "rifle 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
.30-30 Winchester
The .30-30 Winchester/.30 Winchester Center Fire/7.62×51mmR cartridge was first marketed in early 1895 for the Winchester Model 1894 lever-action rifle. The .30-30 , as it is most commonly known, was the USA's first small-bore, sporting rifle cartridge designed for smokeless powder. The .30-30 is...
.
The first modern smokeless powder cartridge deliberately loaded by a major manufacturer to higher than standard pressure was the .38 ACP
.38 ACP
The .38 ACP also known as the .38 Auto was introduced at the turn of the 20th century for the Browning designed Colt M1900. The cartridge headspaces on the rim. It had first been used in his Model 1897 prototype, which Colt did not produce...
, originally introduced in 1900. This cartridge provided performance similar to other .38 caliber cartridges of the time. However, in 1929 the cartridge was renamed the .38 Super Automatic, or just .38 Super
.38 Super
The .38 Super or .38 Super Automatic is a pistol cartridge that fires a diameter bullet. The Super was introduced in the late 1920s as a higher pressure loading of the .38 ACP or .38 Auto. The old .38 ACP propelled a bullet at . The improved .38 Super Auto pushed the same bullet at...
, 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. It was commonly found in the M1911 pistol, and was also available in the Thompson submachine gun
Thompson submachine gun
The Thompson is an American submachine gun, invented by John T. Thompson in 1919, that became infamous during the Prohibition era. It was a common sight in the media of the time, being used by both law enforcement officers and criminals...
. 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
International Practical Shooting Confederation
The International Practical Shooting Confederation is a shooting sport based on the concept of practical shooting. Accuracy, power and speed are all required to achieve a maximum score.-Founding and organization:...
. A similar move to a high pressure loading was done on the .38 Special
.38 Special
The .38 Smith & Wesson Special is a rimmed, centerfire cartridge designed by Smith & Wesson. It is most commonly used in revolvers, although some semi-automatic pistols and carbines also use this round...
in 1930, producing the .38-44 HV loadings, and eventually leading to the development of the .357 Magnum
.357 Magnum
The .357 S&W Magnum , or simply .357 Magnum, is a revolver cartridge created by Elmer Keith, Phillip B. Sharpe, Colonel D. B. Wesson of firearms manufacturer Smith & Wesson, and Winchester. It is based upon Smith & Wesson's earlier .38 Special cartridge. The .357 Magnum cartridge was introduced in...
.
Standards
In the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, standards related to arms and ammunition are maintained and published by the Sporting Arms and Ammunition Manufacturers' Institute
Sporting Arms and Ammunition Manufacturers' Institute
The Sporting Arms and Ammunition Manufacturers' Institute is an association of American firearms and ammunition manufacturers...
(SAAMI), which publishes standard internal pressures of calibers, formerly measured in copper units of pressure
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 convention to indicate that the pressure values were...
and currently in psi
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
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%. 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.
It must be noted that "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 was 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 boosted with +P pressures are the 9 mm Luger9 mm Luger Parabellum
The 9×19mm Parabellum cartridge was designed by Georg Luger and introduced in 1902 by the German weapons manufacturer Deutsche Waffen- und Munitionsfabriken for their Luger semi-automatic pistol...
, .45 ACP
.45 ACP
The .45 ACP , also known as the .45 Auto by C.I.P., is a cartridge designed by John Browning in 1904, for use in his prototype Colt semi-automatic .45 pistol and eventually the M1911 pistol adopted by the United States Army in 1911.-Design and history:The U.S...
, and .38 Special
.38 Special
The .38 Smith & Wesson Special is a rimmed, centerfire cartridge designed by Smith & Wesson. It is most commonly used in revolvers, although some semi-automatic pistols and carbines also use this round...
, which are all cartridges that date from the dawn of the 20th century. There has been significant improvement in metallurgy
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
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
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
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 mm Luger | 35,000 | 38,500 | 10% increase |
| .38 Special .38 Special The .38 Smith & Wesson Special is a rimmed, centerfire cartridge designed by Smith & Wesson. It is most commonly used in revolvers, although some semi-automatic pistols and carbines also use this round... |
17,000 | 18,500 | 9% increase |
| .45 ACP .45 ACP The .45 ACP , also known as the .45 Auto by C.I.P., is a cartridge designed by John Browning in 1904, for use in his prototype Colt semi-automatic .45 pistol and eventually the M1911 pistol adopted by the United States Army in 1911.-Design and history:The U.S... |
21,000 | 23,000 | 9.5% increase |
| .38 Auto | 26,500 | 36,500 | 38% increase to make .38 Super .38 Super The .38 Super or .38 Super Automatic is a pistol cartridge that fires a diameter bullet. The Super was introduced in the late 1920s as a higher pressure loading of the .38 ACP or .38 Auto. The old .38 ACP propelled a bullet at . The improved .38 Super Auto pushed the same bullet at... |
| .45 Colt .45 Colt The .45 Colt cartridge is a handgun cartridge dating to 1872. It began as a black powder revolver round developed for the Colt Single Action Army revolver, but is offered as a magnum level handgun hunting round in modern usage. This cartridge was adopted by the U.S. Army in 1873 and served as the... |
14,000 | 27,500 | 96.4% increase, For use in certain modern revolvers and lever rifles |
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 × 19 mm +P+ loading as having a pressure of 42,000 psi, an 18% increase over the standard pressure of 35,000 psi, 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
.45 Colt
The .45 Colt cartridge is a handgun cartridge dating to 1872. It began as a black powder revolver round developed for the Colt Single Action Army revolver, but is offered as a magnum level handgun hunting round in modern usage. This cartridge was adopted by the U.S. Army in 1873 and served as the...
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 black powder 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
Ruger Blackhawk
The Ruger Blackhawk is a 6-shot or 8-shot, single-action revolver manufactured by Sturm, Ruger. It is produced in a variety of finishes, calibers, and barrel lengths.-History:In the early 1950s, Westerns were popular in movies and television...
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 blackpowder cartridges, such as the .45 Colt and .45-70.45-70
The .45-70 rifle cartridge, also known as .45-70 Government, was developed at the U.S. Army's Springfield Armory for use in the Springfield Model 1873...
(both dating from 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
Handloading or reloading is the process of loading firearm cartridges or shotgun shells by assembling the individual components , rather than purchasing completely assembled, factory-loaded cartridges...
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 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 cartridges. The following table lists some non-SAAMI +P loads for which the manufacturer publishes pressure information.
| Cartridge | Standard pressure | +P pressure | Notes |
|---|---|---|---|
| .44 Magnum | 36,000 | 43,500 | 21% increase, Garrett Cartridge, Ruger and Dan Wesson DA revolvers, long-frame single action conversions |
| .45 Colt | 14,000 | 23,500 | 68% increase, Accurate Powder loading manual, Ruger and T/C only |
| .45-70 | 28,000 | 35,000 | 25% increase, Garrett Cartridge, modern rifles including lever and break actions |
| .45-70 | 28,000 | 40,000 | 43% increase, Accurate Powder loading manual, Ruger and similar high strength actions |
+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 headstampHeadstamp
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 Luger +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
Recoil is the backward momentum of a gun when it is discharged. In technical terms, the recoil caused by the gun exactly balances the forward momentum of the projectile and exhaust gasses, according to Newton's third law...
, this can effect firearm function. For example, recoil operated
Recoil operation
Recoil operation is an operating mechanism used in locked-breech, autoloading firearms. As the name implies, these actions use the force of recoil to provide energy to cycle the action...
firearms may fail to function if the velocity of the recoiling parts is too high; in lightweight revolver
Revolver
A revolver is a repeating firearm that has a cylinder containing multiple chambers and at least one barrel for firing. The first revolver ever made was built by Elisha Collier in 1818. The percussion cap revolver was invented by Samuel Colt in 1836. This weapon became known as the Colt Paterson...
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
Muzzle flash
Muzzle blast is the term used to describe the release of high temperature, high pressure gases expelled from the muzzle of a firearm when it is discharged. Muzzle flash is the term used to describe the visible light of the muzzle blast. The blast and flash are caused by the combustion products of...
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 internal ballisticsInternal ballistics
Internal ballistics, a subfield of ballistics, is the study of a projectile's behavior from the time its propellant's igniter is initiated until it exits the gun barrel...
). 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 powderSmokeless powderSmokeless powder is the name given to a number of propellants used in firearms and artillery which produce negligible smoke when fired, unlike the older gunpowder which they replaced...
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
.40 S&W
The .40 S&W is a rimless pistol cartridge developed jointly by major American firearms manufacturers Winchester and Smith & Wesson. The .40 S&W was developed from the ground up as a law enforcement cartridge designed to duplicate performance of the FBI's reduced velocity 10mm cartridge which could...
, 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.