Overpressure ammunition

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Overpressure ammunition, commonly designated as +P or +P+ (pronounced Plus-P or Plus-P-Plus), is

defensive, or hunting purposes. Because of this, +P ammunition is typically found in handgun calibers which might be used for paramilitary forces, armed security, and defensive purposes
.

+P vs. magnum cartridges

Magnum cartridges, such as the .357 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 Magnum is slightly longer than the .38 Special, 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. It is not an advisable practice to utilize +P ammunition in firearms of questionable quality or in a state of disrepair. In such cases, the margin of safety may be eroded to the extent that hazards or malfunctions will result.

History

The burning characteristics of

black powder used in early cartridges meant that these cartridges operated at lower chamber pressures, generally under 25,000 psi (170 MPa). 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, which made rifles with nominal powder capacities from 70 grains (.45-70) to 110 grains (.45-110).[1]

With the advent of

lever-action rifles. The rifle actions could handle much higher pressures, than their revolver counterparts. This led to "machine gun only" loads in these calibers, which provided far more velocity and energy than in rifles, but were not safe in revolvers, due to the higher chamber pressures that were being generated. These loads were eventually dropped due to a combination of safety concerns, and newer smokeless powder rifle cartridges that offered even higher velocities, such as the .30-30 Winchester.[2]

The first modern smokeless powder cartridge deliberately loaded by a major manufacturer to higher than standard pressure was the

.38-44 High Velocity, which was manufactured from 1930 to 1966; which eventually lead to the development and production of the .357 Magnum in 1935.[2][3]

Standards

In the

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%.[4] 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

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 (psi) +P pressure (psi) Notes
9mm Parabellum
35,000 psi (240 MPa) 38,500 psi (265 MPa) 10.00% increase
.38 Special 17,500 psi (121 MPa) 20,000 psi (140 MPa) 14.29% increase
.45 ACP 21,000 psi (140 MPa) 23,000 psi (160 MPa) 9.52% increase
.38 ACP 26,500 psi (183 MPa) 36,500 psi (252 MPa) 37.74% increase
.257 Roberts 54,000 psi (370 MPa) 58,000 psi (400 MPa) 7.41% increase

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 (290 MPa), a 20% increase over the standard pressure of 35,000 psi (240 MPa), and the .38 Special +P+ as 22,000 psi (150 MPa), a 25.71% increase over the standard pressure of 17,500 psi (121 MPa).[5]

Small ammunition makers and reloading guides will often include special loads for specific purposes, such as the below listed

.41 Magnum and .44 Magnum
, the pressure can be pushed far higher with no ill effects. However, since these loads, with nearly double the chamber pressure, could destroy a firearm intended for use with black powder level loads, although they are less commonly encountered.

Custom and handloaded overpressure cartridges

Some older cartridges, especially those that were originally

Thompson/Center Contenders", "Use only in modern Marlin and Winchester lever-action rifles" or in "Ruger No. 1 and Ruger No. 3" single-shot rifles.[6][7]

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.[8] The following table lists some non-SAAMI +P loads for which the manufacturer publishes pressure information.

Cartridge Standard pressure (psi) +P pressure (psi) Notes
.44 Magnum 36,000 psi (250 MPa) 43,500 psi (300 MPa) 20.83% increase, Garrett Cartridge, Ruger and Dan Wesson DA revolvers, long-frame single action conversions[6]
.45 Colt 14,000 psi (97 MPa) 23,500 psi (162 MPa) 67.86% increase, Accurate Powder loading manual, Ruger and T/C only[9]
.45-70 28,000 psi (190 MPa) 35,000 psi (240 MPa) 25.00% increase, Garrett Cartridge, modern rifles including lever and break actions[6]
.45-70 28,000 psi (190 MPa) 40,000 psi (280 MPa) 42.86% increase, Accurate Powder loading manual, Ruger and similar high strength actions[10]

+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 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.[11] 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 affect 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.[12] 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.

+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 material
  • Bullet shape
  • Bullet diameter
  • Bullet seating depth
  • Test barrel diameter
  • Test barrel chamber shape
  • Bullet hardness
  • Friction in bore
  • Crimp strength
  • Smokeless powder 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.[13]

References

  1. ^ "The Shiloh Sharps .45-110". Retrieved 2 October 2014.
  2. ^ .
  3. ^ Chuck Taylor (May 2000). ".38-44 HV: The Original Magnum - revolver round". Guns Magazine.
  4. ^ "How are proof pressures determined?". Retrieved 2 October 2014.
  5. ^ "What is +P and +P+ ammunition?". Retrieved 2 October 2014.
  6. ^ a b c "Garrett Cartridge". Retrieved 2 October 2014.
  7. ^ "Condensed Load Guide Version 3.2.2" (PDF). Accurate Powder. Archived from the original (PDF) on 2008-05-16.
  8. ^ Hydra-Shok ballistics Archived 2007-02-24 at the Wayback Machine
  9. ^ Accurate Powder. ".45 Colt (Ruger & T/C only)" (PDF). Archived from the original (PDF) on 2008-11-20.
  10. ^ Accurate Powder. ".45-70 High Pressure" (PDF). Archived from the original (PDF) on 2007-09-30.
  11. ^ "HK USP Operators Manual" (PDF). Archived from the original (PDF) on 13 April 2004. Retrieved 2 October 2014.
  12. ^ "Titanium Taurus" in Small Arms Review Archived 2007-01-30 at the Wayback Machine
  13. ^ "How do changing various components affect chamber pressure and velocity?". Retrieved 2 October 2014.