While browsing for ammunition at your local gun store, you may have noticed that certain boxes of ammunition feature the label +P. You might have also found the term in some of your guns’ instruction manuals, either advising against its use or indicating that it is safe to shoot.
Find out what exactly is +P ammunition, how it differs from regular ammunition, what it does to your guns, and what you can do with it.
What is +P Ammo?
+P stands for overpressure, which refers to ammo of a specific caliber designed to produce a higher chamber pressure than usual.
To fully understand what overpressure means and how it affects your firearm, it is essential to understand what chamber pressure is and what constitutes standard and non-standard pressure levels.
Chamber pressure
The term chamber pressure refers to the maximum chamber pressure (Pmax) a standard cartridge of a given caliber is expected to produce when firing the cartridge and igniting the gunpowder it contains.
In the United States, the Sporting Arms and Ammunition Manufacturers Institute (SAAMI) is the standards organization responsible for testing the reliability, safety, and interchangeability of all commercially available ammunition in the country. SAAMI publishes an extensive collection of data and standards information, including the pressure statistics for every cartridge they tested.
For example, the Pmax of 9x19mm Luger is 35,000 psi, meaning that all 9x19mm ammunition sold in the United States must not exceed 35,000 psi of pressure in the chamber when fired. Actual pressure varies from cartridge to cartridge due to minute differences in gunpowder quantities.
Specifications of +P Ammunition
Generally, overpressure ammunition is any brand or type of ammo that the manufacturer has intentionally made to exceed the corresponding caliber’s Pmax. There are no differences in external dimensions, making it possible to chamber standard and +P ammunition of a given caliber in the same firearms.
However, the exact amount of pressure needed to be considered +P varies from caliber to caliber, and SAAMI does not offer +P standards for all cartridges.
Examples of cartridges with SAAMI +P standards include:
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9x19mm Luger: 35,000 psi (Standard Pmax), 38,500 psi (+P Pmax). The difference is exactly 10%.
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.45 ACP: 21,000 psi (Standard), 23,000 psi (+P). Here, the difference is only 9.52%.
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.38 Special: 17,500 psi (Standard), 20,000 psi (+P). For this cartridge, +P ammunition develops 14.29% more pressure than standard.
Despite being available in a wide range of velocities and bullet weights, many other commonly available pistol cartridges (e.g., .40 S&W, 10mm Auto) do not have +P designations. In addition, the +P moniker is virtually nonexistent in rifle and shotgun ammunition.
What about +P+?
Some firearm enthusiasts and ammo manufacturers may market their ammunition as +P+, typically denoting a Pmax that exceeds even the SAAMI +P standard. This designation is unofficial and not recognized by the organization and has no consistent definition.
For example, while you can be sure that 9mm +P ammunition in the United States may not exceed 38,500 psi, there is no way to know the Pmax of a 9mm +P+ brand unless the manufacturer provides specific numbers.
The .38 Super case
.38 Super (metric designation: 9x23mm SR) ammunition sold today typically possesses the +P designation, despite the lack of a non-+P equivalent on the market.
The reason behind the +P designation for this particular caliber is due to the cartridge’s dimensions: .38 Super is dimensionally identical to .38 ACP, a cartridge introduced at the turn of the 20th century. In other words, you can consider .38 Super to be equivalent to .38 ACP +P.
Standard .38 ACP has a Pmax of 26,500 psi, whereas .38 Super is rated at 36,500 psi (over 37% higher). While you can chamber and fire .38 Super in a .38 ACP firearm, doing so carries a high risk of destroying it and causing severe injuries.
Performance of +P Ammunition
The most common ways to obtain a higher chamber pressure are to increase the quantity of gunpowder inside the case or to use a faster-burning, more powerful propellant.
Assuming all other factors are equal (same projectile, same barrel length, etc.), more pressure translates into increased velocity and higher energy output. Effectively, +P ammunition is more powerful, but increasing the power is a double-edged sword; it is not necessarily more lethal or effective.
Advantages of +P ammo
The primary advantage of a higher velocity is to increase the amount of energy on impact. For self-defense ammunition, which typically uses hollow-point bullets, more velocity also means more reliable expansion.
FBI testing protocols have determined that ideal self-defense ammunition should expand as much as possible while penetrating between 12 and 18” in standard ballistic gelatin. If standard-pressure ammo cannot meet these standards in your firearm, +P may give it the extra performance it needs.
Although not as critical for self-defense, a higher velocity helps make the bullet trajectory flatter, potentially making it slightly easier to hit a target at extended ranges.
Disadvantages of +P ammo
The primary drawbacks of +P ammunition are the logical costs of increasing the amount of chamber pressure: higher recoil, louder muzzle blast, and more muzzle flash.
The exact amount of extra recoil and muzzle blast depends on your firearm, specifically, its weight and barrel length. However, 9mm +P ammunition generally develops more across the board, which can make your gun harder to control.
More recoil means having to spend more time to re-acquire your sights after shooting. A louder and brighter muzzle blast can also disorient you more in a self-defense situation.
Additionally, more power also translates into increased wear and tear on your gun. Even if your particular model is designed to shoot +P ammo safely, regularly shooting +P out of your firearm means you should expect a shorter service life.
Should I Use 9mm +P Ammunition?
When choosing ammunition for a self-defense 9x19mm firearm, safety should be your priority. Verify that your gun can safely shoot +P ammo before loading. You can find whether your gun can safely shoot +P ammunition by checking the instruction manual or contacting the manufacturer.
If your gun can safely shoot +P ammunition, the choice is yours. Test a particular brand at the range and get an idea of its performance. If you find the advantages of +P outweigh the potential drawbacks, don’t forget to practice regularly with both +P and standard-pressure ammo.
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